Dien Hoa Ly Thuyet_se

8/8/2019 Dien Hoa Ly Thuyet_se

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TRNG AI HOC CONG NGHIEP THANH PHO HO CH MINHKHOA CONG NGHE HOA HOC

&

BAI GIANG

IN HA L THUYT

Ngi soan : ang Kim Triet

TP. HCM, 01-2005


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CHNG I :

C S LY THUYET VE CAC QUA TRNH IEN CC

I. LP IEN TCH KEP:1. M au:

Khi cho 2 pha tiep xuc nhau th gia chung se tao thanh be mat phanchia pha. Khi tao thanh be mat phan chia pha thng co s phan bo lai ien

tch gia cac pha. Khi ay be mat phan chia pha se tao thanh lp ien tch

kep va xuat hien bc nhay ien the gia cac pha.

Cac trng hp phan bo lai ien tch :

- Chuyen ien tch qua be mat phan chia gia cac pha . V du : Khi

nhung mot thanh kim loai vao trong dung dch th nhng ion kim loai se

chuyen vao dung dch va be mat dung dch ni tiep xuc ( be mat phan

chia pha) se tch ien dng, con be mat kim loai se tch ien am.-Hap phu khong eu nhau cac ion trai dau (hap phu chon loc). V du :

Hap phu ion Cl- tren be mat kim loai tr. Khi ay, be mat kim loai tch ien

am va se hut ien tch dng cua dung dch va lp ien tch kep hnh thanh.

- Hap phu va nh hng cac phan t lng cc.

Tren cung mot be mat phan chia gia cac pha co the xay ra hai hoac nhieu

trng hp tren. Cho nen be mat phan chia gia hai pha co the bao gom

nhieu lp, nhng ta van goi lp ien tch hnh thanh tren be mat phan chia

gia cac pha la lp ien tch kep.a.The ben trong, the ben ngoai, the be mat:.The ben trong (the Galvani) : La ien the ben trong cua pha so vi

mot iem xa vo cung trong chan khong khong tch ien.

Theo nh ngha tren e chuyen mot ien tch e nao o t vo cc vao

ben trong cua mot pha can phai ton mot cong la e.Thc ra ieu o ch thc hien c trong tng tng v can phai co

mot ien tch co tnh chat ac biet. ien tch o phai het sc nho e khi no

xuat hien va bien mat khong anh hng g en phan bo ien tch trong phava ac biet khong c tac dung len ien tch o mot lc hoa hoc nao. Thc

ra nghiem khac ma noi th cac lc hoa hoc ay cung co tnh chat ien.

. The ben ngoai () hay con goi la the Volta : La ien the cua motiem tren be mat cua pha so vi mot iem xa vo cung trong chan

khong khong ien tch.


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Nh vay e chuyen mot ien tch e t vo cung en be mat pha can

phai ton mot cong la e.The ben ngoai xuat hien la do tha ien tch t do tren be mat.The Volta bang khong khi be mat khong mang ien tch.

.The be mat ( ) : Xuat hien khi tren be mat co s nh hng cac

lng cc cua dung moi hoac khi co s hap thu ac biet cac ion, nguyen t

hay phan t. Gia the ben trong, the ben ngoai, the be mat co moi lien he

sau :

= + The be mat se trung vi the ben trong, khi the ben ngoai bang khong

ngha la khi tren be mat khong co ien tch t do.

= 0 va = b. Hieu the Galvani va hieu the Volta :.Hieu the Galvani 1,2 : La hieu so ien the ben trong cua 2 pha.

1,2 = 1 - 2Sc ien ong E cua mot mach bao gom nhieu pha tiep xuc vi nhau

bang hieu so the ben trong cua 2 pha cuoi giong het nhau cua mach (1 va 1).

o chnh la hieu the Galvani.

E = = 1 - 1V du : Trong mach ben (H.1) ta co the viet :

MeIII MeI ien giai I ien giai II MeII MeIII

1 2 3 4 5 6 7=1

E = 1MeIII - 7MeIIINoi mot cach khac sc ien ong E cua mach bang tong so ai so cua

cac hieu ien the Galvani gia cac pha noi tiep nhau cua mach.

ieu o the hien ro rang hnh 1. Ta khong o c trc tiep tng

hieu so ien the ben trong rieng biet. Ngc lai tong E co the o c.

. Hieu the Volta ( 1,2 ) :Hieu the Volta 1,2 gia pha 1 va 2 la hieu so cua cac the ben ngoai

cua 2 pha o.

1,2 = 1- 2Trong mach co 2 pha au va cuoi giong nhau 1 va 1 ta cung co the

viet:

1- 1= 1 - 1 = EE = 1- 1= i,i + 1


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He thc nay co the kiem tra bang thc nghiem v hieu the volta gia

cac pha co the o c.

. S lien he gia the Volta va ien the ien cc :

Bc nhay ien the nay sinh gia cac pha ngha la gia cac iem

trong kim loai va trong dung dch goi la ien the ien cc tuyet oi.

Hien nay ta khong o c ien the ien cc tuyet oi. Do o danh t

ien the ien cc tha nhan la e ch sc ien ong cua 1 pha gom ien cc

a cho va ien cc hyro trang thai tieu chuan ngha la khi +OH

a3

= 1 va

2HP = 1amt. Do o tren thc te ien the ien cc Men+/Me nao o se la sc

ien ong cua pin :

H2 Pt/H3O+( +

OHa

3= 1), H2O//Me

n+, H2O/Me (I)

Ta xet s lien he gia the volta va ien the ien cc, tc la xet s

lien he gia the volta va sc ien ong E cua pin tren.

Gia s kim loai 1 la Pt nhung trong dung dch L1 cha ion H3O+ vi

+OH

a3

= 1 va2H

P = 1amt, con 2 la kim loai nhung trong dung dch L2 cha ion

Men+ can bang vi Me va kim loai 2 va 2 la 2 kim loai giong nhau.

Hnh 2 tng ng vi pin (I) sc ien

ong cua mach (hnh 2 ) Chnh la sc ien

ong cua pin (I) hay noi cach khac i chnh la

ien the ien cc Men+/Me .

Ta co the viet :

E = 2- 2= i,i + 1 hay viet mot cach khac:

E = 2,L 2 + L 2 ,L 1 + L 1 ,1 + 1,2= (

2,L2

2

) (1,L

1

- 1

) +L

2 , L 1 .L 1 ,L 2 la th khuyech tan gia L1 va L2. Thn L 1 ,L 2 nho , boqua.

E = (2,L 2 2) (1,L 1 - 1)Neu lay ien cc hydro lam ien cc tieu chuan vi 2H+/H 2 = 0 th :

1,L 1 - 1 = 0

2'

E

2 1

Hnh 2


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Ta thay E =2,L 2 2 tc la ien the ien cc khong phai ch labc nhay ien the dung dch ien cc ma bao gom 2 hieu so ien the

trong o mot la bc nhay dung dch ien cc va phan khac la mot phan

cua hieu so ien the tiep xuc kim loai kim loai.

c. The ien hoa :Trong nhieu trng hp khong the dung the hoa hoc ma phai dung

th ien hoa ( i ). Gia th hoa hoc va th ien hoa co he thc :

i = i + Zi FF : So Faraday

: The ben trong (the Galvani)Zi : Gia tr ai so cua ien tch. oi vi cac phan t khong tch

ien Zi = 0

2. S dung phng phap nhiet ong e nghien cu lp kep:a. Thuyet Gui - Sapman:

Theo Hemtholtz th lp ien tch kep co the hnh dung nh mot tu

ien phang song song. Pha dung dch ch co mot lp ion day ac ep sat vao

ien cc con pha ien cc co mot lp ien tch trai dau.

dd x

+ -

+ -

+-

+ -

+ -

+ -

+ -

+ -

S o cau tao Bien thien ien the theo khoang cach

kieu HelmtholtzMau ien tch kep cua Helmtholtz con qua gian n. No khong giai

thch c mot so hien tng nh :

- ien dung cua lp kep phu thuoc vao nong o chat ien giai va ien

the ien cc.

