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NATIONAL QUALIFICATIONS CURRICULUM SUPPORT

Chemistry

Staff Notes forUnit 2: Principles of Chemicl

Rections

!A"#ANC$" %I&%$R'

&(in )hitt*erUni(ersity of $+in,-r.h

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Acknowledgements

This +oc-ment is pro+-ce+ ,y Lernin. n+ Techin. Scotln+ s prt of the Ntionl

Q-lifictions s-pport pro.rmme for Chemistry/ &rtef-l thn*s re +-e to the $+-ction

"i(ision of the Royl Society of Chemistry0 Scottish Re.ion0 for comment on the +rfts/ The

s-pport of the %i.her Still "e(elopment Unit n+ the e+itoril +(ice of "o-.ls 1-chnn

re lso c*nole+.e+/

The -thor n+ p-,lisher re .rtef-l to the folloin. for permission to repro+-ce

photo.r phs: Mrc "y0 Lrence 1er*eley Ntionl L,ortory0 USA 3hy+ro.en fire 0 p.e

2456 Interntionl F-el Cells n+ 1llr+ Poer Systems 3f-el cell im.es0 p.es 78975/

First p-,lishe+ 2;;4

$lectronic (ersion 2;;2

< Lernin. n+ Techin. Scotln+ 2;;4

This p-,liction my ,e repro+-ce+ in hole or in prt for e+-ctionl p-rposes ,y

e+-ctionl est,lishments in Scotln+ pro(i+e+ tht no profit ccr-es t ny st.e/

IS1N 4 =7>77 >4? 4

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CONTENTS

Section 1:Thermochemistry 4

Section 2:Rection fesi,ility 4

Section 3:$B-ili,ri 2>

Section 4:F-el cells 74

Section 5: inetics 7>

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ote on s!"et#

TechersDlect-rers re +(ise+ tht it is their responsi,ility

to t*e notice of employersE re.-ltions ith re.r+ to the

sfe prctices to ,e folloe+/ )here necessry0 prior to

-sin. chemicls0 the rele(nt +(ice my ,e cons-lte+ in

the Haz ar do us Chem ical s Manua l 3SS$RC50 either in

pr int e+ for m or in C" ROM (ersion /

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SECTION 1

Int%od&ction

Thermochemistry is the ,rnch of thermo+ynmics tht +els ith the

interchn.e of het in chemicl processes/

The science of t'e%mod#n!mics itself s +e(elope+ thro-.h necessity in

the nineteenth cent-ry n+ le+ +irectly to the stem en.ine/ It is

mcroscopic science0 in tht it llos -s to +el ith properties of mtter

itho-t ny ss-mptions ,o-t the nt-re of mtter itself/ Thermo+ynmics

eGmines the reltionships ,eteen to physicl B-ntities 9 ener.y n+entropy/ Ene%g#my ,e re.r+e+ s the cpcity to +o or*0 hile ent%o(#

my ,e re.r+e+ s mes-re of the +isor+er of system/

In the chemicl conteGt0 the reltionship ,eteen these properties is the

+ri(in. force ,ehin+ chemicl rections n+ +etermines their fesi,ility/

Since ener.y is either relese+ or t*en in0 n+ since the le(el of +isor+er is

chn.e+ in ll chemicl processes0 thermo+ynmics en,les -s to pre+ict

hether rection my occ-r or not itho-t the nee+ to consi+er str-ct-re0

,on+in. or rection mechnism/

There re limittions to the prcticl scope of thermochemistry tht sho-l+

,e ,orne in min+/ Consi+ert ion of the het chn.e in the co-rse of

rection is only one prt of the story/ Altho-.h hy+ro.en n+ oGy.en ill

rect to relese .ret +el of ener.y -n+er the correct con+itions0 ,oth

.ses ill coeGist in+efinitely itho-t rection/ Thermo+ynmics tells -s

,o-t the potentialfor chemicl chn.e0 not the rateof chemicl chn.e 9

tht is the +omin of chemicl *inetics/ 1ec-se it is s-ch commonly hel+

misconception tht the potentil for chn.e +epen+s on the relese of ener.y0

it sho-l+ lso ,e hel+ in the ,c* of oneEs min+ tht it is not ener.y ,-t

entropy tht is the finl r,iter of chemicl chn.e/

$ner.y is trnsferre+ s either het or or*0 hich0 hile fmilir0 re

not lys esily +efine+/ One of the most -sef-l +efinitions is +eri(e+

from the mechnicl fshion in hich ener.y is trnsferre+ either s

het or or*/ %et is the trnsfer of ener.y s +isor+erly motion0 s

the res-lt of tempert-re +ifference ,eteen the system n+ its

s-rro-n+in.s/ )or* is the trnsfer of ener.y s or+erly motion/ In

mechnicl terms0 or* is +-e to ener.y ,ein. eGpen+e+ .inst n

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opposin. force/ The totl or* is eB-l to the pro+-ct of the force n+ the

+istnce mo(e+ .inst it/ )or* in chemicl or ,iolo.icl systems .enerlly

mnifests itself in only limite+ n-m,er of forms/ The most commonlyenco-ntere+ re press-re9(ol-me or* n+ electricl or*/

Ent'!l(#

The mHority of chemicl rections0 n+ lmost ll ,iochemicl processes in

li(in. or.nisms0 re performe+ -n+er constnt press-re con+itions n+

in(ol(e relti(ely smll (ol-me chn.es/ )hen process t*es plce -n+er

these con+itions0 -sef-l mes-re of het trnsfer is enthlpy0 +enote+ ,y

the letter H/

The ri.oro-s +efinition of enthlpy0 H0 is ,eyon+ this teGt0 ,-t chn.e in H

3H5 my ,e con(eniently +efine+ s the het .i(en o-t or t*en in +-rin.

rection 3or ny other process5 t constnt press-re/ In other or+s0 it is the

het chn.e -n+er the most commonly enco-ntere+ rection con+itions/

St!te "&nctions !nd (!t' "&nctions

An importnt clssifiction of thermo+ynmic properties is hether

they re re.r+e+ s stte f-nctions or pth f-nctions/ If the (l-e

of prtic-lr property for system +epen+s solely on the stte ofthe system t tht t ime0 then s-ch property is referre+ to s

st!te "&nction / $Gmples of stte f-nctions re (ol-me0 press-re0

internl ener.y n+ entropy/ )here property +oes not +epen+ on

the stte of the system0 ,-t on the pth ,y hich system in one

stte is chn.e+ into nother stte0 then tht property is referre+ to

s (!t' "&nction / The +istinction is importnt ,ec-se in

performin. clc-ltions on stte f-nctions e nee+ t*e no cco-nt

of ho ny stte of interest s prepre+/

This +istinction my ,e compre+ to ltit-+e/ A person t the ,seof mo-ntin hs specific ltit-+e0 s +oes one t the s-mmit/

The mo-nt of or* tht is +one in clim,in. from the ,se to the

s-mmit +epen+s on the ro-te tht is t*en0 n+ this or* is pth

f-nction/ Th-s0 person clim,in. from Fort )illim to the s-mmit

of 1en Ne(is chn.es their ltit-+e 3 stte f-nction5 ,y 8;;; feet

hether they t*e the most +irect ro-te or hether they tr(el (i

Uln 1tor0 ltho-.h the mo-nt of or* tht is +one 3 pth

f-nction5 ill clerly ,e +ifferent in ech cse

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)%o(e%ties o" ent'!l(#

$nthlpy is stte f-nction/ In other or+s system possesses +efine+(l-e for ny prtic-lr system t ny specific con+itions of tempert-re n+

press-re/ %oe(er0 prcticl consi+ert ions men tht its ,sol-te (l-e in

system cnnot ,e *non0 since there o-l+ ,e contri,-tions to the ,sol-te

enthlpy from0 for eGmple0 n-cler ,in+in. ener.ies/ Fort-ntely0 changes

in enthlpy cn ,e mes-re+0 n+ re +enote+ ,y the sym,ol H0 here

HJ Hfinl

9 Hinit il

/

$nthlpy chn.es my res-lt from either physicl processes 3e/./ het loss to

col+er ,o+y5 or chemicl processes 3e/./ het pro+-ce+ +-rin. chemicl

rection5/

An increse in the enthlpy of system le+s to n increse in its

tempert-re 3n+ (ice (ers50 n+ is referre+ to s n endot'e%mic process/

Loss of het from system loers its tempert-re n+ is referre+ to s n

e*ot'e%mic process/ The si.n of Hin+ictes hether het is lost or .ine+

,y the system/ For n eGothermic process0 here het is lost from the

system0 Hhs ne.ti(e (l-e/ Con(ersely0 for n en+othermic process in

hich het is .ine+ by the system0 His positi(e/ The si.n of Hin+ictes

the +irection of het flo n+ sho-l+ lys ,e eGplicitly stte+0 e/./

HJ K2/8 * mol94 /

For chemicl rections0 the most ,sic reltionship follos +irectly from the

fct tht enthlpy is stte f-nction/ The enthlpy chn.e tht ccompnies

chemicl rection is eB-l to the +ifference ,eteen the enthlpy of the

pro+-cts n+ tht of the rectnts:

H rection J H pro+-cts 9 H rectnts

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St!nd!%d st!te

The enthlpy chn.es ssocite+ ith ny rection re0 to (ryin.+e.rees0 +epen+ent on tempert-re0 press-re0 n+ the sttes of the

rectnts n+ pro+-cts/ In or+er to -tilise enthlpy mes-rements0

therefore0 it is importnt tht the con+itions -n+er hich the rection

t*es plce re i+enticl/ For this reson0 it is con(enient to specify

st!nd!%d st!tefor s-,stnce/ The stn+r+ stte for s-,stnce is

+efine+ s ,ein. the pure 0 undiluted substancet 1 atmosphere pressure

n+ t specif ied temperature/ The tempert-re +oes not form prt of the

+efinition of the stn+r+ stte0 ,-t for historicl resons +t re

.enerlly B-ote+ for 2>=/47 327C5/ For sol-tions0 the +efinition of the

stn+r+ stte of s-,stnce is 4 mol l94 concentrtion/

Table 1

Some common enthalpy changes (note that the enthalpy change is not the

same as the free energy change see page 25!

St!nd!%d ent'!l(# c'!nges

+&!ntit# Ent'!l(# !ssoci!ted ot!tion E*!m(le

wit':

$nthlpy of ionistion $lectron loss from Hi

N3.5 NK3.5 K e93.5

species in the .s phse

$nthlpy of electron &in of n electron Hec

F23.5 K e93.5 F93.5

.in

$nthlpy of (poristion #poristion of H(

%2O3l5 %

2O3.5

s-,stnce

$nthlpy of s-,limtion S-,limtion of Hs-,

CO23s5 CO

23.5

s-,stnce

$nthlpy of rection Any specifie+ chemicl H 2Fe2O

?3s5 K ?n3s5 Fe3s5 K ?nO3s5

rection

$nthlpy of com,-stion Complete oGi+tion of Hc

%23.5 K O

23.5 %

2O3.5

s-,stnce

$nthlpy of formtion Formtion of s-,stnce Hf

C3s5 K 2%23.5 C%

83.5

from its elements intheir stn+r+ stte/

$nthlpy of sol-tion "issol-tion of Hsol

NCl3s5 NK3B5 K Cl93B5

s-,stnce in specifie+B-ntity of sol(ent

$nthlpy of sol(tion Sol(tion of .seo-s ions Hsol(

NK3.5 K Cl3.5 NK3B5 K Cl93B5

from n ionic s-,stnce

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The +efinition of stn+r+ stte llos -s to +efine the st!nd!%d ent'!l(#

c'!nges the enthlpy chn.e tht comes ,o-t hen rectnts in their

stn+r+ sttes re con(erte+ into pro+-cts in their stn+r+ sttes/ Theenthlpy chn.e my ,e the res-lt of either physicl process0 s-ch s

meltin.0 or chemicl process0 s-ch s oGi+tion/ The stn+r+ enthlpy

chn.e for process is +enote+ s H2>=

0 ith the s-,script +enotin. the

tempert-re/

In or+er to i+ concise +isc-ssion0 n-m,er of chemicl n+ physicl

processes re .i(en specif ic nmes/ Thermo+ynmiclly0 there re no

+ifferences ,eteen the processes0 since they ll refer to the het chn.e s

the components in the system con(ert from one form to nother/ The only

reson for the -se of these specific terms is con(enience n+ ,re(ity/ Aselection of the more importnt processes is liste+ in T,le 4/