- Co ton tai mot the ien ong nho hn M la trai dau vi M.


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aM=10

Sau Helmtholtz th Gui - Sapman a phat trien lp ien tch kep.

Theo ho th cac ion co chuyen ong nhiet t do, mat khac cac ion cung dau

se ay nhau nen cau tao cua phan lp ien tch nam pha dung dch khong

day ac nh Helmtholtz quan niem ma co cau tao khuyech tan. Vi mot ien

cc phan cc ly tng th co the noi rang gia mot iem bat ky nao o trong

lp kep va mot iem trong the tch dung dch, co ton tai mot can bang :

i lp kep =

S

i

x

1

l1

+

+

+

+

++

+ +

+

+

+

+

+

++

+

+

-

-

-

-

-

-

- -

-

-

- - -

-

-

-

-

q = 16 + 6s

S o cau tao kieu Gui-Sapman Phan boien thetheo khoang cach

Trong o :

i lp kep, is

: The ien hoa cua cac ion o trong lp kep va trong thetch dung dch.

Biet :

i lp kep = io + RT ln Ci + Zi F

is = ios+ RT ln Cis + Zi F s

Trong o :

Ci, Cis : la nong o cua ion trong lp ien tch kep va trong the tch

dung dch.

ien thes tha nhan bang khong, x la khoang cach en ien cc.Do o co the viet :

i lp kep = io + RTlnCi + ZiF = ios + RT ln CisGan ung coi : Io = ios ta co the viet :

RTlnSCi

Ci = -ZiF


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lnSCi

Ci = -Zi RT

F

at f =RT

F ta co : lnSCi

Ci=Zif

Do o :SCi

Ci = e -Zif rut ra : Ci = Cis.e-Zif

Phng trnh tren cho biet qui luat phan bo ion trong dung dch vatrong lp ien tch kep.

Theo Gui-Sapman ien tch khuyech tan tong cong phan bo pha

dung dch di tac dung cua lc tnh ien va chuyen ong nhiet tai x = 0 la :

q = -222

0

2/1

ZfSh

DRTCiS

D : hang so ien moi

0 : ien the tai x =0

ien dung vi phan cua lp kep :

C =2

cosh2

2/122

M

S

M

qkt zf

RT

CiFDZ

=

T cong thc tren ta thay ien dung cua lp kep phu thuoc vao nong

o chat ien giai va ien the ien cc. ieu nay thuyet Hemtholzt khong giai

thch c.

b. Ly thuyet SternTrong ly thuyet Gui- Sapman cac ion coi nh cac ien tch iem co

the tiep can ien cc en mot khoang cach nho bao nhieu cung c. Nhngtrong thc te th cac ion eu co kch thc xac nh nen theo Stern th cac ion

ch co the tiep can ien cc en mot mat phang tiep can cc ai nao o. Mat

phang nay la chung cho ca Cation va anion. Nh vay lp ien tch kep chia

lam 2 khu vc :

-Lp day ac nam gia mat phang ien cc va mat phang tiep can cc

ai. Ta goi la lp Helmtholtz hay la lp ben trong.

-Lp khuech tan trai rong t mat phang tiep can cc ai vao sau trong

dung dch.Stern thay can phai phan biet hai mau lp ien tch kep.

Mau Stern khong co hap phu Mau Stern co s hap phu ac biet cac

ion


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tao thanh lp khuech tan khong the tiep can en ien cc gan hn mot

khoang x = x2. Mat phang qua x2 va song song vi ien cc c goi la mat

phang Helmtholtz ngoai, ien the tai mat phang o ky hieu la o.

Mat phang ben trong va ben ngoai khac

nhau khong phai ch khoang cach ien cc.

Mat phang ben trong la mot lp ion nam trong

h the nang ac biet. Nhng ion o mat het

hoac mot phan vo hydro hoa. Chuyen ion o

vao trong dung dch phai ton mot nang lng

e thang cong hap phu ac biet cua s tac

dung cua ion vi ien cc. Mat khac chuyen ion t dung dch vao mat phang

Helmtholtz ben trong ton 1 cong- Cong kh hydro hoa. Chuyen ion vao gan

ien cc hn x1 vap phai sc ay cua ien tch ien cc. Mat phang tiep cancc ai ngoai khong phai la mot lp ma ch la gii han co the tiep can c

cua cac ion chuyen ong nhiet.

Grame chng minh rang neu nh khong co s hap phu ac biet th qM

= - q2 va lp kep coi nh hai tu ien mac noi tiep.

21

111

CCC+=

C : ien dung vi phan cua lp kep

C1 : ien dung vi phan cua lp kep day acC2 : ien dung vi phan cua lp kep khuech tan

3. Cac phng phap nghien cu lp kep:a. Phng phap ien mao quan:. Phng trnh Lpman : Phng phap ien mao quan da tren phep

o s phu thuoc sc cang be mat cua kim loai long nh Hg vao ien the va

nong o chat ien giai. ay la phng phap tin cay e nghien cu s hap phu

ien hoa tai be mat phan chia ien cc va dung dch. ieu kien e cho s olng trong phng phap ien mao quan c gian n la tren ien cc

khong xay ra mot phan ng ien hoa nao. Khi ay toan bo ien tch en be

mat ien cc ch dung e tao nen lp kep. Ta goi ien cc ay la ien cc

phan cc ly tng. Co nhieu kim loai co the dung lam ien cc phan cc ly

tng nhng trong dung dch nc tot nhat la dung Hg v qua th H2 tren Hg

rat ln.

0

x1 x2

0 vi E > EZ va qM < 0 vi E

EZ (qM > 0) con Cation b hut vao ien cc khi

E < EZ (qM< 0 ). Cac ion b hut vao ien ccse ay nhau, do o e tang them mot n v be

mat phan chia ien cc- dung dch oi hoi mot cong nho hn khi khong co tac

dung tnh ien gia cac ion va ien cc (qM = 0, cac ion khong b hut vao

ien cc ). Do o sc cang be mat se giam i khi tang gia tr tuyet oi cua qMva ng cong mao quan se cc ai tai ien the iem khong tch ien.

Anh hng cua s hap phu cac ion va phan t trung hoa en dangng cong mao quan:

Dang cua ng cong mao quan phu thuoc rat nhieu vao s hap phu

cac ion va chat trung hoa tren be mat ien cc s phu thuoc o manh yeu

khac nhau tuy thuoc ban chat ion va phan t trung hoa.