Me!s&%ing ent'!l(# c'!nges

$nthlpy o-l+ clerly ,e of limite+ importnce if e ere not ,le to

mes-re enthlpy chn.es/ Fort-ntely0 there re n-m,er of

eGperimentl metho+s tht llo -s to B-ntify the enthlpy chn.es tht

ccompny lmost ll concei(,le chemicl rections or physicl

processes0 s-ch s melt in. or ,oil in./ The metho+s my ,e ,ro+ly

cte.orise+ s +irect or in+irect/ ,i%ect met'ods0 s the nme s-..ests0

in(ol(es the +irect mes-rement of the enthlpy ssocite+ ith the

rection or process/ The most common metho+ for this -ses

clorimeter0 s-ch s ,om, clorimeter/ Indi%ect met'odsre -se+ to

+etermine enthlpy chn.es for processes tht re +iffic-lt0 or prcticlly

impossi,le0 to mes-re ,se+ on Hess-s l!w0 n+ in(ol(e clc-ltin. the

enthlpy chn.e of interest in+irectly from enthlpy chn.es tht cn ,e

cc-rtely mes-re+/

,i%ect met'ods

"irect metho+s re reser(e+ for rections tht cn ,e performe+ ith hi.h+e.ree of selecti(ity 3i/e/ the rection procee+s clenly n+ completely to

pro+-cts itho-t formin. ,ypro+-cts50 ith the mes-rements ,ein.

performe+ -sin. clorimetry 9 literlly to mes-re hetE 3,ein. mon.rel

or+ from the Ltin calor0 het0 n+ the &ree* metron0 mes-re5/

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The most fmilir techniB-e in clorimetry 3if not lys the ,est

-n+erstoo+5 is the ,om, clorimeter0 in hich mteril is ,-rnt in ,om,

t constnt (ol-me in (ery hi.h press-re oGy.en tmosphere/ The ,om, isplce+ in (ol-me of ter0 n+ the het from the rection increses the

tempert-re of the ter/ 1y *noin. the tempert-re chn.e in the ter

n+ the het reB-ire+ to c-se this tempert-re chn.e 3the clorimeter

h(in. pre(io-sly ,een cli,rte+ -sin. *non s-,stnce of n electric

heter50 the het .i(en o-t from the rection cn ,e +etermine+/

It is not lys pprecite+ tht the het mes-re+ ,y this metho+ +oes not

necessrily pro(i+e +irect (l-e for the enthlpy chn.e/ This is ,ec-se

enthlpy chn.es re mes-re+ t constnt press-re0 n+ this metho+ pro(i+es

mes-re of the het .i(en o-t t constnt (ol-me/ The +ifference ,eteen

the to (l-es rises ,ec-se t constnt press-re some ener.y hs to ,e -se+

-p in p-shin. ,c* the tmosphere/

Sensiti/it#

For most people0 their eGperience of clorimeters is of poorly repro+-ci,le

mes-rements of hi.hly eGothermic rections/ The le(el of sophistiction

n+ cc-rcy in mo+ern clorimetry is therefore someht s-rprisin.

3mo+ern ,ein. relti(e term 9 hi.h le(el of cc-rcy hs ,een (il,le

for t lest 7; yers5/ The rpi+ly .roin. interest in ,iolo.icl

rections0 for eGmple0 hs le+ to some interestin. pproches to theclorimetry of rections tht relese only smll mo-nts of het/ The

eGperimentl mes-rement of Hfor these rections is clle+

mic%oc!lo%imet%# / In the 4>7;s the ,iophysicist T % 1ein.er s

lr.ely responsi,le for the 'e!t0&%st c!lo%imet%#techniB-e/ Usin. this

metho+0 1ein.er mes-re+ the het e(ol(e+ hen 7 mmol of +enosine

triphosphte 3ATP5 s hy+rolyse+ in the presence of the enyme myosin

to ,e ;/? / So sensiti(e is the techniB-e tht it s necessry to

compenste for the het e(ol(e+ s protons relese+ ,y the rection recte+

ith the ,-ffer sol-tion/ The +(ent of mo+ern electronics n+ hi.hly

sensiti(e tempert-re sensors hs m+e e(en this eGtror+inrychie(ement seem insensiti(e/

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Hess-s l!w

)e h(e lre+y +escri,e+ enthlpy s stte f-nction0 n+ it follos fromthis tht the ,sol-te enthlpies ssocite+ ith the rectnts n+ pro+-cts in

rection re in+epen+ent of the process ,y hich they ere forme+/

ConseB-ently0 the o(erll enthlpy chn.e +-rin. the co-rse of rection

3clc-lte+ s H rectnts 9 H pro+-cts5 is in+epen+ent of the rection

pthy/ Hess-s l!w o" const!nt 'e!t s&mm!tion is reco.nition of this

fct0 n+ sttes tht:

"#he o$erall enthalpy change for a reaction is e%ual to the sum of the

enthalpy changes for the indi$idual steps in the reaction measured at

the same temperature!&

This l is prtic-lrly -sef-l hen mes-rement of specific enthlpy

chn.e is imprcticl or -nfesi,le/ A clssic eGmple of this is the

mes-rement of the enthlpy chn.e ssocite+ ith the ,-rnin. of cr,on to

form cr,on monoGi+e/ The +iffic-lty ith this mes-rement is tht

formtion of some cr,on +ioGi+e cnnot ,e pre(ente+0 m*in. +irect

eGperimentl mes-rement of the enthlpy chn.e impossi,le/ %oe(er0 the

enthlpy chn.e my ,e mes-re+ in+irectly0 ,y consi+erin. the +irect 3one

step5 rection s n in+irect 3tostep5 process 3Fi.-re 45/

Figure 1

H4J H

29 H

?J 924 * mol94

C K O2

CO K O2

H2J 9?>8 * mol94 H

?J 942? * mol94

CO2

%essEs l in+ictes tht the totalenthlpy chn.e ,y either pth is i+enticl0

in hich cse e my rite H4J H

2K H

?0 so lloin. -s to o,tin

(l-e for H4itho-t the nee+ for +irect mes-rement/

It is lso possi,le to clc-lte the enthlpy ,y ++ition or s-,trction of

rections0 n+ in this eGmple this my ,e +one s follos:

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4/ C3s5 K O23.5 CO

23.5 HJ 9?>8 * mol94

2/ CO3.5 K O23.5 CO23.5 HJ 942? * mol94

492 C3s5K O23.5K CO

23.5 CO

23.5 K CO3.5K O

23.5 HJ 9?>8 93942?5 * mol94

C3s5 K O23.5 CO3.5 HJ 924 * mol94

This pproch is effecti(e0 ,-t cn ,ecome -niel+y n+ conf-sin. hen

se(erl rections re ++e+ n+ s-,trcte+/ It is often esier n+ more

instr-cti(e to -se .rphicl metho+s0 ,-t the ,est pproch is the one tht the

st-+ent fin+s esiest to (is-lise n+ this is s-,Hecti(e +ecision for thetecher n+ st-+ent/

T'e ent'!l(# o" "o%m!tion

As +efine+ in T,le 40 the enthlpy of formtion for s-,stnce is the

enthlpy ssocite+ ith the formtion of s-,stnce in its stn+r+ stte

from its elements in their stn+r+ sttes/ The -sef-lness of this concept is

most re+ily pprecite+ hen it is -se+ in conH-nction ith %essEs l/

)hile0 in principle0 it is possi,le to t,-lte enthlpy (l-es for ll

mes-r,le rections0 s-ch +t re limite+ n+ o-l+ ,e (ery +iffic-lt toin+eG n+ ccess/ T,les listin. the enthlpies of formtion of i+e rn.e

of mterils0 on the other hn+0 my ,e fo-n+ in the chemicl litert-re n+

re more re+ily ccessi,le thn the enthlpy chn.e ssocite+ ith

specific rection/ For ny rection0 it is possi,le to constr-ct rection

pthys tht procee+ (i the elementl components of ,oth the rectnts n+

the pro+-cts 3Fi.-re 25/ The (l-e for H rection is re+ily clc-lte+ from:

H rection J Hfpro+-cts 9 H

frectnts

%ence0 for the eGmple rection in Fi.-re 20 the rection enthlpy is .i(en

,y:

Hrection J

!Hf3C%

?COO C%

?5 K H

f3%

2O5' 9 !H

f3C%

?COO %5 K H

f3C%

?O%5'

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Figure 2

H

rectionC%

?COO % K C%

?O% C%

?COO C%

?K %

2O

Hf3C%

?COO % K C%

?O%5 H

f3C%

?COO C%

?K %

2O5

8%2K 4O

2K ?C

T'e ent'!l(# o" com&stion

The enthlpy of com,-stion is the enthlpy ssocite+ ith the ,-rnin. of

3-s-lly one mole of5 s-,stnce in oGy.en/ This is -sef-l B-ntity0 s it is

possi,le to -sef-lly com,ine the enthlpy of com,-stion n+ %essEs l0 H-st

s %essEs l is -sef-l in com,intion ith the enthlpy of formtion/ The

o,(io-s limittion of this metho+ is tht it my only ,e pplie+ to rections or

processes tht in(ol(e com,-st i,le s-,stnces / T*e the eGmple of the

rection ,eteen methne n+ ter0 hich my ,e -se+ to .enerte

hy+ro.en for -se s f-el in crs poere+ -sin. f-el cells 3see Section 85/ The

ener.y reB-ire+ to ccomplish this my ,e clc-lte+ -sin. t,-lte+ (l-es

for enthlpies of com,-stion:

C%8K %

2O CO K?%

2

)e my ++ s-fficient oGy.en to ,oth si+es of the eB-tion to formlly

com,-st the rectnts n+ pro+-cts 3Fi.-re ?5

Figure 3

H rectionC%

8K %

2O K 2O

2 CO K ?%

2K 2O

2

Hc3C%

85 H

c3CO K ?%

25

CO2K ?%

2O

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The o(erll enthlpy of rection is -nffecte+ ,y this ltertion0 ,-t

Hrection my no ,e clc-lte+ -sin. %essEs l 3note the chn.e of si.n

s compre+ to the pre(io-s eGpression5:

H rection J 9Hcpro+-cts K H

crectnts

From hich it my ,e seen tht:

Hrection J Hc3C%

8K ?%

25 9 H

c3CO K ?%

25

The +(nt.e of this metho+ is tht enthlpies of com,-stion re more

re+ily o,tine+ thn hets of formtion/ The +is+(nt.e is tht it cn only

,e pplie+ to rections in(ol(in. com,-sti,le s-,stnces0 rest riction tht.enerlly lso eGcl-+es mterils in sol-tion/

Ent'!l(# o" sol/!tion

The enthlpy of sol(tion0 Hsol(

0 is nother specific type of enthlpy chn.e/

In this cse0 hoe(er0 the process is n lmost p-rely hypotheticl cse0

hich is +iffic-lt 3if not impossi,le5 to .enerte eGperimentlly0 n+ is lmost

completely impossi,le to mes-re/ The enthlpy of sol(tion is +efine+ s

the enthlpy chn.e hen mole of .seo-s ions from n ionic s-,stnce is

sol(te+ in n infinite mo-nt of sol(ent0 for eGmple:

N K3.5 K Cl93.5 NK3B5 K Cl93B5

$nthlpies of sol(tion re clc-lte+ -sin. %essEs l/

Ent'!l(# o" sol&tion

The enthlpy of sol-tion0 Hso l

0 is the enthlpy chn.e ssocite+ ith the

+issol-tion 3+issol(in.5 of s-,stnce in sol(ent/ It is most often B-ote+

for mole of s-,stnce +issol(in. in n infinite mo-nt of sol(ent0 for

eGmple:

NCl3s5 NK3B5 K Cl93B5

The enthlpy of sol-tion my ,e +etermine+ ,y mes-rement of the enthlpy

ssocite+ ith finite rtios of sol-te:sol(ent0 folloe+ ,y eGtrpoltion of

these (l-es to the (l-e ssocite+ ith n infinite B-ntity of sol(ent/

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T'e 6o%n7H!e% c#cle

The 1orn9%,er cycle is specific ppliction of the first l ofthermo+ynmics 9 tht ener.y my not ,e crete+ or +estroye+ 9 -sin. %essEs

l/ The cycleE 9 series of rections tht form close+ pth 9 llos the

l!ttice ent'!l(#of n ionic soli+ to ,e clc-lte+/ This is the enthlpy

ssocite+ ith the +irect com,intion of .seo-s ions to form n ionic

lttice:

nMmK3.5 K

mn93.5 M

n

m3s5

1ec-se +irect mes-rement of this process is .enerlly imprcticl0 n

in+irect pth is crete+/ Usin. Cl s n eGmple0 the cycle ill-strte+ inFi.-re 8 is o,tine+/

Figure 4

#he 'ornHaber cycle for Cl

3s5 K NCl23.5

3.5 K NCl23.5

K3.5 K NCl23.5 K eF

K3.5 K Cl3.5 K eF

Cl3s5

K3.5 K ClF3.5

%f 3Cl5

%s35

%i 35

N%+3Cl25%ec 3Cl5

%l3Cl5

If Cl is t*en s the strtin. point0 the folloin. eGpression my ,e

.enerte+0 s enthlpy is stte f-nction n+ the enthlpy chn.e o(er the

complete cycle m-st therefore eB-l ero:

9Hf3Cl5 KH

s35 K H

i35 K H

+3Cl

25 K H

ec3Cl5 K H

l3Cl5 J ;

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rerrn.in. .i(es:

Hl3Cl5 J Hf3Cl5 9 Hs35 9 Hi35 9 H+3Cl25 9 Hec3Cl5

The terms on the ri.hthn+ si+e of this eB-tion my ll ,e o,tine+ ,y

+irect physicl or spectroscopic metho+s0 .i(in. (l-e for the lttice

enthlpy:

Hl3Cl5 J 98?4 9 => 9 84> 9 428 9 39?8>5 J 948 * mol 94

6ond ent'!l(#

In the co-rse of chemicl rection0 some 3or ll5 of the ,on+s re ,ro*en inthe rectnts0 n+ ne ,on+s re crete+ s the pro+-cts form/ Since0 ,y

+efinition of chemicl process0 this is essentilly the only so-rce of ener.y

chn.e in the process0 it sho-l+ 9 t lest in principle 9 ,e possi,le to

clc-lte this ener.y chn.e/ If the ener.y of the ,on+s in the rectnts is

clc-lte+ n+ s-,trcte+ from tht of the pro+-cts0 this sho-l+ .i(e the

ener.y chn.e in the rection/

For eGmple0 the +ecomposition of %2O

23.5 t*es plce to .i(e ter n+

oGy.en:

2%2O

23.5 2%

2O3.5 K O

23.5

On the lefthn+ si+e of the eB-tion there re 2 2 %9O ,on+s n+

2 O9O ,on+s/ On the ri.hthn+ si+e there re 2 2 %9O ,on+s n+

4 OJO ,on+/

If the ener.y of n 9@ ,on+ is +enote+ )39@50 then the ener.y chn.e for

this rection is .i(en ,y:

2 2 )3%9O5 K )3OJO5 9 2 2 )3%9O5 K 2 )3O9O5

The pro,lem in s-ch clc-ltions is tht the ener.y of ,on+ ,eteen to

toms0 for eGmple %9O0 (ries ,eteen +ifferent molec-les/ Consi+er the

,re*in. of the O9% ,on+s in ter0 for eGmple:

%9O%3.5 %3.5 K O%3.5 HJ K8>2 * mol94

O9%3.5 %3.5 K O:3.5 HJ K82 * mol94

In some molec-les0 the ener.y reB-ire+ to ,re* the ,on+ 3the ond

ent'!l(#5 is .reter thn K8>2 * mol94 0 heres in others it is less thn

K82 * mol94

/

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Another common eGmple is the stepise ,re*in. of C9% ,on+s in C%8:

C% 8 C% ? K %C%

? C%

2K %

C%2 C% K %

C% C K %

The H(l-es ssocite+ ith ech step of this process pro.ressi(ely

+ecrese/ It is o,(io-s tht there is no sin.le (l-e for the O9% or C9% ,on+

enthlpies0 n+ since clc-ltion of the ener.y of specific ,on+ is not

tri(il0 it is con(enient 3if less cc-rte5 to -se !/e%!ge ond ent'!l(#

(l-es/ A selection of (er.e ,on+ ener.ies is .i(en in T,le 2 for ,on+s

,eteen se(erl first ro elements/ Notice tht the (er.e ,on+ enthlpy for+o-,le n+ triple ,on+s ,eteen elements is not tice or three times tht of

sin.le ,on+0 since the nt-re of the ,on+in. in m-ltiple ,on+s is B-ite

+ifferent from tht of sin.le ,on+s/

For the +ecomposition of hy+ro.en peroGi+e0 these (er.e (l-es yiel+ n

enthlpy chn.e of:

!32 2 8?5 K 32 485' 9 !32 2 8?5 K 8>'

J 34=72 K 2>25 9 34=72 K 8>5

J 2488 9 2?8>

J 92;7 * mol94

This compres reson,ly ell ith the eGperimentl (l-e of 4> * mol94 /

The +ifference rises ,ec-se the (er.e ,on+ enthlpies +o not represent

the precise (l-e of the ,on+ enthlpies of the molec-les/ For mny prcticl

p-rposes0 hoe(er0 the clc-lte+ (l-e is s-fficiently close to llo semi

B-ntitti(e e(l-tion of the rection/ In this eGmple0 the clc-ltion

+emonstrtes tht the rection is hi.hly eGothermic0 hich is clerly in ccor+

ith eGperiment/

The +ecomposition of %2O23B5 is slo t room tempert-re ,-t my ,e

+emonstrte+ more effecti(ely ith the -se of piece of li(er 3clfEs li(er0

for eGmple0 from ,-tcher5/ The li(er is choppe+ into smll ch-n*s n+

++e+ to ,e*er of hy+ro.en peroGi+e sol-tion/ $nymes in the li(er

ctlyse the eGothermic +ecomposition rection to ter n+ oGy.en/ The

rise in tempert-re cn ,e mes-re+ n+ -se+ to clc-lte n pproGimte

(l-e for the enthlpy chn.e/ The +ifference from the (l-e clc-lte+

,o(e o-l+ ,e +-e to ,oth eGperimentl error n+ enthlpy chn.es

ssocite+ ith chn.es in stte/

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Table 2

*$erage bond enthalpies for bonds bet+een some common elements

( indicates a single bond, - a double bond and a triple bond

H C O 8 Cl

8? 842 ?== 8? 77 8?4 H

?8= 9 ?;7 9 ?; 9 8=8 ??= C

42 J 4? J 8? J 8? J

=?= =>;

4? 9 47 2; 2;;

8;> J

>8

48 9 4=7 2;? O

8> J

477 278 8

282 Cl

1on+ enthlpies cn ,e +etermine+ eGperimentlly to hi.h le(el of cc-rcy

-sin. spectroscopic metho+s tht re ,eyon+ the scope of the A+(nce+

%i.her/ These techniB-es in(ol(e mes-rement of the ener.y reB-ire+ to

(i,rte the ,on+ to (ryin. mplit-+es n+ eGtrpoltin. this ,eh(io-r to the

point t hich the (i,rtion is infinitely lr.e0 si t-tion tht correspon+s to

complete seprtion of the toms/

In the eGmple of methne0 it is possi,le to mes-re the ,on+ enthlpy ,y this

metho+0 n+ since the ,on+s re in+istin.-ish,le0 it is not s-rprisin. tht

ech ,on+ is fo-n+ to h(e the sme (l-e for the ,on+ enthlpy/ If the

,on+s re ,ro*en in the stepise fshion .i(en ,o(e0 then fo-r +ifferent

,on+ enthlpies re o,ser(e+/ Note0 hoe(er0 tht ,y %essEs l the s-m of

the fo-r ,on+ enthlpies from the stepise processes m-st eB-l the s-m of

fo-r of the (er.eE ,on+ ener.ies/ This my ,e represente+ s shon in

Fi.-re 7/

Figure 5

C K 8%

H8 C% K ?%

H?

C% 2K 2%

H2

8 H(er.e

C% ?K %

H4

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It is possi,le to +etermine ,on+ enthlpies ,y -sin. enthlpies of formtion0

often in conH-nction ith ,on+ enthlpies tht h(e ,een more esily

+etermine+0 s-ch s those of +itomic molec-les 3% 20 N20 Cl20 etc/5/

For eGmple0 the (er.e O9% ,on+ enthlpy in %2O my ,e +etermine+

-sin. %essEs l s shon in Fi.-re /

Figure 6

2% K O

,on+ enthlpy of %2

%2K O 2 O9% ,on+

enthlpy

,on+ enthlpy of O2

%2

K O2

Hf3%

2O5

%2O

From this it is e(i+ent tht:

2 O9% ,on+ enthlpy

J Hf3%

2O5 K 39 ,on+ enthlpy of O

25 K 39,on+ enthlpy of %

25

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SECTION 2

Re!ction "e!siilit#

Chaos umpire sits,

*nd by decision more embroils the fray

'y +hich he reigns. ne/t him high arbiter

Chance go$erns all!

ohn Milton0 0aradise ost0 1oo* II0 line >;/

ohn Milton s so close to the tr-th in this B-ote tht it is pro,,ly the most

poetic (ersion of the secon+ l of thermo+ynmics in the $n.lish ln.-.e/)e ill come to tht l shortly0 ,-t let -s first set it in conteGt/

It isnEt ,y cci+ent tht chos is the res-lt of ne.lect in ny system0 ,e it

chemicl system or filin. system/ )e re -se+ to the i+e tht some thin.s

hppen spontneo-sly0 hile other thin.s +onEt/ If (se is plce+ on the

e+.e of t,le0 it is only mtter of time ,efore it is *noc*e+ off0 flls to the

floor n+ shtters into h-n+re+s of pieces ith the relese of crshin.

so-n+/ If ,ro*en (se is plce+ t the foot of t,le0 e o-l+ ne(er

eGpect so-n+ (es to spontneo-sly concentrte on those pieces n+ for

them to H-mp -p n+ reform n -n,lemishe+ (se on the t,le/ Tht is the

y of thin.s in o-r corner of the -ni(erse t lest0 n+ it is ho e

reco.nise the pss.e of time/ )e cn .enerlly tell hen film is ,ein. r-n

,c*r+s ,ec-se in o-r eGperience thin.s +o not ten+ to fll -pr+s0

stem +oes not ten+ spontneo-sly to enter stem en.ineEs eGh-st n+

ripples in pon+ +o not ten+ to r-n into one nother n+ thro o-t stone

from the ,ottom of the pon+ in the process/

In short0 the -ni(erse is r-nnin. +on ,y ,ecomin. incresin.ly +isor+ere+/

This +isor+er is not limite+ to physiclly tn.i,le o,Hects 9 ener.y is lso

r-nnin. +on into hi.her stte of +isor+er: the or+ere+ electricl ener.y

tht flos into o-r homes is con(erte+ to li.ht ener.y tht is spre+ ro-n+the room0 n+ con(erte+ into lo.r+e het ener.y s it is ,sor,e+ ,y the

lls n+ fittin.s in the room/ This het is little -se for nythin.0 ,-t this

.in is the y of the -ni(erse/ $(ent-lly0 ll the ener.y ill ,e

+istri,-te+ -niformly thro-.ho-t the -ni(erse0 ren+erin. ny chemicl or

e(en physicl e(ents impossi,le/

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REACTIO 8EASI6I9ITY

T'e d%i/ing "o%ce "o% c'!nge

isorder is important because it is the single factor that determines +hetherany physical process, including a chemical reaction, ta3es place

spontaneously! 3Spontneo-sE is ct-lly technicl term 9 see the ,oG on

p.e 4> for n ,rief eGplntion of ht it mens/5 A process ill only t*e

plce spontneo-sly if it increses the +isor+er in the -ni(erse/

It is common misconception tht rections ten+ to mo(e tor+s the loest

ener.y le(el/ In fct0 ll rections n+ physicl processes mo(e tor+s the

stte tht .enertes the lr.est mo-nt of +isor+er in the -ni(erse/ This

+isor+er pplies to ,oth the sptil +isor+er of the molec-les n+ the +isor+er

of the ener.y/ To eGmples tht my ,e esily repro+-ce+ in the l,ortory

+emonstrte tht the concept of incresin. +isor+er is more importnt thn

tht of +ecresin. ener.y/

Consi+er first to iron ,loc*s0 one t 2;;C n+ one t ;C/ If they re

,ro-.ht to.ether0 het ill flo spontneo-sly from the hot ,loc* to the col+

,loc*/ Clerly0 the hot ,loc* is mo(in. to loer ener.y stte0 ,-t t the

sme time the col+ ,loc* is mo(in. to hi.her ener.y stte/ The het0 on

the other hn+0 is ,ein. +istri,-te+ more e(enly 3n+ therefore more

rn+omly5 thro-.ho-t the ,loc*s/

NeGt consi+er the cse of hy+ro.en cyni+e .s relese+ from .s cyl in+er,ottle into room/ At first0 the hy+ro.en cyni+e molec-les re or+ere+0 in

tht they occ-py ell+efine+ (ol-me/ After relese they +iff-se thro-.h

the room spontneo-sly0 ,-t note tht there is no chn.e in ener.y hen to

.ses miG0 so neither the hy+ro.en cyni+e nor the ir cn ,e re.r+e+ s

mo(in. to loener.y stte/ Once .in0 the process is mo(in. tor+s

stte of hi.her +isor+er/ This eGmple my ,e +emonstrte+ ,y fillin. smll

plstic ,. ith mins .s n+ lloin. this .s to +iff-se thro-.h the

clssroom/ The smell of the .s 3ct-lly +-e to the tertiry ,-tyl thiol

3C%?5

?CS%0 hich is ++e+ to m*e .s smell5 rpi+ly per(+es the room0

shoin. tht the spre+ of the .s is rpi+ n+ spontneo-s/ %y+ro.encyni+e sho-l+ not ,e -se+ to +emonstrte this process0 e(en ith +isr-pti(e

st-+ents/

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S(ont!neo&s !nd non0s(ont!neo&s (%ocesses