+ oi vi anion co the chia lam 2 nhom :

- Nhng anion khong hoat ong be mat nh : F -, CO-, OH-, SO42-,HPO4

2- th sc cang be mat thay oi rat t. ien the iem khong tch

ien EZ khong thay oi.

- Nhng anion hoat ong be mat nh : NO-, NO3-, CNS-, I-, Br- ha

thap sc cang tren be mat tch ien dng hoac khong tch ien.

Lng anion b hap phu thuoc vao ien tch be mat. Khi be mat tch

ien dng th hap phu ln, be mat tch ien am yeu th t hap phu.

Tang ien tch am cua be mat len (tang ien the ien cc u am) th

lc ay tnh ien ln hn lc hap phu ac biet, cac anion se b nha rakhoi be mat ien cc. Do o ien the E u am ng cong mao

quan cua dung dch co chat hoat ong be mat va khong se trung nhau

va khi o dang cua ng cong mao quan t phu thuoc ban chat chat

ien giai ien the u am. Khi hap phu cac anion ien the EZ se

chuyen ve pha am hn.

+ oi vi Cation

,q

+q

-E

Ez0

-q -q

q=0


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- Khac vi anion cac Cation vo c hap phu yeu (tr Tl+). con cac

Cation hu c hap phu manh len be mat Hg. V du : Cac Cation

[(CH3)4 N]+, [(C2H5)4N]+, [(C4H9)4N]+. Cac Cation hap phu tren bemat tch ien am va tang len khi tang ien tch am be mat. Khi ien

tch be mat u dng cac cation b nha ra.

420

400

380

360

340

320

300

280

260

240

220

KOH

NaClNa SO

KCNS [(C H ) N]KI

-E -E

2

4 9 4+

4

ng cong mao qun trong dung dch cc ng cong mao qun

cht in gii khc nhau( Hap ph anion). trong d.d.cc cht in gii

khc nhau(Hap ph Cation)

+ Khi ta cho vao dung dch chat ien giai tr

nhng hp chat hu c trung hoa th sc cang

be mat cung ha thap xuong. S ha thap sc

cang be mat do hap phu cac chat hu c

thng xay ra ien the iem khong tch

ien hoac be mat tch ien yeu. Khi be mat

tch ien am hoac dng manh th cac chat

hu c b nha va ng cong mao quan cua

dung dch sach va dung dch co chat hoat

ong be mat trung nhau.

Muon biet anh hng cua ien tch be

mat en s hap phu cac phan t chat hu c

ta xet nang lng cua tu ien theo ly thuyettnh ien th nang lng W cua tu la :

W = 22 MqD

d

Ta thay khi qM cang ln th W cang ln . Neu qM khong oi th W giamkhi tang D (Hang so ien moi) hoac giam d.

KNO

ru

ng cong mao quan khi co hap phu

cac chat hu c trung hoa

D : Hang so ien moid : khoang cach gia 2 ban tu ien

-E

3


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mao quan

He thong co khuynh hng t nhien la giam nang lng cua mnh e co

nang lng cc tieu, nen lp ien tch kep co khuynh hng hut vao

nhng phan t dung moi (nc) co hang so ien moi ln va ay cac chat

hoat ong be mat (cac phan t hu c) co phan t ln (d ln). Do o he thong

tang D giam d e ha thap W xuong.

o ng cong mao quan bang ien cc thuy ngan:e o s phu thuoc sc cang be mat vao ien the ta dung dung cu

nh hnh ve :

Bnh thuy tinh A o ay dung dch chat ien

giai. Nhung ong thuy tinh K co mao quan C

vao dung dch o. Bnh B noi vi ong K

bang mot ong cao su e co the thay oi

chieu cao cua bnh B. Ong K va bnh B

cha ay thuy ngan. ien the c a vaobe mat phan chia ien cc thuy ngan va

dung dch nh mot he thong ien hoa bao

gom mao quan C va ien cc so sanh

Calomen.

Nh a biet, sc cang be mat phu thuoc vao ien the ap at tren be

mat phan chia ien cc Hg- dung dch. Bay gi ta thay oi ien the ien cc

va ieu chnh o cao h cua cot thuy ngan sao cho be mat phan chia gi

nguyen v tr. Trong ieu kien o sc cang be mat can bang vi lc trongtrng theo phng trnh :

2r cos = r2 ghHg . (*)

Trong o :

r : Ban knh mao quan

: tiep xuc : Sc cang be mat : Ty trong cua Hg

g : gia toc trong trng

h : chieu cao cua cot Hg

o chieu cao h cac ien the khac nhau suy ra theo phng trnh (*).Do o ta ve c ng cong ien mao quan.

b. Phng phap o ien dung cua lp kep bang dong ien xoaychieu:

B k

A C

+ -

S o ng cong ien mao quan

2


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Lp kep co the coi nh mot tu ien, mot ban cua no la be mat kim

loai tch ien con be mat kia la lp ion trai dau nam cach be mat ien cc

mot khoang d bang ban knh cua ion a b solrat hoa.

Trong trng hp lp kep ch co lp day ac ma khong co lp khuech

tan th 1 = 0, khi ay ta co :

C =d

Dqdc

dc

4=

Trong o :

C : ien dung cua 1cm2 be mat

q /c :Mat o ien tch tren be mat kim loai

D : Hang so ien moi

d : Khoang cach gia cac ban cua tu ien.

Vi tu ien thng gom hai ban kim loai trong o q/c va d la hang so th

ien dung tch phan xac nh theo cong thc tren trung vi ien dung vi phan:

C =d

dq

Trong ien hoa ta ch o c bien thien ien the d va bien thien dtng ng, ngha la o c ien dung vi phan.

Lu y rang ch trong trng hp ien cc phan cc ly tng th toan bo

ien lng a vao mi dung e nap lp kep. Con tren ien cc khong phai

la ien cc phan cc ly tng th mot phan ien lng a vao ien cc se btieu hao cho phan ng ien hoa tren be mat ien cc. Do o ien cc co the

coi nh mot tu ien b ro ien, ngha la mot tu ien mac song song vi mot

ien tr R.

C

R

lk

S o tng ng cua lp kep tren be mat ien cc khong phan cc

ly tng

Co 2 phng phap o ien dung bang dong ien xoay chieu

- Phng phap cau can bang

- Phng phap so sanh


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. Phng phap cau can bang:e o ien dung cua lp kep ta dung cau o xoay chieu (hnh) Khi o

ta thu c ien dung cua bnh ien phan.

r : May phat dao ong

Zi : Cac tong tr cua cau

O : Dao ong ky

D : Cuon cam e ngan dong xoay

chieu

S o tng ng cua bnh ien phan nh hnh ve :Cphu

Ck-a

Cx Rx

S o tng ng cua bnh ien phan

Trong o :

CX : ien dung cua lp kep cua ien cc nghien cu

RX : ien tr cua dung dch trong dung dch ien phan

CPhu : ien dung cua ien cc phu

Ck a : ien dung gia Anot va Catot

V ien cc Catot va anot cach nhau rat xa nen Ck a rat nho va v Ck

a mac song song trong mach tren nen ta co the bo qua Ck a.

ien cc nghien cu va ien cc phu mac noi tiep nen ien dung

tong cong o c C co the xac nh bang phng trnh :

phux

phux

dd

phuxdd

CC

CCC

CCC

+=

+=

.