Any process my ,e +efine+ s ,ein. either spontneo-s or nonspontneo-s/ A s(ont!neo&s (%ocessis process tht hs nt-rl

ten+ency to occ-r0 itho-t the nee+ for inp-t of or* into the system/

$Gmples re the eGpnsion of .s into (c--m0 ,ll rollin. +on

hill or the flo of het from hot ,o+y to col+ one/ Spontneo-sE is

forml +efinition n+ is not -se+ here in the colloB-il sense/ If

process is +escri,e+ s spontneo-s0 it +oes not men tht it is either fst

or rn+om/ For spontneo-s process to t*e plce0 the system m-st ,e

t position here it is re+y for chn.e to come ,o-t itho-t the nee+

for or* to ,e +one on it/ A spontneo-s process my ,e -se+ to +o

or* on nother system/

A non0s(ont!neo&s (%ocess is process tht +oes not h(e nt-rl

ten+ency to occ-r/ $Gmples mi.ht incl-+e the compression of .s into

smller (ol-me0 the risin. of ei.ht .inst .r(ity0 or the flo of

het from col+ ,o+y to hotter one in refri.ertion system/ For

nonspontneo-s process to ,e ,ro-.ht ,o-t0 ener.y in the form of or*

m-st ,e inp-t into system/ In the cse of ,ll on hill0 the

spontneo-s process is for the ,ll to roll -n+er the infl-ence of .r(ity

to the ,se of the slope0 relesin. ener.y s het in the process/ The

re(erse process 9 tht the ,ll t*es in het from the s-rro-n+in.s n+

rolls -p the slope 9 +oes not occ-r spontneo-sly/ Note tht ltho-.h theprocess +oes not occ-r nt-rlly0 it is possi,le to effect non

spontneo-s process0 ,-t or* m-st ,e p-t into the system for this to

come ,o-t/ In the eGmple .i(en0 mechnicl or* m-st ,e +one in

or+er for the ,ll to ,e rise+ .inst .r(ity/ In ny system0 the re(erse

of spontneo-s process m-st ,e nonspontneo-s/

Ent%o(#

Altho-.h +isor+er is rther +iff-se term0 it is possi,le to B-ntify it in termsof ent%o(#0 hich is thermo+ynmic property of system/ $ntropy is

+enote+ ,y Sn+0 li*e enthlpy n+ internl ener.y0 it is stte f-nction/ In

other or+s0 specifie+ mo-nt of s-,stnce t specifie+ tempert-re n+

press-re ill h(e specific (l-e of entropy0 no mtter ho those con+itions

ere reche+/

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REACTIO 8EASI6I9ITY

St!tistic!l de"inition o" ent%o(#

It is possi,le to +efine entropy in sttisticl terms0 so pro(i+in. n insi.ht

into the rel menin. of entropy n+ entropy chn.es/ For ny system0 the

entropy is .i(en ,y the 6oltm!nn e;&!tion:

SJ 31ln3+5

here +is the n-m,er of possi,le confi.-rtions of the system n+ 31is

6oltm!nn-s const!nt/ This +efinition llos entropy to ,e -n+erstoo+

s mes-re of the +isor+er in system/ If e t*e0 for eGmple0

hypotheticl crystl continin. siG42

I42

I molec-les0 then the n-m,er ofys in hich the molec-les cn ,e rrn.e+ if the crystl is perfectly

or+ere+ is 4/ If to molec-les re re(erse+0 so incresin. the +isor+er0

then the n-m,er of +istin.-ish,le rrn.ements increses to 47/

Re(ersin. three of the molec-les re+-ces the n-m,er of possi,le

rrn.ements to 4;/ If ll rrn.ements re ener.eticlly eB-i(lent0 the

most pro,,le sit-tion is the one ith the hi.hest n-m,er of possi,le

confi.-rtions0 n+ hence this is the most +isor+ere+E/ This lso mens

tht the perfectly or+ere+ sit-tion0 hich hs the loest n-m,er of

poss i,le confi.-rtions n+ loest entropy0 is lso the mos t impro,,le/

T'e second l!w o" t'e%mod#n!mics

The secon+ l of thermo+ynmics is resttement of o-r pre(io-s

+isc-ssion0 n+ sttes tht:

"for a spontaneous process, the total entropy change in a system and in the

system&s surroundings +ill increase&

In other or+s0 process tht increses the o(erll entropy of the -ni(erse is

process tht is thermo+ynmiclly possi,le/ This +oes not men tht more

or+ere+ systems cnnot ,e .enerte+ 9 it is o,(io-s tht lr.e n-m,er n+rn.e of processes tht le+ to increse+ or+er t*e plce e(ery +y in o-r

on +irect eGperience/ Ice is more or+ere+ thn ter0 yet e form ice in o-r

freeers/ Similrly0 e cn crystllise slt crystls o-t of +isor+ere+

sol-tion/ In ll s-ch cses0 hoe(er0 e re only loo*in. t prt of the story

9 remem,er tht it is the totalentropy tht m-st increse/ Ice forms in o-r

freeers ,ec-se het is p-mpe+ o-t into the s-rro-n+in.s0 hetin. them -p

n+ incresin. the +isor+er there/ Slt crystllises o-t of sol-tion s the

sol-tion releses het to the s-rro-n+in.s n+ s the ter molec-les

e(porte0 the effect of ,oth of hich is to increse the entropy of the

s-rro-n+in.s/

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)e cn .enerlise this +isc-ssion/ Clerly0 it is possi,le in spontneo-s

process for the sys tem to ,ecome incresin.ly +isor+ere+0 ,-t e(en here it

+oes not0 then pro(i+in. tht the s-rro-n+in.s ,ecome more +isor+ere+ tocompenste0 the process my ,e spontneo-s/

)e cn increse or +ecrese entropy in the s-rro-n+in.s ,y .i(in. o-t het or

t*in. it in to the system 3-s-lly chemicl rection in this conteGt5 of

interest/ If rection0 for eGmple0 is eGothermic0 then het is +issipte+ into

the s-rro-n+in.s0 m*in. the motion of the molec-les there more chotic0 n+

the entropy of the s-rro-n+in.s increses 3Fi.-res = n+ =c5/ If rection is

en+othermic0 het is t*en in from the s-rro-n+in.s n+ this +ecreses the

rn+om motion there 3Fi.-re =,5/ The res-lt is tht the entropy in the

s-rro-n+in.s +ecreses/

Figure 8a

4ire+or3s, e/othermic reactions in

+hich Ssy st em

6 and %76, so

Ssu rr ou nd in gs

6

Figure 8b

N89dissol$ing in +ater, an

endothermic process in +hich

Ssy st em

6 and %6,

so Ssu rr ou nd in gs

76

Figure 8c

H2burning in o/ygen to gi$e li%uid

+ater, an e/othermic reaction in

+hich Ssy st em

76 and %776, so

Ssu rr ou nd in gs

6

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T'e t'i%d l!w o" t'e%mod#n!mics

The thir+ l of thermo+ynmics sttes tht:

"the entropy of a perfectly crystalline solid at the absolute zero of

temperature is zero&

For perfectly crystlline soli+0 there cn ,e only one possi,le sptil

confi.-rtion of the components of the crystl0 n+ s the mteril is t the

,sol-te ero of tempert-re0 there re no +ynmic chn.es in the crystl

either/ In other or+s0 the entropy of the crystl is eB-l to ero/

In prctice0 ,sol-te ero cnnot ,e reche+ n+ perfect crystlline soli+s

cnnot ,e m+e0 ,-t it is still possi,le to pply the thir+ l/ For most

prcticl p-rposes0 the entropy of mter ils +rops to infinitesimlly smll

(l-es t lo tempert-re0 so tht it my con(eniently ,e m+e eB-l to ero

t the lo tempert-res tht cn ,e ro-tinely chie(e+ in the l,ortory/

This ,ecomes importnt ,ec-se it m*es possi,le the mes-rement of

entropy chn.es from reference point/ 3Mes-rin. the entropy chn.es is

,eyon+ the scope of A+(nce+ %i.her0 ,-t it my ,e note+ tht they my ,e

performe+ (ery esily thro-.h het cpcity ,y plottin. CpD#.inst #/ The

entropy chn.e ,eteen ny to tempert-res is the re -n+er the c-r(e

,eteen the to tempert-res/5 Unli*e enthlpy n+ internl ener.y0therefore0 entropy hs mes-r,le ,sol-te (l-e/

It is possi,le to plot entropy s f-nction of tempert-re for s-,stnce

3Fi.-re >5 n+ this yiel+s n-m,er of interestin. points/

There is .enerl increse in the entropy of s-,stnce s the tempert-re

increses/ This is not s-rprisin.0 since the hi.her ener.y tht is (il,le

t hi.her tempert-res increses the motion of the molec-les in the

s-,stnce/ The increse+ +isor+er tht this cretes trnsltes into n

increse in entropy/

For most s-,stnces0 there re to points t hich the entropy .oes

thro-.h step increse0 tht is it increses ,y finite mo-nt o(er n

infinitesimlly smll tempert-re/ These to points re esily i+entifie+

s the meltin. n+ ,oilin. points/ At ech of these points there is n

increse in the specil +isor+er of the molec-les 9 in other or+s0 they re

m-ch more free to mo(e ro-n+ ,o(e these tempert-res thn they re

,elo them/ The increse+ +isor+er .in trnsltes into n increse in

entropy/

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Figure 9

#he $ariation of entropy +ith temperature for chloromethane

)%edicting ent%o(# c'!nges

The prece+in. points ,o-t entropy lso llo -s to inspect rections n+

pre+ict the li*ely chn.e in entropy/ Consi+er0 for eGmple0 the rection:

CCO?3s5 CO3s5 K CO

23.5

%ere one mole of relti(ely or+ere+ soli+ is con(erte+ into one mole of similrly or+ere+ soli+ n+ one mole of hi.hly +isor+ere+ .s/ The rection

is pre+icte+ to yiel+ n increse in entropy0 n+ in+ee+ SBfor this rection is

fo-n+ ,y eGperiment to ,e K4;/ 94 mol94 /

Similrly0 nitro.en n+ hy+ro.en rect in the %,er91osch process to yiel+

mmoni:

N23.5 K ?%

23.5 2N%

?3.5

In this cse0 e h(e rection tht procee+s from 8 mol of hi.hly +isor+ere+.s on the lefthn+ si+e of the eB-tion to 2 mol of hi.hly +isor+ere+ .s on

the ri.hthn+ si+e of the eB-tion/ )e pre+ict +ecrese in entropy s the

rection t*es plce0 pre+iction tht is .in ,orne o-t ,y eGperiment s

SJ 94>=/= 94 mol94 / Altho-.h this or*s .inst the ,ility to perform

the rection0 this pro,lem is o(ercome in the in+-stril %,er process ,y the

-se of hi.h press-res ,-t mo+erte tempert-res/

24 CHEMISTRY

l iB-i+

;

; #D

.s

?;;

SD9 4 mo l9 4

soli+

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REACTIO 8EASI6I9ITY

In thir+ eGmple0 e t*e the rection ,eteen hy+ro.en .s n+ oGy.en

.s to .i(e liB-i+ ter/ In this cse0 e eGmine the rection n+ see tht

e procee+ from 4 mol of +isor+ere+ .s to 4 mol of relti(ely or+ere+liB-i+:

%23.5 K O

23.5 %

2O3l5

)e pre+ict0 n+ fin+0 tht the entropy chn.e is ne.ti(e

3SJ 9?2 94 mol945/ Altho-.h the entropy +ecreses for the system0 it is

importnt to pprecite tht the secon+ l of thermo+ynmics 3tht entropy

lys increses5 refers to n isolatedsystem/ Most eGperimentl systems

cnnot ,e re.r+e+ s ,ein. isolte+0 in hich cse the -ni(erse effecti(ely

,ecomes the isolte+ system/ In this cse0 the totl entropy chn.e is the s-mof the entropy chn.e in the s ystem n+ in the s-rro-n+in.s0 n+ this totl

m-st ,e .reter thn or eB-l to ero to comply ith the secon+ l of

thermo+ynmics:

Stotl

K Ss-rro-n+in.s

J Stotl

n+

Stotl

;

Another eGmple is the entropy chn.e s hy+ro.en n+ fl-orine .ses rect

to .enerte liB-i+ hy+ro.en fl-ori+e/ This is fo-n+ to ,e 924; 94 mol94 /

Altho-.h this represents +ecrese in entropy0 the rection procee+s

spontneo-sly ,ec-se the totl entropy chn.e is .reter thn ero/ The

positi(e totl entropy chn.e rises ,ec-se the rection is eGothermic n+

the het lost to the s-rro-n+in.s increses the +isor+er there0 c-sin.