111

C

S o cau can bang e o c ien dung lp kep

cD

d

iz1

11i

z2

iz3

3

iz444

b

a

~


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T cong thc tren ta thay rang khi hai tu ien mac noi tiep th ch xac

nh c ien dung cua tu ien co gia tr ien dung be nhat. That vay neu

CX


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18

Nguyen tac cua phng phap la trong khi cho mot dong ien xoay

chieu co cng o khong oi i i qua ta o ien the ri X tren bnh ienphan va m tren ien dung mau.

Khi o ien dung can tm se bang :

C =22

2 1

1

x

x

x

m

CRi

Cmi

CmCm

+

=

Cthc nghiem =1.

222 +xxzx

CR

C

RX va CX la cac thanh phan ien tr va ien dung mac noi tiep cua

bnh ien phan. Nhng thanh phan o se ng vi ien tr dung dch va ien

dung cua lp kep khi tren ien cc khong co phan ng ien hoa nao xay ra.

T phng trnh tren ta thay rang Cthc nghiem ch bang CX khi tan so

goc thap va ien tr dung dch nho, tc la khi bat ang thc sau cnghiem ung :

RX2CX

2X2


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19

II. ONG HOC QUA TRNH IEN CC :1. ong hoc qua trnh ien cc n gian khong kem theo hap phu

Vat ly va Hoa hoc :a. S phan cc:

Muon hieu ban chat cua khai niem S phan cc ta xet cac v dusau :

-V du 1 : Co mot bnh cha dung dch CuCl2 hoat o aCu++ = 1. Neu ta

nhung vao dung dch o 2 ien cc bang ong, ien cc ong se can bang

vi ion ong trong dung dch va ien the ien cc can bang cua 2 ien cc

bang nhau va bang +0,34v. Noi 2 ien cc vi nguon ien ben ngoai th ien

the ien cc cua ong se dch khoi gia tr can bang. ien the cua ien cc

noi vi cc am cua nguon se co gia tr am hn + 0,34v, con ien cc noi vi

cc dng cua nguon se co gia tr dng hn + 0,34v.

+ +0,34v

> >

Th mot so hang cua phng trnh (*) co the bo qua va khi ay hoacqua trnh anot la chu yeu hoac qua trnh Catot la chu yeu. T phng trnh

(*) rut ra :Qua trnh Catot :

0ln1

ln1

i

nf

i

nf

k

=

Qua trnh anot :

0ln)1(

1ln

)1(

1i

nfi

nfk

=

T 2 phng trnh neu at :


23/44

23

=

=

nfb

ninf

a

303,2

10

oi vi qua trnh catot

va

=

=

nfb

ninf

a

)1(

303,2

)1(

10

oi vi qua trnh anot

Ta co the viet lai mot cach tong quat :

= a + blg i o chnh la phng trnh Tafel.

f. ng cong phan cc hon hp :Ta bieu dien cac phng trnh toc o phan ng bang o th :

Cac ng cong ii , la ng phan cc bo phan. Tai moi gia tr nhat

nh cua ien the ca 2 qua trnh anot va Catot eu xay ra va ii , eu

khac 0. Khi MCbM = th ii = = i0. Dong ien tong bang 0.

+i

i

o

ocb

i

i

i

i

1

2

pha phai cua ien the can bang la qua trnh kh, pha trai la qua trnh

oxy hoa.ng noi cac iem 1, Cb, 2 la ng cong phan cc toan phan cho biet

toc o cua qua trnh anot va Catot phu thuoc vao ien the.

ng cong phan cc toan phan la 1 trong nhng d lieu quan trong e

nghien cu ong hoc qua trnh ien cc. Ta o c ng cong nay bang

thc nghiem.

2. ong hoc qua trnh khuech tan:


24/44

24

a. ac iem cua ng cong phan cc:Muon phong ien ien cc th cac phan t phan ng phai trai qua 4

giai oan. Giai oan II va IV la giai oan khuech tan.

Khi mat o dong ien khong ln lam,

toc o phan ng ien cc khong ln th

toc o khuech tan co the am bao cung cap

cac phan t phan ng en ien cc hoac

thai kp thi cac san pham phan ng khoi

khu vc ien cc. Nhng khi cng o

dong ien tang len kha ln th s khuech

tan cac phan t phan ng en ien cc co

the khong u ln va toan bo qua trnh ien

cc se b toc o khuech tan khong che. Khi ay du co thay oi ien the th qua

trnh van khong tang len c. Ta lay qua trnh Catot lam v du. ng congphan cc gom 3 khu vc :

Khu vc I : Toc o qua trnh do yeu to ien hoa khong che. ng cong phan

cc trong oan I co dang ham so mu.

Khu vc II : Qua oKhu vc III : Khong che do khuech tan

Neu trong dung dch co hai hoac nhieu phan t co the kh Catot th

ng cong phan cc co dang sau :

V du : Co hai ion MeI++ va MeII++ cung ton tai trong dung dch. ienthe can bang cua chung la ICb va IICb. Neu ta cho ien the ien cc dchchuyen ve pha am hn th khi ien the vt qua ICb ion MeI++ se phong ienva at ti dong gii han Igh (I). Khi ay

ien the van tang nhng Igh (I) khong tang.

Khi ien the vt qua IICb th MeII++ batau phong ien va dan ti dong gii han

Igh (II), Igh(I) va Igh(II) goi la dong gii han.

Dong ien gii han tong quat la:

Igh(K) = Igh (I) + Igh (II) (1)

b. Toc o khuech tan:Khi phan ng ien cc tien

hanh th nong o cua cac phan t phan

Cb-

i

I

II III

Cac khu vc cua ng cong phan cc

i

i

i

gh(I)

I IICb Cb

gh(II)


25/44

25

ng nam trong vung cach be mat ien cc la se giam xuong, cang tang thigian th khoang cach b thay oi nong o cang tang len.

Neu v ly do nao o on nh th theo nh luat Fick 1, ta co :

= Dd

dm

( )

CCoDC = (2)

Phn b nng o cht phn ng theokhong cach n in cc thi gian in

phn (t1


26/44

26

Khi dong ien iK i qua th nong o chat phan ng sat ien cc

giam xuong en C. ien the ien cc luc o :

i,k = 0 + CZF

RT

(gia thiet rang phan cc hoa hoc khong ang ke)

Theo cong thc (3) ta co :id(K) = ZF )( CCo

D

at K = ZF

Dv vi mot che o lam viec nhat nh, mot dung

dch nhat nh th Z, D, la nhng hang so.Rut ra :

C = C0 -K

iKd )( (5)

Khi C = 0 ta co :igh(K) = KC0 do o : C0 =

K

kigh )((5)

The gia tr cua C0 vao phng trnh (5) ta co :

C =K

idi

K

i

K

i KKghKdKgh )()()()( = (6)

Ta biet rang dch chuyen ien the khi co phan cc nong o :

nong o = i,K - Cb

Nen : nong o =Co

C

ZF

RTln (7)

The gia tr cua C, C0 trong cac cong thc (5)va (6) vao cong thc (7) ta co :

nong o =

K

ighK

idigh

ZF

RT

K

KK

)(

)()(

ln

=)(

)()(ln

K

KK

igh

idigh

ZF

RT

hay nong o = )1ln()(

)(

K

K

igh

id

ZF

RT (8)

Theo cong thc tren th nong o se tien ti - khi id(K) = igh(K). Nhng thctenong o khong tien ti vo cung v co ion khac tiep tuc phong ien.

d. Anh hng cua dong ien di c va chat ien giai tr en mat odong gii han :

Chat phan ng chuyen ong en ien cc bang hai cach :

- Do khuech tan : dong khuech tan id.


27/44

27

-Do chuyen ong cua ion en ien cc di tac dung cua ien trng :

Dong di c im

+ Trng hp Cation phong ien Catot :

Goi toc o cua Catot la iK. Khi ay dong khuech tan va di c cung chieu.

iK = id + im

im = i Kt+

t+ : la so chuyen van cua Cation

id = iK iKt+ = iK (1 t+) = iKt-

t - : la so chuyen van cua anion

+ Trng hp anion phong ien anot :

id = iA(1 t - ) = iAt+

iA : Toc o anot

+ Trng hp anion phong ien Catot :

V du : Cr2O7- + 14H+ + 6e = 2Cr3+ + 7H2OChieu chuyen ong cua ion di tac dung cua ien trng va khuech tan

ngc chieu nhau.

iK = id im = id ikt_id = iK (t - + 1)

+ Trng hp Cation phong ien anot :

V du : Fe2+ Fe3+ + eTng t ta co :

id = iA (t+ + 1)Thay gia tr cua id phng trnh (3) vao ta co :

*Trng hp Cation phong ien Catot :

ZF ( ) = CCoD

iK(1 t+ )

iK = ( )CCoD

t

ZF + 1

Khi C = 0

igh(K) = Co

D

t

ZF

+1 *Trng hp anion phong ien anot :


28/44

28

igh(A) = CoD

t

ZF

1

*Trng hp Cation phong ien anot :

igh (A) = ++ 1tZF

CoD

*Trng hp anion phong ien Catot :igh(K) = Co

D

t

ZF

+ 1

Khi trong dung dch co nhng chat ien giai tr khong tham gia vao qua

trnh ien cc ma ch ong vai tro chuyen ien th dong ien cua cac ion

tham gia phan ng ien cc se nho i. V du :

igh(K) = CoD

t

ZF

+1

Trong o :'xx

x

+=

x, x : o dan ien rieng cua ion tham gia va khong tham gia qua trnh ien

cc.

Khi x >> x th 0

igh(K) = ZF CoD

d .Phng trnh khuech tan khong on nh oi vi ien cc phang: phan tren ta a xet qua trnh khuech tan on nh ngha la trong qua trnh

o toc o khuech tan khong thay oi theo thi gian. Trong phan nay ta xetqua trnh khuech tan khong on nh, trong o toc o cua qua trnh thay oi rat

nhieu theo thi gian.

Xet qua trnh khuech tan khong on nh v :

- Qua trnh nay thng gap trong thc te.- Xet e biet c khi nao th xay ra khuech tan on nh, khong on

nh.

Ta xet qua trnh khuech tan khong on nh gian n nhat la vi ien cc

phang nhung trong dung dch khong b khuay tron. Gia s tren ien cc

phang thang goc vi truc x co xay ra phan ng ien

hoa sau :

0 + ne D R

e tien li ta xet ien cc phang co kch

thc 1 x 1cm2. V chieu day cua lp khuech tan

thng dao ong trong khoang 10-2 10-3 cm nen

x

y

z


29/44

29

kch thc cua ien cc ln hn kch thc cua lp khuech tan nhieu. Do o

khong can xet en qua trnh tien hanh ra ien cc va gia thuyet rang nong

o cua chat phan ng ch phu thuoc khoang cach x en ien cc ma khong

phu thuoc vao cac toa o z,y.

Trong trng hp nay ta coi be mat ien cc la vo cung ln va kch

thc cua thung ien phan theo chieu x cung coi la vo cung ln so vi lp

khuech tan va do o tai khoang cach x u ln nong o cua chat phan ng

trong dung dch se khong thay oi.

e n gian ta gia thiet rang khi phan cc th nong o cua dang oxy

hoa cua chat phan ng b thay oi con dang kh th khong.

Theo nh luat Fick th 2 ta co bien thien nong o vat chat 0 theo thi gian :

x

txCD

t

txC2

2

00 ),(),(

=

Trong o :

t : Thi gian t khi bat au ien phan

x : khoang cach ti ien cc

Nong o dang oxy hoa Co (x,t) phu thuoc ca vao toa o (x) va thi

gian (t) phan ng.

Giai phng trnh vi phan tren va rut gon ta co :

x

txCo ),(x=0 = 2/12/12/1

tD

CC

o

bm

oo

Cobm : Nong o chat oxy hoa be mat ien cc.

Gradien nong o cua chat b kh tren be mat ien cc (x = 0) ty le nghch

vi can so bac 2 cua thi gian t.

Hnh ve bieu dien s phan bo nong o cua vat chat 0 be mat catot tai cac

thi iem t1< t2< t3< t4 sau khi atlen ien cc mot ien the khong

oi.

Qua hnh ve ta thay rang tngt nh phng trnh tren Gradiennong o be mat ien cc cung

giam theo thi gian t. Mat o dong

ien se bang :

i d = ZFD0 (x

txCo

),(

)x = 0


30/44

30

i d = 2/12/12/1

)(

t

CCZFDbm

ooo

Phan bo nong o chat b kh be mat catot tai

thi iem khac nhau khi cho vao ien cc mot

ien the khong oi.

Theo phng trnh tren th mat o dong ien i giam theo can so bac 2cua thi gian t.

Neu nh ien the a vao ien cc am hn ien the0 nhieu th Cobm =0 va khi ay co dong ien khuech tan gii han qua ien cc :

i gh(K) = 2/12/12/1

0

t

CZFD o

Quan he bac nhat gia id va 2/11

ta c thc nghiem xac nh la ung.

Do o thay rang cang tang thi gian ien phan th i d cang giam va khong

the co che o khuech tan on nh c khi t th id0 va nong o chatphan ng be mat ien cc va trong dung dch bang nhau.

Quan he gia igh va 2/11

trat thuan tien e xac nh he so khuech tan be

mat ien cc.

Phng trnh dong khuech tan gii han tren anot cung tng t nh phng

trnh tren catot.

T hai phng trnh tren ta thay rang thi iem ong mach ngha la khi t =

0, mat o dong ien ban au, i (t = 0) = . Tren thc te th i (t = 0) khong theln vo cung c v khi ay phan ng ion hoa se khong che toc o cua quatrnh.

d. Phng trnh khuech tan khong on nh oi vi ien cc hnh cau:Ky hieu ban knh hnh cau la ro. V hnh cau oi

xng nen hng trong khong gian khong quan trong. Tai

cac iem cach eu tam ien cc th nong o va Gradien

nong o bang nhau. Do o co the xet nong o bat ky

iem nao o trong dung dch tai thi iem bat ky nh la

ham cua hai bien so : Thi gian t va o dai cua vect ban knh r (ngha la

khoang cach ti tam hnh cau )

Khi r = ro th :

(r

trCo

),(

)r = ro =o

o

or

C

tD

C+

0

Phng trnh tren gom 2 thanh phan :

0


31/44

31

Thanh phan th nhat tng ng vi bieu thc cua dong khuech tan en

ien cc phang, thanh phan nay ty le vi t .

Thanh phan th hai la hang so.