Ss-rro-n+in.s

to ,e positi(e n+ of .reter m.nit-+e thn Ssystem

/

St!nd!%d ent%o(# c'!nge

Any noneB-ili,ri-m process le+s to chn.e in entropy/ As entropy is stte f-nction0 entropy chn.es my ,e clc-lte+ from the stn+r+ entropies

of the initil n+ finl sttes of the system:

Spr oc es s

J Sfinl stte

9 Sinitil stte

For chemicl rection0 for eGmple0 the st!nd!%d ent%o(# o" %e!ction is the

+ifference ,eteen the stn+r+ entropies of the rectnts n+ pro+-cts0 n+

my ,e clc-lte+ from:

S

rection J S

rectnts 9 S

pro+-cts

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This eGpression resem,les those -se+ ith other stte f-nctions0 s-ch s

enthlpy0 n+ +espite the sli.htly simpler form0 the similrity ith

eGpressions for enthlpy is e(en closer thn is initilly e(i+ent/ In the cse ofenthlpy0 for eGmple0 the correspon+in. eB-tion is

H rection J Hpro+-cts 9 Hrectnts/ %ere0 Hfis the enthlpy of

formtion of s-,stnce from n arbitrary standard(l-e of ero0 heres in

the eGpression for the entropy chn.e0 Srepresents the entropy of formtion

of s-,stnce from n absolute(l-e of ero/

8%ee ene%g#

The totalentropy chn.e +etermines hether or not rection is spontneo-s0

n+ so it is -sef-l to h(e some property tht B-ntifies the totl entropy/ Ames-re of the totl entropy chn.e is the "%ee ene%g#/ The

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8%ee ene%g# !nd tot!l ent%o(# c'!nge

The totl entropy chn.e tht ccompnies process is the s-m of theentropy chn.e in the system n+ in the s-rro-n+in.s:

Stotl

J Ssystem

K Ss-rro-n+in.s

%oe(er0 Ss-rro-n+in.s

is relte+ to the enthlpy chn.e in the system t

constnt press-re thro-.h the reltionship Ss-rro-n+in.s

J %D#0 here %is the

het ++e+ to the s-rro-n+in.s/ If the het lost ,y the system is Hsystem

0

the het .ine+ ,y the s-rro-n+in.s m-st ,e 9Hsystem

0 therefore Ss-rro-n+in.s

J 9 Hsystem

D#:

Stotl

J Ssystem

K 9Hsystem

D#

If this eGpression is m-ltiplie+ ,y 9#0 it yiel+s the reltionship:

9#Stotl

J Hsystem

9 #Ssystem

The B-ntity 9#Stotl

is referre+ to s the &i,,s free ener.y chn.e0 :/

Since Stotl

m-st ,e posi ti$e 3incresin. entropy5 for spontneo-s

process0 it follos tht :system

m-st ,e negati$efor process to ,e

spontneo-s/

An importnt property of the free ener.y is tht it represents the mGim-m

mo-nt of or* tht my ,e o,tine+ from process/ This +iffers from the

het tht my ,e o,tine+ from process ,ec-se the totl entropy chn.e

m-st ,e .reter thn ero/ For eGmple0 in the cse of rection for hich

Ssystem

is ne.ti(e0 some het m-st ,e lost to the s-rro-n+in.s n+ contri,-te

to Ss-rro-n+in.s

in or+er tht Stotl

is .reter thn ero/ The (l-e of the het

tht is then -n(il,le for con(ersion into or* is .i(en ,y #Ssystem

/

8%ee ene%g# !nd s(ont!neit#

For spontneo-s process0 Sof the -ni(erse is positi(e n+ :for the

system is therefore ne.ti(e/ The reltionship :J H9 #Sllos

pre+iction of the con+it ions -n+er hich rection is spontneo-s/ As #

m-st ,e positi(e0 the reltionships my ,e s-mme+ -p s in T,le ?/

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Table 3

4ree energy and the spontaneity of reactions

Hs#stem

Ss#stem

S(ont!neo&s= S(ont!neit# "!/o&%ed #

Ne.ti(e Positi(e Un+er l l con+itions All con+it ions

Ne.ti(e Ne.ti(e If V#SV W VHV Lo tempert-res

Positi(e Positi(e If V#SV X VHV %i.h tempert-res

Positi(e Ne.ti(e Ne(er No con+itions

Tempert-re hs mHor impct on the spontneity of some rections0 s

in+icte+ in T,le ?/ For n eGothermic rection 3HW;5 here SW;0 V#SV

3i/e/ the ,sol-te (l-e of #S5 ill ,e less thn VHV pro(i+e+ tht #is smll0n+ s-ch rection ill ,e spontneo-s t loer tempert-res/ Con(ersely0

in n en+othermic rection 3HX;50 if SX;0 V#SV ill ,e .reter thn VHV

pro(i+e+ tht #is lr.e0 n+ s-ch rection ill ,ecome spontneo-s t

hi.her tempert-res/ In ,oth cses0 the tempert-re t hich the rection

,ecomes spontneo-s is simply .i(en ,y #J HDS/

)%o(e%ties !nd !((lic!tions o" t'e

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rection t*es plce -n+er con+itions of constnt composition/ The

+ifference my ,e ill-strte+ ,y the rection:

cyclopropne0 C?%

3.5 propene0 C

?%

3.5

If one mole of p-re rectnts rect to .enerte one mole of pro+-cts0 then :J

984/ * mol94 / This is the stn+r+ rection free ener.y chn.e/ If the free

ener.y chn.e is no mes-re+ hen one mole of rectnts .i(es one mole of

pro+-cts ithin miGt-re of one mill ion moles of cyclopropne n+ to mil lion

moles of propene0 it is fo-n+ to ,e 9?>/> * mol94 / This rection free ener.y is

.i(en the sym,ol :/ The +ifference ,eteen :n+ :rises ,ec-se of the

+ifferent con+itions -n+er hich the to rections t*e plce/

E;&ili%i! !nd "%ee ene%g#

)e h(e lre+y +escri,e+ ho the -ni(erse nt-rlly mo(es tor+s position

of mGim-m +isor+er 3i/e/ mGim-m entropy5 n+ tht this is the +ri(in. force

for chn.e in ech in+i(i+-l chemicl rection or process/ Achie(in. the most

+isor+ere+ stte of the system itself clerly plys n importnt prt in

+eterminin. the position of mGim-m o$erallentropy/ This is prtic-lrly tr-e

hen the (l-e for H3hich is responsi,le for entropy chn.es in the

s-rro-n+in.s5 is smll/ It follos then tht in s-ch rections there is stron.

+ri(in. force for the system to rech the most +isor+ere+ stte/

)hen e ttempt to -se ":Bto +etermine the most +isor+ere+ stte of the

system0 e imme+itely enco-nter limittion of this B-ntity/ 1ec-se :

only compres the properties t the strt n+ finish of rection0 it cn le+

-s to ,elie(e tht the mGim-m entropy is t the point here e h(e either

p-re rectnts 3if :is positi(e5 or p-re pro+-cts 3if :is ne.ti(e5/

)hen e loo* more closely t the system0 hoe(er0 it is re+ily pprecite+

tht the mGim-m +isor+er 9 the mGim-m entropy 9 is not o,tine+ hen it

is compose+ of p-re pro+-cts or p-re rectnts0 ,-t hen it consists of

miGt-re of the to/

It follos tht n interme+ite composition often represents the most

+isor+ere+ stte of the system0 n+ the system ill spontneo-sly pro.ress to

this position/ At this point0 the rection is in stte of eB-ili,ri-m 3see

Section ?50 here e o,ser(e stte of constnt composition tht is

comprise+ of ,oth rectnts n+ pro+-cts/

)e cn represent this r.-ment in plots of free ener.y .inst rection

composition/ )hen the free ener.y is t its minim-m (l-e0 the totl entropy

is mGimise+0 n+ the system ill ten+ to mo(e spontneo-sl y tor+ s thi s

pos ition /

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E+>I9I6RIA

SECTION 3

E;&ili%i!

Rections re lmost -ni(erslly ritten so s to imply tht rectnts pro+-ce

pro+-cts in oney process/ Rections tht relly +o ,eh(e in this y

re in the minority/ The mHority of rections0 s-ch s mny of the

,iochemicl rections ithin o-r on ,o+ies0 re not li*e this/ For s-ch

rections it is necessry to t*e cco-nt of the fct tht the re(erse rection is

,oth poss i,le n+ importnt/ An eGmple is the +issocition of e* ci+0

s-ch s ethnoic ci+0 C%?COO %/ The forr+ rection0 the one tht e

o-l+ normlly consi+er to .o to completion0 is .i(en ,y6

C%?COO %3l5 C%

?COO93B5 K %K3B5

If the rection co-l+ only procee+ in one +irection0 then the ion o-l+

,ecome f-lly +issocite+/ It so hppens tht the ethnote ion0 C%?COO93B50

is cp,le of rectin. ith proton to .i(e ethnoic ci+:

C%?COO93B5 K %K3B5 C%

?COO %3l5

This is clerly the re(erse of the first rection0 n+ ,oth rections t*e plcesim-ltneo-sly in smple of ethnoic ci+/ )e in+icte this ,y -sin. the

eB-ili,ri-m sym,ol0 0 th-s:

C%?COO93B5 K %K3B5 C%

?COO %3l5

In chemicl system s-ch s this0 therefore0 there re to rections0 one

formin. pro+-cts n+ competin. re(erse rection tht reforms rectnts/

At some point0 the n-m,er of forr+ rections tht t*e plce in .i(en

time eB-ls the n-m,er of re(erse rections tht t*e plce in tht sme

perio+/ )hen this t*es plce0 the concentrtions of rectnts n+ pro+-cts

+o not chn.e0 n+ the system is in stte of e;&ili%i&m/ The stte of

eB-ili,ri-m0 here the concentrtion of rectnts n+ pro+-cts reches stte

of ,lnce0 sho-l+ not ,e compre+ to ,lncin. pir of scles/ It is

e/tremely important, and should be $ery strongly emphasised0 tht ltho-.h

the concentrtions of rectnts n+ pro+-cts no lon.er chn.e0 the rection

+oes not stop ,-t reches stte of d#n!mic e;&ili%i&m0 in hich the rtes

of forr+ n+ re(erse rections re eB-l/

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E;&ili%i&m !t ! (!%t# 7 !n !n!log#

The concept of +ynmic eB-ili,ri-m0 s enco-ntere+ in chemicleB-ili,ri-m0 cn ,e +iffic-lt concept for st-+ents to .rsp/ The i+e tht

chemicl system cn h(e constnt composition0 ,-t tht the rection

hs not stoppe+ seems t first to ,e contr+ictory/

An nlo.y cn ,e +rn ith .ro-p of people t prty/ As the prty

strts0 most of the .-ests my strt off in the li(in. room0 ith none of

them in the *itchen/ As the prty pro.resses0 proportion of the .-ests

my +eci+e to .o into the *itchen n+ spen+ time there/ From this point

on0 the relti(e n-m,er of people in the li(in. room n+ the *itchen

remins reson,ly constnt0 ,-t e cn imme+itely reco.nise tht the

sit-tion is not sttic0 s the sme people +o not remin in the *itchen

thro-.ho-t the prty/ There is flo of people ,eteen the *itchen n+

li(in. room0 ,-t if for ech person tht mo(es into the *itchen nother

person mo(es o-t into the li(in. room0 then there re st,le pop-ltions in

ech/ This is stte of +ynmic eB-ili,ri-m0 n+ if the li(in. room is the

nlo.-e of the rectnts0 the *itchen cn ,e re.r+e+ s the nlo.-e of

the pro+-cts/

Some comments on e;&ili%i!