Khi t nho th so hang th nhat ln hn so hang th hai nhieu va s khuech

tan en be mat hnh cau giong nh en be mat phang. Khi t tang th so hang

th nhat giam v phan dong ien tng oi cua so hang th hai tang len. Neu

tiep tuc tang thi gian th dong ien khong giam ti khong ma ch ti 1 gia tr

khong oi.

` igh =o

oo

r

CZFD

Ngha la khuech tan chuyen t trang thai khong on nh thanh trang

thai on nh (khong phu thuoc t)

Ngi ta cung chng minh c rang, s thiet lap trang thai khuech

tan on nh khong phai do dang hnh cau cua ien cc quyet nh ma do kchthc co han cua ien cc.

Vi ien cc hnh cau ban knh 1mm khi Do co gia tr bnh thng th

thi gian can thiet e khuech tan chuyen thanh on nh la 300 giay. Do o

trong khoang 10 giay au tien sau khi ien phan, chieu day cua lp khuech

tan con nho so vi ban knh hnh cau th co the ap dung c nh luat

khuech tan en ien cc phang cho s khuech tan en ien cc hnh cau.

3. Phng trnh ong hoc tong quat cho ca hai khu vc khong che kch

ong va khong che khuech tan: phan tren ta thay, ng cong phan cc toan phan gom3 khu vc chnh :

- Khu vc ong hoc (khong che kch ong )

- Khu vc qua o (khong che hon hp )

- Khu vc khuech tan (khong che khuech tan)

Phng trnh tong quat cho ca 3 khu vc la :

i =phonggh

ghphong

ii

ii

+

T phng trnh tren ta thay :

- Neu iphong >> igh th i = igh tc la toc o cua qua trnh b km ham bi

giai oan khuech tan.

- Neu igh >> iphong th i = iphong tc la toc o cua qua trnh b km ham bi

giai oan phong ien.

Cung t phng tren ta co the viet :


32/44

32

phonggh

phonggh

ii

ii

i .

1 +=

hay :ghphong iii

111 +=

CHNG II : ONG HOC MOT SO QUA TRNH IEN CCI. ong hoc qua trnh thoat H2 :

Theo Frumkin th qua the cua phan ng ien hoa hoc la ai lng bieu

dien s chuyen ien the ien cc gay ra bi phan cc ien hoa hoc. Ngi ta

phan biet 2 loai qua the anot A va qua the catot K.A = Cb - iA (< 0 )

Cb : ien the ien cc H2 trang thai can bang.

iA

: ien the ien cc anot a b phan cc ( v du : Phan ng ion hoaphan t H2)

oi vi phan ng Catot (khi ay ien the dch chuyen ve pha am hn) th :

K = Cb - iK (K > 0)Theo nh ngha tren th A se co gia tr am khi phan cc anot con K co

gia tr dng khi phan cc Catot.

Theo Tafel khi mat o dong ien u ln th gia qua the cua H2 va mat o

dong ien co phng trnh sau :

K = a + blgiKa va b la hai he so khong phu thuoc vao mat o dong ien.

Nhan xet :

-oi vi phan ln kim loai phng trnh tren phu hp vi thc nghiem trong

khoang mat o kha rong t 10-9 10 A/m2 ( vi ien cc Hg trong dung dchHCl)

Tuy nhien theo phng trnh Tafel th khi iK tien ti 0 th tien ti -,trong khi o theo nh ngha khi iK tien ti 0 th

2H tien ti khong.

Trong thc te khi phan cc nho :

2H = kiK

Trong o : k la hang so

Khi phan cc nho qua the phu thuoc mat o dong ien theo quan he bacnhat.

-Hang so a phu thuoc vao ban chat ien cc , trang thai be mat cua no,

thanh phan chat ien gia va nhiet o. Hang so a co gia tr nam trong khoang


33/44

33

t 0,3- 1,7volt. Gia tr cua a ac biet cao vi mot so kim loai nh: Ch, Thuy

ngan, kem, Catmi, thiec.

-Hang so b t phu thuoc vao ban chat kim loai va thanh phan dung dch.

-Cac so lieu thc nghiem cho thay rang qua the phu thuoc vao nhiet o,

thanh phan dung dch. Trong khoang ap suat t 1-200atm qua the khong phu

thuoc vao ap suat. Levin va Dunberphoc thay rang qua the giam khi chuyen

t dung dch nc sang ru trong cac ieu kien tng ng. Qua the phuthuoc rat nhieu vao pH dung dch. Trong khoang pH = 2-7 qua the tang t

0,055-0,058 ( 22oC) khi tang 1 n v pH.

-Cac anion hoat ong be mat nh Cl-, Br-, I- lam giam qua the Hydro khi

mat o dong ien khong ln. Khi mat o dong ien ln cac anion b ay ra

khoi be mat ien cc (ien the u am) do o anh hng cua anion bien mat.

Ngi ta cung thay rang qua the tang len khi be mat ien cc hap phu nhng

ion tch ien dng (v du cac Cation hu c).Hien nay co nhieu gia thuyet ve qua the H2. Tieu chuan e xet gia thuyet

ung nhat la cac phng trnh rut ra t gia thuyet o phai giai thch c cac

so lieu, hien tng thc nghiem a neu tren.

II.ong hoc phan ung thoat Oxy :Phan ng thoat oxy la mot phan ng anot phc tap, bao gom nhieu giai

oan noi tiep nhau :

Trong moi trng kiem :

Trong moi trng kim:M + OH- MOH + e (I)MOH + OH- MO + H2O + e2MO 2M + O2hay : M + OH- MOH + e

2MOH MO + M + 2H2O2 MO 2M + O2

Trong moi trng axt th giai oan (I) la :

M + H2O MOH + H+ + eChat trung gian co the la MO hoac MOH

Hien nay co nhieu y kien khac nhau ve c che cua phan ng thoat oxy

v:

- Phan ng thoat oxy co nhieu phan ng phu.

- Kho o chnh xac ien the thuan nghch tieu chuan cua ien cc oxy.


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34

-Trang thai be mat ien cc thay oi theo thi gian, chon anot on nh

trong ieu kien thoat oxy rat kho khan. Thc vay, muon cho oxy thoat ra t

dung dch axt co aH+ = 1 th ien the ien cc phai dng hn +1,24v. Nhng

phan ln cac kim loai eu b hoa tan trc khi at ti ien the o. V vay

muon nghien cu qua trnh thoat oxy trong moi trng axt phai dung kim

loai nhom Platin, vang va mot so kim loai qu khac. Trong moi trng axt,

cac kim loai o c mot lp oxt ben vng bao ve.

Trong dung dch kiem, ien the thoat oxy am hn (+0,41v khi aOH- = 1) nen

co the dung kim loai nhom Fe, Cd va mot so kim loai khac lam anot. Oxy

thoat ra tren be mat t nhieu b oxy hoa.

-Qua the cua oxy tang len t t theo thi gian (Fe, Pt) hoac nhay vot(Pb,Cu) do o ta phai lay gia tr on nh cua no.

-Tren ng bieu dien

2O

phu thuoc logia thng thay nhieu oan

thang co o ln khac nhau, phan anh s thay oi ot ngot ong hoc cua qua

trnh. Cac oan thang o tuan theo nh luat Tafel :

2O

= a + b logia

Gia tr cua hang so a, b phu thuoc vao nhiet o, vat lieu ien cc va thanh

phan dung dch.