A mHor pro,lem in chemistry0 s in ny s-,Hect0 is tryin. to see the oo+ for

the trees0 n+ hen e consi+er chemicl eB-ili,ri it is (ery esy to ,ecome

conf-se+ ,y the +ifferent eB-ili,ri tht re enco-ntere+/ It often helps to

relise tht ll t ypes of eB-ili,ri re of the sme .enerl form:

chemicls in stte 4 chemicls in stte 2

As chemists e enco-nter se(erl types of eB-ili,ri0 incl-+in.0 for eGmple:

rection eB-ili,ri

%23.5 K I23.5 2%I3.5

phse eB-ili,ri

%2O3s5 %

2O3l5

ci+ +issocition eB-ili,ri

C%?COO %3B5 %K3B5 K C%

?COO93B5

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E+>I9I6RIA

-toprotolysis eB-ili,ri

2%2O3l5 %

?OK3B5 K O%93B5

sol-tion eB-ili,ri

A.Cl3s5 K %2O3l5 A.K3B5 K Cl93B5

The only thin. tht +istin.-ishes these eB-ili,ri from one nother is the

mnner in hich e0 s chemists0 choose to cte.orise them/

T'e e;&ili%i&m const!nt? K

It is importnt to ,e ,le to B-ntify the relti(e concentrtions of rectnts

n+ pro+-cts in n eB-ili,ri-m rection/ Let -s ss-me tht e h(e neB-ili,ri-m of the form:

aA K b1 cCK d"

If the rection is in stte of eB-ili,ri-m0 then e cn +efine B-ntity

*non s the e;&ili%i&m const!ntfor the forr+ rection0 pproGimtely

.i(en ,y:

[ ] [ ]

[ ] [ ]

C D=

A B

c d

a bK

here !A'0 !1'0 !C' n+ !"' re the eB-ili,ri-m concentrtions of the

rectnts n+ pro+-cts0 n+ a0 b0 c0 n+ dre the molr rtios of compo-n+s

A0 10 C n+ " respecti(ely0 s .i(en in the chemicl eB-ili,ri-m/

Un+er ny specifie+ set of physicl con+itions0 the (l-e of is constnt for

.i(en rection/ )hen the system is t eB-ili,ri-m0 the concentrtions of the

re.ents m-st ,e s-ch tht hen their (l-es re plce+ in the eB-tion0 they

eB-l/

Example

The rection ,eteen hy+ro.en n+ io+ine t tempert-re of 7>; is:

%2K I

2 2%I J 2

As ,efore0 e reco.nise tht is .i(en ,y the eGpression:

[ ]

[ ] [ ]

2

2 2

HI=

H IK

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E+>I9I6RIA

From this0 it is esy to see tht the con+itions for eB-ili,ri-m ill ,e met if

!%I' J 2 mol l94 0 !%2' J 2 mol l94 0 n+ !I

2' J 4 mol l94 / This set of

concentrtions0 hoe(er0 is not the only one tht represents stte ofeB-ili,ri-m/ The con+itions for eB-ili,ri-m ill lso ,e met if

!%I' J 7 mol l94 0 !%2' J 7 mol l94 0 n+ !I

2' J 2/7 mol l94 / Nee+less to sy0 there

is n infinite set of rectnt n+ pro+-ct concentrtions tht ct s

mthemticl sol-tions to the eGpression ,o(e/ This ill-strtes the point

tht0 hile is constnt t .i(en tempert-re0 the concentrtions of the

rectnts n+ pro+-cts nee+ not ,e/

Some gene%!l (oints !o&t K

)e strt ith common point of conf-sion/ Nineteenth cent-ryphysicl chemists ere not note+ for their ori.inlity in nmin.

physicl constnts0 n+ the eB-ili,ri-m constnt for rection0

3-pper cse50 is often conf-se+ ith the rectionEs rte constnt0

33loer cse5/ It is orth notin.0 hoe(er0 tht the m.nit-+e of

is .i(en ,y the rtio of the forr+ to ,c*r+ rte constnts0

J 3fD3

,/ This f-rther emphsises the +ynmic nt-re of the chemicl

eB-ili,ri-m/

Another point ,o-t the eB-ili,ri-m constnt is tht Kis dimensionless9

it is p-re n-m,er ith no -nits/ This is nonne.oti,le fct/ There is0

+mitte+ly0 contin-in. conf-sion on this mtter since mny teGt,oo*s0

,iochemist ry co-rses n+ e(en chemistry co-rses t tri,-te ith -nits/

This mis-n+erstn+in. rises from fil-re to pprecite tht the -se of

concentrtions in the eGpression for is n pproGimtion/ 1ec-se this

pro(o*es s-ch stron. opinions in mny people e incl-+e here s-mmry

of the rele(nt ,c*.ro-n+ to the -se of cti(ities/

The ri.oro-s +efinition of -ses !cti/ities0 hich h(e no -nits:

[ ] [ ]

[ ] [ ]

C D

= A B

c d

a b

a a

K a a

here a3A5 is the cti(ity of compo-n+ A0 etc/

The concept of cti(ity is ,eyon+ the scope of the A+(nce+ %i.her

syll,-s0 ,-t ,rief eGplntion for its -se is tht concentrtion

+oes not directlyinfl-ence the physicl properties 3s-ch s electricl

con+-cti(ity5 of sol-tion/ )hile the con+-cti(ity of 4;98 mol l94

sol-tion of NCl my reflect the presence of 4;98 mol l94 of non

interctin. NKn+ Cl9ions0 4 mol l94 sol-tion +oes not sho 4; 8

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E+>I9I6RIA

times the sme con+-cti(ity/ This is ,ec-se0 t hi.h concentrtions0 the

ions interct ith one nother0 n+ the effecti$econcentrtion is less thn

the ct-l concentrtion/ Acti(ity my ,e consi+ere+ to ,e mes-re ofthe effecti(e concentrtion 3or press-re0 for .ses50 relti(e to stn+r+

concentrtion 3or press-re5/

For sol-te0 A0 t lo concentrtions0 the cti(ity of A is appro/imately

eB-l to the concentrtion of A +i(i+e+ ,y the stn+r+ concentrtion/ As

the stn+r+ concentrtion is 4 mol l94 0 this mens tht in sol-tion

continin. ;/;;4 mol l94 of A0 A hs n cti(ity of ;/;;4 mol l94 D4 mol l94

hich is eB-l to ;/;;40 s the -nits cncel o-t/ As this is the cse for ll

components in the eB-ili,ri-m0 ll the -nits cncel o-t0 n+ so itself hs

no -nits/ At this le(el0 it is s-fficient to sy tht is clc-lte+ -sin.

concentrtion0 ,-t itho-t -sin. the +imensions of the concentrtions/

Note0 hoe(er0 tht concentrtion m-st ,e mes-re+ in mol l94 n+

press-re in tmospheres 0 s the stn+r+ concentrt ion n+ press-re is 4 in

ech of these -nits/

$!nis'ing solids? li;&ids !nd sol/ents

The concept of chemicl cti(ity llos -s to eGplin hy p-re liB-i+s

n+ soli+s +ispper from the eB-ili,ri-m eGpression/ As the cti(ity of

mteril my ,e re.r+e+ s the rtio of its effecti(e concentrtion to its

stn+r+ concentrtion0 n+ s soli+ or liB-i+ is only (ery sli.htly+ifferent from its stn+r+ stte0 then it ,ecomes cler tht their cti(ity is

4/ %ence0 for soli+ s-ch s sil(er chlori+e +issol(in. in ter0 e h(e

the eB-ili,ri-m:

A.Cl3B5 K %2O3l5 A.K3B5 K Cl93B5

The f-ll eGpression for the eB-ili,ri-m constnt is:+

2

[Ag ][Cl ]=

[H O][AgCl]K

Altho-.h e rite this -sin. the pproGimtion of concentrtions0 e

reco.nise tht A.Cl is soli+0 n+ so !A.Cl'E J4/ Also0 ltho-.h the

ter contins some +issol(e+ A.Cl0 it is little +ifferent from p-re ter0

n+ so !%2O'E J 4/ %ence0 J !A.K'!Cl9'/

The remin+er of this teGt ill -se the pproGimtion of concentrtion n+

prtil press-re/

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C'!nging t'e conditions 7 9e C'!telie%-s (%inci(le

Le ChtelierEs principle sttes tht:

";hen a system at e%uilibrium is sub

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This is closely relte+ to the eB-ili,ri-m constnt eGpresse+ in terms of

concentrtion/ Note tht concentrtion is mes-re of the n-m,er of

.i(en species per -nit (ol-me 3nD=5/ For n i+el .s0 G0 pG=JnG>#0 n+so p

GJ >#3n

GD=5 J >#!G'/

)e -se Le ChtelierEs principle0 lmost -nittin.ly0 in or.nic

rections/ Consi+er the hy+rolysis n+ formtion of n ester/ In

effect0 e h(e n eB-ili,ri-m ,eteen the ester n+ the cr,oGylic

ci+ n+ lcohol:

R9O% K RCOO % R9CO9O 9R K %2O

lcohol ci+ ester ter

To pr eparen ester0 e employ re.ents0 s-ch s concentrte+

s-lph-ric ci+ or polr or.nic sol(ent0 tht st,ilise the %2O0 so

enco-r.in. the forr+ rection n+ the .enertion of more ester/ To

hydrolysen ester e -se ter s sol(ent hich0 ,y Le ChtelierEs

pr inc ipl e0 le+s to the +o min nc e of the re (e rs e re cti on n+ the

hy+rolysis of the ester to the ci+ n+ lcohol/

Incresin. the o(erll press-re ,y +ecresin. the (ol-me of the continer

c-ses n increse in ll of the prtil press-res/ As the eB-ili,ri-m

constnt in(ol(es more moles of .s on the lefthn+ si+e of the eB-tionthn on the ri.hthn+ si+e0 eB-ili,ri-m is lost n+ the rection B-otient0

n+ the forr+ rection 3N2n+ %

2rectin. to form N%

?5 t*es plce

-ntil the prtil press-res re .in relte+ ,y the eB-ili,ri-m constnt

eGpression .i(en ,o(e/

Exam pl e

At prtic-lr tempert-re the rection is fo-n+ to ,e t eB-ili,ri-m

hen the prtil press-re of ech component is 4 tmosphere 3prtil

pr es s-res m-st ,e mes -r e+ in t mosp he re s 9 see s ome .e ne r l po int s

,o-t 0 ,o(e5:

J 42D3 424?5 J 4

If the (ol-me of the continer is hl(e+0 then the ne press-re of ech

component m-st ,e 2 tm/ Plcin. these ne (l-es in the eGpression

shos tht the ne press-res +o not represent system t eB-ili,ri-m s

they no lon.er eB-l :

3

22

2 2NH

p 2 3 2 3N H

2 4 1 = = =

2 2 32 8

pK

p p

3. CHEMISTRY

3

22

2NH

p 2 3N H

=p

Kp p

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The (l-es in the eGpression m-st no lter in or+er to .i(e the correct (l-e

for/ This cn only ,e +one if p2N% ?increses n+ 3p2N2

p?%25 +ecreses0 i/e/

if the forr+ rection t*es plce/

Loo*in. t Le ChtelierEs principle in this y eGplins hy incresin. the

press-re ,y ++in. heli-m0 for eGmple0 hich plys no prt in the rection0

+oes not infl-ence the position of eB-ili,ri-m/ In this cse0 hile the o(erll

press-re of the sys tem increses0 the prt il press-res of the rection .ses

+oes not chn.e0 n+ so the eB-ili,ri-m is not pert-r,e+/

E*t%emo('iles? !t'letes !nd 9e C'!telie%

Figure 1

"i+ yo- e(er on+er hy yo- ne(er see y*s or +eep se (iper fish t the

ooY The reson is tht these cret-res re eGtremophiles 9 they li(e in

eGtreme con+itions of hi.h ltit-+e n+ (ery +eep ses/ Amon.st other

fctors0 these en(ironments h(e lo oGy.en prtil press-res0 n+ this

mens tht the eB-ili,ri-m ,eteen oGy.ente+ n+ +eoGy.ente+

hemo.lo,in in the l-n.s:

hemo.lo,in K O2 oGyhemo.lo,in

is shifte+ to the left relti(e to tht of n niml t se le(el0 ,ec-se of

Le ChtelierEs principle/ M-scle tiss-e still reB-ires the sme mo-nt of

oGy.en s t se le(el n+ to s-r(i(e in these con+itions these nimls

m-st trnsport s-fficient oGy.en into the ,loo+strem/ These nimls

h(e respon+e+ to this nee+ ,y pro+-cin. lr.er B-ntities of

hemo.lo,in0 hich shifts the eB-ili,ri-m ,c* to the ri.ht n+ llos

hi.her proportion of the (il,le oGy.en to ,e -tilise+/ Unfort-ntely0

this lso mens tht too muchoGy.en is trnsporte+ into the ,o+y t se

le(el0 n+ m*es life -n+er m,ient con+itions (ery +iffic-lt for these

nimls/

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The h-mn ,o+y lso respon+s to oGy.en +eficiency ,y relesin. eGtr

hemo.lo,in0 in the short term/ It is for this reson tht thletes -n+er.o

trinin. t hi.h ltit-+e ,efore competitions 9 the eGtr hemo.lo,in inthe ,loo+ remins in plce for some ee*s fter ,ein. t ltit-+e/ Once

.in0 Le ChtelierEs principle mens tht the eB-ili,ri-m is shifte+ to the

ri.ht compre+ to tht for n -ncclimtise+ thlete0 n+ more oGy.en is

then (il,le in the ,o+y0 hich llos more rpi+ n+ s-stine+

respirtion in the m-scle cells/

C!t!l#sts

The role of ctlyst is to increse the rte of rection t .i(en tempert-re/

There re n-m,er of points ,o-t ctlysts tht sho-l+ ,e pprecite+/

Contrry to pop-lr myth0 hich r-ns lon. the lines of ctlyst increses

the spee+ of rection itho-t t*in. prt in itE0 ctlysts +o t*e prt in

rections 9 they h(e to0 or ho else co-l+ they h(e ny effect on the rte of

rectionY This myth comes ,o-t ,ec-se the ctlyst is re.enerte+ t the

en+ of rection 9 it only appearsnot to h(e ,een in(ol(e+/

F-rther to the pre(io-s point0 the reson hy ctlysts ct to increse rection

rtes is tht they pro(i+e n lternti(e rection pthy/ In the Ho-rney from

rectnts to pro+-cts0 there re ener.y ,rriers to ,e o(ercome/ If n

lternti(e ro-te ith loer ener.y ,rriers ,ecomes (il,le thro-.h the-se of ctlyst0 then it my ,e possi,le to mo(e from rectnts to pro+-cts

more esily/

The ctlyst pro(i+es n lternti(e ro-te ,eteen rectnts n+ pro+-cts0 n+

+oes not infl-ence the ener.y +ifference ,eteen them/ As this is the cse0

ctlyst cnnot m*e possi,le rection tht o-l+ not otherise t*e plce/

F-rthermore0 s the position of n eB-ili,ri-m is infl-ence+ ,y

thermo+ynmics 3recll tht the eB-ili,ri-m constnt my ,e clc-lte+ from

:J 9>#ln 50 here rection +oes t*e plce n+ reches con+ition of

eB-ili,ri-m the position of eB-ili,ri-m is not ltere+ ,y the presence of thectlyst/ The eB-ili,ri-m con+ition is chie(e+ more rpi+ly0 hoe(er0 in the

presence of ctlys t/

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C!t!l#sts 7 %o(es "o% molec&l!% mo&nt!inee%ing

The role of ctlyst my ,e compre+ to the Ho-rney cross mo-ntinfrom one (lley to nother 3Fi.-re 445/ In the norml co-rse of e(ents0

o-r mo-ntineer cn only tr(el from A to 1 o(er the ri+.e0 hich is slo

,ec-se it reB-ires lr.e .in in ltit-+e n+ therefore lot of ener.y/

If0 on the other hn+0 o-r mo-ntineer hs ccess to ropes0 he cn t*e n

lternti(e ro-te -p the lo cliffs into smll (lley ,eteen the to

mHor (lleys 9 n option tht he +oes not h(e itho-t ropes/ This ro-te

reB-ires less ener.y0 s he only hs to m*e mo+est .in in ltit-+e n+

so he cn procee+ m-ch fster into the neGt (lley to .et to 1/

The first ro-te in this nlo.y is the eB-i(lent of n -nctlyse+ rection/

The rope cts s the ctlyst in pro(i+in. n lternti(e0 n+ m-ch fster0

ro-te/

Figure 11

A

B

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,emonst%!ting t'e e""ect o" c!t!l#sis !nd t'e in"l&ence o" s&%"!ce !%e!

%y+ro.en peroGi+e +ecomposes sloly0 o(er perio+ of ee*s t roomtempert-re0 into ter n+ oGy.en:

%2O

2%

2O K O

2

This +ecomposition cn ,e ctlyse+ ,y lr.e n-m,er of ctlysts/

These eGperiments +emonstrte this n+ the effect of the s-rfce re of

the ctlyst/

O-r ,o+ies0 n+ those of ll li(in. or.nisms0 contin nt-rl ctlysts 9

enymes/ One of these0 ctlse0 cn ,e fo-n+ in pottoes/ Ctlse myct s ctlyst in the +ecomposition of hy+ro.en peroGi+e/ Plce

pproGimtely 7; cm?of hy+ro.en peroGi+e sol-tion 3sy ?; (ol-me

+il-te+ to 4; (ol-me5 in 4;;; cm?plstic mes-rin. cylin+er0 n+ ++

fe +rops of shin.-p liB-i+/ )hen piece of potto is ++e+0

col-mn of ,-,,les is forme+ s oGy.en is relese+ in the +ecomposition

process/

If the pre(io-s eGperiment is repete+ -sin. piece of potto cr-she+ ith

little sn+0 the col-mn of ,-,,les forms0 ,-t m-ch fster thn in the

pre(io-s eGperiment/ This increse+ ctlyt ic cti(ity is +-e to the hi.her

s-rfce re of the potto0 hich is cp,le of interctin. ith morehy+ro.en peroGi+e molec-les per secon+ thn the lo s-rfce re piece/

The +ecomposition is lso relti(ely fst hen mn.nese +ioGi+e0 MnO20

is -se+0 +-e to the hi.h s-rfce re of the ctlyst/

In finl0 +rmtic0 +emonstrtion0 the -ltimte hi.h s-rfce re is -se+/

The io+i+e ion lso ctlyses hy+ro.en peroGi+e +ecomposition0 n+ if n

B-eo-s sol-tion of I is ++e+ to %2O

23B5 prepre+ s in pre(io-s

+emonstrtions0 e(ery I9ion cts s ctlytic centre/ The rection no

procee+s $eryrpi+ly0 n+ the col-mn of sop ,-,,les literlly flies o-t of

the mes-rin. cylin+er 3er .o..les5

One finl point ,o-t ctlyse+ rections is tht the rection profile is often

incorrectly ill-strte+ s simple loerin. of the ener.y ,rrier relti(e to

the -nctlyse+ rection/ This simplifiction .i(es n incorrect impression of

the ctlytic process/ 1ec-se the ctlyst t*es prt in the rection it forms

t lest one interme+ite species0 n+ so there hs to ,e t lest one (lley in

the rection profile ,eteen the rectnts n+ pro+-cts/

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)'!se e;&ili%i!

0artit ion coefficient)e strte+ this section ,y reco.nisin. tht ll eB-ili,ri t*e similr form:

chemicls in stte 4 chemicls in stte 2

The prtition coefficient is mes-re of the +istri,-tion of sol-te ,eteen

to immisci,le sol(ents0 n+ is s imply specific cse of n eB-ili,ri-m:

sol-te in sol(ent 4 sol-te in sol(ent 2

As ith ny eB-ili,ri-m it my ,e +escri,e+ ,y n eB-ili,ri-m constnt0 0

here

J !sol-te'sol(ent2

D!sol-te'sol(ent4

This is clle+ the prtition coefficient0 n+ .i(es mes-re of the relti(e

sol-,ility of sol-te in the to sol(ents/

Sol/ent e*t%!ction

Sol(ent eGtrction is the process of p-rifyin. compo-n+ ,y -sin. its

+ifferentil sol-,ility ,eteen to sol(ents/ This is prcticl eGmple of-sin. the prtition coefficient/ Fi.-re 42 pro(i+es schemtic representtion

of the process 9 in rel sit-tions ,etter eGtrction is pro(i+e+ ,y repetin.

the process se(erl times/

Figure 12

Schematic representation of sol$ent e/traction

Sol(ent A

Sol(ent 1

miGt-re P-rifie+

compo-n+

The to components of the

miGt-re m-st h(e +ifferent

prtition coefficients in thesol(ents A n+ 1:

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MiGt-re

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Example

S-ppose tht miGt-re of ;/4 mol ech of to compo-n+s0 A n+ 10 is

eGtrcte+ ,eteen ?;; cm?

+ichloromethne n+ ?;; cm?

ter/ S-pposef-rther tht A hs prtition coefficient of ;/2 ,eteen the to sol(ents0 n+

tht 1 hs prtition coefficient of 7/

For compo-n+ A0 if mis the n-m,er of moles of A in ter0 then ;/49m

moles of A re +issol(e+ in +ichloromethne:

!A'ter

D!A'+ichloromethne

J ;/2

3mD;/?5D3;/49m5D;/? J ;/2

mD3;/49m5 J ;/2

mJ ;/;4

Similrly for 10 here n is the n-m,er of moles of 1 +issol(e+ in the ter:

!1'ter

D!1'+ichloromethne

J 7

3nD;/?5D3;/49n5D;/? J 7

nD3;/49n5 J 7

nJ ;/;=

The mteril in the B-eo-s lyer0 therefore0 is enriche+ in 1 relti(e to A/

Rther thn the 4:4 molr rtio tht e strte+ ith0 the mteril tht is

eGtrcte+ in the B-eo-s lyer contins 8/=:4 rtio of 1:A/ Usin.

clc-ltions nlo.o-s to those H-st .i(en it is stri.htforr+ to sho tht0

,y repetin. this process0 this rtio cn ,e increse+ to 2?:4 n+ then to

44;:4/

C'%om!tog%!('#

Altho-.h it is possi,le to repete+ly eGtrct compo-n+s ,eteen to

sol(ents0 it is timecons-min. n+ cn reB-ire mny eGtrctions to o,tin

s-fficiently p-re pro+-cts/

Chromto.rphy is nlo.o-s to sol(ent eGtrction0 in tht it is ,se+

on the -se of the prtition coefficients/ The +ifference is tht one of the

phses 3the moile ('!se5 is sol(ent0 in hich the sol-te cn +issol(e0

n+ the other phse is soli+ 3the st!tion!%# ('!se5 to hich the sol-te

cn +here/ At ny one time there ill ,e n eB-ili,ri-m0

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represente+ ,y the prtition coefficient0 for the sol-te +istri,-tion ,eteen

the sttionry n+ mo,ile phses/ The mo,ile phse tr(els o(er the

sttionry phse n+ crries ech component of the smple ith it0 ,-t ech t +ifferent rte/ As some sol-tes spen+ lon.er on the sttionry phse thn

others 3i/e/ those tht h(e hi.h prtition coefficient ,eteen the sttionry

phse n+ the mo,ile phse50 they ill tr(el lon. the chromto.rphy

col-mn more sloly thn those tht spen+ .reter proportion of the time in

the mo,ile phse 3i/e/ those tht h(e smll prtition coefficient ,eteen the

sttionry phse n+ the mo,ile phse5/

S'o((ing c'%om!tog%!('#

The process of chromto.rphy cn ,e compre+ to co-ple on

shoppin. trip on ,-sy street/ There is mo,ile phse 9 the cro+s on

the p(ement ho ten+ to +r o-r shoppers ith them lon. the len.th

of the street 9 n+ there is sttionry phse 9 the shops/

S-ppose tht o-r co-ple h(e +ifferent personlities 9 one 3A5 is n (i+

shopper hile their more rel-ctnt prtner 315 hs little interest in

shoppin./ Person A ill therefore spen+ lon. perio+s in the shops0 n+

less time in the street 9 they h(e lr.e prtition coefficient ,eteen the

shop n+ the street/ On the other hn+0 person 1 ill spen+ lon. perio+s

in the street0 n+ less time in the shops 9 they h(e smll prtition

coefficient ,eteen the shop n+ the street/

As o-r co-ple strt off t one en+ of the street0 A ill strt to spen+ fe

min-tes in shop0 hile 1 is +rn +on the street ,y the cro+/ The

co-ple re no sli.htly seprte+/ Altho-.h A comes o-t of this first

shop n+ is lso +rn +on the street ,y the cro+0 A ill soon +rop

into nother shop n+ the process repets/ The co-ple ,ecome

incresin.ly seprte+0 n+ lon.er streets ten+ to c-se .reter

seprtions/

1 mayoccsionlly spen+ some time in the shops0 ,-t since this mo-ntsto m-ch less time thn tht spent ,y A0 it is not s-fficient for 1 to ctch