-Trong khoang mat o dong trung bnh, qua the oxy trong dung dch

dang kiem tang theo day : Co, Fe, Cu, Ni, Pb, Au, Pt.- Khi trong dung dch co cacCation la th qua the cua oxy cung tang len.

- Qua the cua oxy con phu thuoc ban chat cua lp oxt tao thanh tren bemat ien cc. V du khi chuyen t -PbO2 sang -PbO2 th a giam 0,4v.

III.ong hoc qua trnh kh Oxy :Phan ng kh oxy Catot la mot phan ng thng gap trong thc te, nhat

la trong an mon kim loai.

Nghien cu bang cc pho thay qua trnh kh oxy gom 2 song :

- Song th nhat ng vi phan ng :

O2 + 2H+ + 2e H2O2

- Song th hai:

H2O2 + 2H+ +2e 2H2O


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35

ng cong cc pho cua qt kh O2 tren Hg.

Trong phan ng kh oxy Ca ot giai oan hoa hoc khong khong che quatrnh.

Nguyen nhan gay qua the chu yeu do cham phong ien va khuyech tan.

Qua trnh phong ien cua Oxy gom 2 giai oan noi tiep :Trong moi trng trung tnh va kiem :

O2 + 2H2O + 2e " H2O2 + 2OH-

H2O2 + 2e " 2OH-

Trong moi trng axt :

O2 + 2H+ + 2e " H2O2H2O2 + 2H

+ + 2e " 2H2O

ng cong phan cc co hai song khi can phai co qua the ln e kh

H2O2. Khi ay s kh oxy thanh H2O2 a b khong che bi s khuech tan oxy

va dong kh oxy at ti gii han. ien the u am H2O2 mi tiep tuc b khtiep va dong kh H2O2 cung at gia tr gii han.

IV . S ket tua ien kim loai:1. ieu kien xuat hien pha mi :

Khi cac phan ng ien cc tien hanh thng kem theo s xuat hiencac pha mi. V du : Khi kh ion Hydro th co bot kh tao thanh, khi kh cac

ion kim loai se xuat hien tinh the kim loai.

S tao thanh pha mi thng bat au t tao mam tinh the (ran ) hoac

giot au tien (long). Xuat hien mam tinh the tren be mat catot kim loai

thng gap tr ngai ln, do o oi hoi ton mot nang lng ln. Cung v vay

muon ket tinh muoi t trong dung dch, ngng tu giot chat long t pha hi th

nong o muoi va ap suat hi phai ln hn bao hoa tc la phai at mot o qua

bao hoa nhat nh. Trong trng hp ket tua ien kim loai th can phai dchchuyen ien the ve pha am so vi can bang th ion kim loai mi b kh thanh

nguyen t kim loai va tao thanh pha mi.

Cong tao mam tinh the c xac nh theo cong thc :

A3 = 222

23

ln

166

3

1

Cs

CTR

V

0,5

0

+5

+

-0,5 -1,0 -1,5 -2,0v


36/44

36

Trong o :

: sc cang be mat

V : Th tch phan t

C : Nong o dung dch qua bao hoa

CS : nong o can bang cua dung dch bao hoa

C/CS : o qua bao hoa cua dung dch

T phng trnh tren ta thay cong se giam khi tang o qua bao hoa.

So 6 ng vi 6 mat cua mot tinh the lap phng.

Xac suat W tao thanh tinh the mi quan he vi cong theo phngtrnh sau :

W = Bexp (-RT

A3 )

B : Hang so

Khi A3 giam th xac suat tao mam tinh the tang len.2. Qua th ket tua kim loai ien cc:

Qua trnh ket tua ien kim loai thng tien hanh trong cac dung dchmuoi n hoac phc va noi chung bao gom cac giai oan sau :

[Me(H2O)x]Z+

dung dch D [Me(H2O)x]Z+

lp kep (a)

[Me(H2O)x]Z+

lp kep D MeZ+ + XH2O (b)

MeZ+ + Ze D Menguyen t (c)

Menguyen t D Memam tinh the (d)

Memam tinh the D Me li tinh the (e)Nhng giai oan sau ay co the khong che qua trnh ket tua ien kim

loai.

- Giai oan d hoac e b cham tre : Cham ket tinh

- Giai oan c b cham tre : Cham phong ien

a. Ly thuyet cham ket tinh:Volmer gia thiet rang, trong qua trnh ien ket tinh kim loai th qua

the ong vai tro nh o qua bao hoa khi ket tinh tinh the t dung dch.

Qua trnh ket tua ien kim loai co the b khong che bi toc o taothanh mam tinh the hai hoac ba chieu.

. Toc o tao thanh tinh the ba chieu khong che ong hocqua trnh ket tua ien :

Mam tinh the ba chieu la mot v the mi xuat hien trong pha cu.

Mam nay phai co kch thc u ln th mi ton tai can bang vi pha cu.


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Cong can thiet e tao thanh mam tinh the ba chieu co the tnh theo

phng trnh:

A3 = 222

23

ln

166

3

1

Cs

CTR

V

ong hoc qua trnh ket tua ien kim loai se b khong che bi toc o taomam tinh the ba chieu khi kim loai ket tua tren be mat ien cc la hoac tren

ien cc cung loai nhng b thu ong hoa hay ngo oc. Trong trng hp nay

qua th cua kim loai ong vai tro nh o qua bao hoa :

ZF = RTln C/CSToc o tao thanh mam tinh the ba chieu co the bieu dien bang

phng trnh :

i = Ke RTA /3

Thay A3 t phng trnh tren va RTlnC/CS = ZF vao ta co :1/2 = a blogi

Vi a = KV

FZln

3223

22

, b =

23

22

32

3,2

V

FZ

S tao thanh mam tinh the ba chieu co y ngha rat ln vi ong hoc

qua trnh chuyen pha. No thng xay ra trong trng hp ket tua kim loai

tren be mat ien cc hay la tinh the mi sinh ra khong the ln len c na

nen muon tao thanh pha mi phai tao mam tinh the ba chieu.

Hnh ve trnh bay quan

he ien the Thi gian cua qua

trnh ket tua ien kim loai tren

be mat ien cc la.

Bien thien ien the ien cc theo thgian khi ket tua kim loai tren ien cc la.

i, i, Cb ien the ng vi mat o dongien i, iva can bang (i= 0)

Ban au v phai nap lp kep va ien cc la nen can phai dch chuyen


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ien the ien cc ve pha am ti mot qua the ban au la + u e taothanh mam tinh the au tien.

Nhng khi a co mot lp tinh the mi tren ien cc th qua the giam

xuong con . V be mat ien cc khong phai la la na.Neu ngat dong th ien the ien cc dan dan tr ve ien the can bang

Cb.. Tao mam tinh the hai chieu khong che qua trnh ong hoc:Khi a co mam tinh the roi th cac tinh the ln len se tien hanh theo

tng lp do tao thanh cac mam tinh the hai chieu.

Neu nh be mat phat trien cua

tinh the c bieu dien bang hnh ve th

nang lng co li nhat khi phan t

chiem v tr III, v tr II kem li hn va

toi nhat la v tr I v tr tng ng vithi gian au qua trnh phat trien tinh

the. Con khi tren be mat a co tap hp

nhng phan t cau tao th co kha nang

ien nhieu lan vao v tr th III la v tr co li nhat ve phng dien nang

lng. Kieu tao mau nh tren goi la tao mam hai chieu.