-p n+ +oes not pre(ent the co-ple seprtin./

The sttionry phse cn ,e lmost ny soli+ mteril0 +epen+in. on the

sol(ent n+ the sol-te0 ltho-.h some re more s-it,le thn others 9

cell-lose 3e/./ pper50 l-min n+ silic re mon. the most common/ The

mo,ile phse cn ,e ny liB-i+ or0 s e ill see lter0 .s/

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Chromto.rphy s +e(elope+ ,y M S Ts(et ro-n+ 4>;0 n+ .ins its

nme from the &ree* or+s 3chromtos .rphos50 colo-r

ritin.E/ Ts(et or*e+ on the hi.hly colo-re+ chlorophylls n+ crotenes0n+ these re still ro-tinely -se+ s +emonstrtions/ A smll mo-nt of fl-i+0

eGtrcte+ from .rss or le(es0 is +roppe+ pproGimtely 4 cm from the ,se

of 4; cm strip of ,lottin. pper/ The strip is plce+ in 27; cm ?,e*er

continin. ;/7 cm +epth of li.ht petrole-m ether/ As the liB-i+ 3the mo,ile

phse5 mo(es -p the pper 3the stt ionry phse5 +ifferent pi.ments re

crrie+ ith the sol(ent t +ifferent rtes/ The orn.e9yello crotenes re

trnsporte+ frthest -p the pper0 ith the yello Gnthophylls shoin. only

smll mo(ement y from the ori.inl spot/ Finlly0 the .reen

chlorophylls remin lmost -nmo(e+ from the ori.inl spot/

Chromto.rphy is h-.e m-lti,illion +ollr in+-stry 9 it is -ni(erslly

-se+ to seprte components from rection/ In the l,ortory it cn ,e -se+

s (ery simple metho+ of seprtin. the pro+-cts from ne rection/

Thinlyer chromto.rphy 3TLC5 most commonly -ses cell-lose0 silic or

l-min on n l-mini-m sheet in plce of ,lottin. pper/ After seprtion0

the sheet cn ,e c-t -p to physiclly seprte the +ifferent ,n+s/ Therefter0

ech ,n+ my ,e re+issol(e+ n+ nlyse+ 9 mo+ern nlyticl metho+s

reB-ire only milli.rms of mteril for .oo+ B-lity nlysis/

Chromto.rphy nee+ not ,e performe+ -sin. liB-i+s s the mo,ile phse 9 it

is lso possi,le to -se .ses/ &s chromto.rphy 3&C5 s +e(elope+ from

or* +-rin. the Mnhtten ProHect0 the US tomic ,om, proHect in )orl+

)r II/ In or+er to mn-fct-re ,om,0 it is necessry to seprte 2? 7U from2? =U/ This is ccomplishe+ ,y con(ertin. -rni-m to .seo-s UF

n+

lloin. this to +iff-se +on eGtremely lon. pc*e+ t-,es 9 the t-,e reB-ire+

is se(erl *ilometres in len.th/ The 2? =U spen+s lon.er in the col-mns thn

the 2? 7U0 n+ this llos the to isotopes to ,e seprte+/ Altho-.h it or*s

ell0 in this form this is n eGtremely inefficient n+ timecons-min.

techniB-e/ Mo+ern +eri(ti(es of this techniB-e re eGtremely rpi+ n+efficient0 n+ cn seprte mterils tht co-l+ not ,e seprte+ ,y ny other

metho+/ 1y -sin. chirl mterils 3i/e/ mteril in only one of its to

mirror im.e forms5 in the sttionry phse0 it is e(en possi,le to seprte

chirl molec-les from their mirror im.es/

In &C0 the sttionry phse is pc*e+ col-mn or cote+ cpillry tht is

hel+ in n o(en0 ith n inert .s s-ch s %20 %e or N

2s the mo,ile

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phse crrier .s/ $ch component of the smple is crrie+ thro-.h the

col-mn ,y the crrier .s t +ifferent rte0 n+ is +etecte+ s it eGits the en+

of the col-mn/ A rn.e of +etector types my ,e -se+0 the simplest of hichmerely +etects the presence of chemicl in the .s strem/ In this cse0

chemicls pper s pe*s in the si.nl from the +etector0 n+ re i+entifie+

,y the len.th of time tht they spen+ on the col-mn 3the retention time50 these

h(in. pre(io-sly ,een scertine+ -sin. p-re chemicls/ An eGmple of this

is shon in Fi.-re 0 hich +isplys the res-lts for ine0 ech component

h(in. ,een i+entifie+ ,y its retention time on the col-mn/ Notice the (ery

smll mo-nt of mteril tht is reB-ire+ for this techniB-e 9 4 cm ?/ Notice

lso tht the chemicl hr+ +t sheets for mny of the components of ine

m*e .rim re+in.

Figure !

:as chromatograph of a 1 cm9sample of +ine

It is lso possi,le to -se more sophisticte+ +etectors0 common one ,ein.

the mss spectrometer in com,intion ith .s chromto.rph0 *non s

&CMS/ This s -se+ in the nlysis of mterils from the Mrtin s-rfce

on the #i*in. Mrs ln+ers in the 4>;s/

&C is no n eGtremely sensiti(e techniB-e0 ith femtomole sensiti(ity0 n+

fin+s n eGtremely i+e rn.e of pplictions: +r-. nlysis 3e/./ forensic

science50 sports testin.0 en(ironmentl monitorin. 3e/. ir poll-tnts from cr

eGh-st .ses0 pestici+e resi+-es50 B-lity control in phrmce-ticls

mn-fct-re0 ,rein.0 +istilltion etc/0 to nme ,-t fe/

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An inno(ti(e +etector hs ,een employe+ ,y reserchers in n ttempt to

i+entify coc*roch pheromones/ Pheromones re nt-rl scents tht h(e

stron.ly ttrcti(e poer for specific species/ To i+entify the specificchemicls tht cte+ s coc*roch pheromones0 coc*roch s liB-i+ise+

n+ psse+ thro-.h &C col-mn/ The chromto.rphy col-mn seprte+ o-t

ech of se(erl tho-sn+ chemicl components of the coc*roch0 n+ these

ere psse+ thro-.h the col-mn in t-rn/ As it s only necessry to i+entify

the pheromone0 ll tht s reB-ire+ s sensiti(e +etector tht o-l+

i+entify hich of these myri+ chemicls s the pheromone/ Fort-ntely0

Nt-re herself h+ pro(i+e+ the ,est pheromone +etector of ll 9 coc*roch

ntenn/ Once the retention time of the pheromone h+ ,een +etermine+ ,y

monitorin. the mo(ement of n ntenn0 it s possi,le to isolte lr.er

B-ntities of the chemicl for more +etile+ nlysis/ Once nlyse+0 the

pheromone s chemiclly synthesise+ n+ -se+ to trp coc*roches/

Acid7!se e;&ili%i!

In 1rZnste+9Lory theory0 n !cidis proton 3%K5 +onor n+ !seis

proton 3%K5 cceptor/ $Gmples of ci+s re %Cl0 C%?COO%0 %

?OKn+ %

2O/

$Gmples of ,ses re N%?0 C%

?COO90 %

2O n+ O%9/ %

2O cn therefore ct

s either n ci+ or ,se/ A rection ,eteen n ci+ n+ ,se 3!n !cid7

!se %e!ction5 in(ol(es the eGchn.e of proton0 for eGmple:

%Cl K N%?Cl9K N% 8K

ci+ ,se

These +efinitions pply -n+er ll con+itions0 ,-t the most importnt ci+9

,se systems -se ter s sol(ent/ In this cse0 eB-ili,ri re set -p in

ter0 hich for %A 3 .enerl ci+5 is:

%A3B5 K %2O3l5 %

?OK3B5 K A93B5

e/./ %Cl3B5 K %2O3l5 %

?OK3B5 K Cl93B5

n+ for 1 3 .enerl ,se5 is:

13B5 K %2O3l5 O%93B5 K 1%K3B5

e/./ N%?3B5 K %

2O3l5 O%93B5 K N%

8

K3B5

In the first eB-ili,ri-m the %2O molec-le cts s ,se0 ccepts proton

n+ forms the '#d%oni&m ion0 %?OK0 hich is the hy+rte+ form of the

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proton in sol-tion/ In the secon+ eB-ili,ri-m the %2O molec-le cts s n

ci+ n+ forms the '#d%o*ide ion0 O%9/ The ,se tht res-lts from the

trnsfer of proton from the ci+ is clle+ the con@&g!te !seof the ci+/ A9is the conH-.te ,se of %A n+ O%9is the conH-.te ,se of %

2O/ Similrly0

the ci+ tht res-lts from the cceptnce of proton ,y the ,se is clle+ the

con@&g!te !cidof the ,se/ This mens tht 1%Kis the conH-.te ci+ of 1

n+ %?OKis the conH-.te ci+ of %

2O/

If n ci+ is re.r+e+ s stron. ci+ 3i/e/ it re+ily +ontes proton50 then it

is lys the cse tht its conH-.te ,se is e* ,se 3i/e/ it +oes not

re+ily ccept proton5/ An eGmple of this is %2SO

80 hich is stron. ci+

hile its conH-.te0 %SO980 is e* ,se/ Con(ersely0 e* ci+ +ontes

proton to .enerte stron. ,se0 n eGmple ,ein. the e* ci+0 ceticci+0 C%

?COO%0 hich hs (ery stron. conH-.te ,se0 C%

?COO9/

Note tht the concept of conH-.te ci+s n+ ,ses is only relti(e one since

species cn ,e0 for eGmple0 ,se or conH-.te ,se0 +epen+in. on the

specific eB-ili,ri-m of interest/ T*e0 for eGmple0 the ethnote ion0

C%?COO9:

C%?COO%3B5 K %

2O3l5 %

?OK3B5 K C%

?COO93B5

C% ?COO

9

3B5 K %2O3l5 O%

9

3B5 K C%?COO%3B5

In the first eB-ili,ri-m0 the ethnote ion is ctin. s conH-.te ,se6 in the

secon+0 it is ctin. s ,se/ The ion is the sme in ,oth cses0 it is only its

relti(e position in the eB-ili,ri-m tht +etermines hether or not it is

re.r+e+ s conH-.te species/

T'e !cidit# const!nt? K!

It is -sef-l to compre the relti(e proton +ontin. stren.th of n ci+0 n+

this is +one ,y consi+erin. the ci+ +issocition eB-ili,ri-m:

%A3B5 K %2O3l5 %

?OK3B5 K A93B5

As ith ny other eB-ili,ri-m0 the position of this eB-ili,ri-m my ,e

mes-re+ thro-.h the eB-ili,ri-m constnt:

J !%?OK'!A9'D!%A'!%

2O'

Strictly0 e sho-l+ -se cti(ity0 n+ s the ter is the sol(ent0 its cti(ity

is pproGimtely eB-l to 4/ This eB-ili,ri-m constnt is no +ifferent

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from ny other0 prt from its specificity to this prtic-lr type of eB-ili,ri-m0

n+ the eB-ili,ri-m constnt is referre+ to s the !cidit# const!nt? K!:

J !%

?OK'!A9'D!%A'

Clerly0 stron. ci+ ill .i(e rise to relti(ely hi.h (l-es of !%?OK' n+

!A9'0 n+ so for stron. ci+ ill ,e lr.er thn for e* ci+/

As (ries o(er s-ch lr.e rn.e 3se(erl tens of or+ers of m.nit-+e50 it is

con(enient to -se the ne.ti(e of the lo.rithm of 0 *non s p

3cf/ the p%

scle5:

pJ 9lo. 3

5

Indic!to%s

An in+ictor for -se in ci+9,se rections is comprise+ of e* ci+ tht

+isplys +istinctly +ifferent colo-rs in its protonte+ n+ +eprotonte+ forms/

An in+ictor hs n eB-ili,ri-m of the .enerl form:

%In %K K In9

ci+ colo-r ,se colo-r

here In9is typiclly lr.e0 compleG0 or.nic molec-le/ This eB-ili,ri-m

hs n eB-ili,ri-m constnt0in

0 .i(en ,y:

in

J !%K'!In9'D!%In'

This my ,e rerrn.e+ to:

!%In'D!In9' J !%K'Din

If the concentrtion of %K3B5 is hi.h 3i/e/ it is n ci+ic sol-tion50 then %In

ill +ominte the sol-tion n+ the ci+ colo-r ill ,e most pprent/ If theconcentrtion of %K3B5 is lo0 then In9ill +ominte n+ the sol-tion ill

h(e the colo-r of the ,se species/

Clerly0 if ,oth forms re in sol-tion0 then the colo-r ill ,e miGt-re of the

ci+ n+ ,se forms/ &enerlly0 the colo-r ill ,e +istinctly tht of the ci+

form if !%In'X4;!In9' n+ of the ,se form if !In9'X4;!%In'/

For the ci+ic colo-r:

4; J !%K'Din

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,-fferin. ci+s n+ ,ses/

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Figure 13

#itration cur$e for a +ea3 acid and strong base

S t o i c h i o m e t r i c p o i n t

B u f f e r r e g i o n

p H

V o l u m e o f t i t r a n t

1 2

1 0

8

6

4

2V

v / 2 v / 2

Mny rections re hi.hly sensiti(e to hy+ro.en ion concentrtion 3p%5/

1-ffer sol-tions re (ery importnt n+ re i+ely -se+ in chemistry n+

,iochemistry in or+er tht p% con+it ions my ,e mintine+ (ery specif iclly

+-rin. the co-rse of rection or process/ Moreo(er0 niml n+ plnt

met,olisms re lso hi.hly sensiti(e to p%0 n+ m-ch of the ,o+yEs chemicl

efforts re p-t into mintinin. the optim-m p% for certin enymes/ For

eGmple0 the +i.esti(e enyme trypsin opertes most effecti(ely in the p%

rn.e 2980 heres l*line phosphtse is most effecti(e t p% >944/

Of ll the ,-