Neu goi A2 la cong can thiet e tao mam tinh the hai chieu ben vng,

ta co :

i = K1. e RTA /2 i : toc o tao mam tinh the hai chieu

K1 : Hang so

A2 =

Cs

CRT

S

ln

2

: sc cang bien

S : Be mat phan t

o bao hoa can thiet e tao mam tinh the lien quan vi qua the theocong thc :

=Cs

C

ZF

RTln

Sau khi bien oi va rut gon, ta co :

=iba ln

1

hay iba ln

1 =

Vi :


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39

a = 12 ln KSZFRT

, b =

S

ZFRT2

Nhng cac ion phong ien tren ien cc khong phai bat k cho nao

tuy y ma ch ni li nhat ve mat nang lng. Sau o nguyen t con phai

dch chuyen tren be mat ien cc va tm cho thch hp e chuyen vao mang

li tinh the. Cho nen chung can phai thang tr lc cua moi trng bao quanhtrung tam phat trien.

e thang tr lc o phai co mot qua the nhat nh. Khi ay gia qua

the va mat o dong ien co quan he bac 1 :

= kiVolmer chia kim loai thanh 2 nhom :

- Nhom kim loai co phan cc nho nh : Hg, Cu, Zn, Ag, Bi (phan cc

ket tinh la chu yeu ).

- Nhom kim loai co phan cc ln gom kim loai nhom sat.Ch chiem v tr trung gian .Thuy ngan ch co phan cc nong o.

Kim loai nhom sat phan cc gay ra bi cham phong ien.

b. Ly thuyet cham phong ien : xa ien the can bang toc o phan ng nghch co the bo qua va co

phng trnh Tafel :

= a + blogi

a =-2,303 omi

ZF

RTlog

, b = 2,303

ZF

RT

iom : Dong ien trao oi cua phan ng:

MeZ+ + Ze D Me

Qua the tang khi giam iom v vay nhom sat co io

m nho nhat nen qua

the cung ln nhat va qua trnh phong ien cua cac ion nhom sat b khong che

bi s cham phong ien.

3. Ly thuyet ve s phong ien ong thi cua cac cation kimloai :

Trong dung dch bao gi cung co nhieu ion. Do o trong khi ien

phan khong phai bao gi cung ch co 1 ion ma nhieu ion ong thi phong

ien. Nghien cu cac qui luat ong thi phong ien cua ion co y ngha ky

thuat rat quan trong. Nh no ta co the ieu che c kim loai co o tinh khiet

cao cha rat t tap chat, co the che tao c cac loai hp kim bang phng

phap ien hoa hoc. Co hai thuyet c ban ve s phong ien ong thi cua ion.


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- Phong ien ong thi cua ion trong he thong ly tng khong lien ket:

Khi ay toc o phong ien cua tng ion rieng biet khong thay oi khi phong

ien ong thi ngha la gia cac ion khong co tac dung tng ho.

- Phong ien ong thi trong he thong ket hp : Khi ay gia cac ion co s

tac dung tng ho do o qui luat kh ion rieng biet b pha v.

a.Phong ien ong thi cua ion trong he thong ly tng khong lien ket:

ieu kien e cac ion phong ien ong thi la ien the ien cc cua

chung phai bang nhau.

o1 + ln1Fn

RTa1-1 = o2 + ln

2Fn

RTa2 - 2 (1)

o1, o2 : ien the ien cc tieu chuan cua ion 1 va 2n1, n2 : hoa tr cua ion 1 va 2

1, 2 : Qua the cua ion 1 va 2.

T hnh ve ta thay tai cung ien thex, toc o phong ien cua cac ion lai1, i2, i1 i2.

Toc o phong ien tong cong :

iK = ii = i1 + i2Trong thc te nhieu khi ch

can 1 ion phong ien, con s phong

ien cua ion khac se co hai v lam

giam hieu suat dong ien hoac giam

o tinh khiet cua san pham.Neu ky hieu A la hieu suat

dong ien cho ion can phong th :

A =K

i

i

i

i

i

i

i=

S o phong ien ong thi cua

cac ion1 va 2

ii : Toc o cua ion can phong

iK : Toc o phong ien tong cong cua cac ion.Thong thng A < 1

V du : Trong dung dch nc, ngoai ion kim loai con co ion H+. Neu ien the

Catot am hn ien the can bang cua ion H+ trong dung dch th H+ se phong

ien ong thi vi ion kim loai.


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AMe = i

i Me, AH =

K

H

i

i, hay AMe =

HMe

Me

ii

i

+

Co khi ta can phai phong ien ong thi cac cation. Trong trng hp o ta

lam cho ien the cac ion phong ien xch lai gan nhau bang cach :

-

Thay oi hoat o cua ion trong dung dch : Nhn cong thc (1) ta thaytang hoat o cua ion len 10 lan dch chuyen ien the 0,029volt vi ion hoa tr

2 va khi tang 1000 lan ch chuyen 0,087v. Do o s xch gan ien the lai se

gap kho khan khi o khac nhau qua nhieu.- Cho cac ion co ien the dng hn phong ien dong gii han.

V du : Zn, Cd co ien the can bang la 0,76v va 0,4v nh vay khi

ien phan Cd se phong ien dong gii han va ien the se at ti ien the

phong ien cua Zn (Trong trng hp nay H2 khong thoat ra v qua the cua

Hydro tren Zn va Cd ln)

i = iCd + iZn

V nong o Zn2+ ln rat nhieu so vi

Cd2+ nen dong di c do Cd2+ khong ang ke,

Cd2+ chuyen chu yeu do khuech tan.

iCd = idCd = Hang so

Do o thay oi ien the co the tao

c hp kim co thanh phan theo y muon.

Tuy nhien t dung phng phap nay

v ket tua ien thng khong at yeu cau.

Bien thien ien the ien cc khi

ket ta hp kim Zn Cd t dungdch

2N ZnSO4 va 0,2N CdZO420oC .

-To thanh phc cht:

Phng php co hieu qua nhat e xch gan ien the la tao thanh phc

chat kim loai co ien the dng hn do o dch chuyen ien the cua no ve

pha am hn. S tao thanh phc chat khong nhng xch gan ien the cua cac

kim loai khac nhau lai ma con thay oi v tr cua chung.

V du : Trong dung dch muoi n gian ien the cua Ag+ dng hn

cua Zn2+ la 1,5v. Trong dung dch Xyanua th ien the thoat cua Ag am hn

cua Zn.

-0,4

4

3

2

1

i

-0,6 -0,7


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Mat khac thay oi qua the cua ion cung lam cho ien the phong ien cua

ion xch lai gan nhau. Khi chuyen t dung dch cha ion n gian sang dung

dch cha ion phc qua the cung tang.

b . Phong ien ong thi trong he thong ket hp:

Trong thc te th cac ion phong ien ong thi luon tac ong lan nhau

nen toc o phong ien cua chung phu thuoc vao cau tao lp ien tch kep,

ong thi vao trang thai cua ion trong dung dch va cau tao lp nen. Cho nen

thay v cong thc (1) ta phai dung cong thc :

o1 + ln1Fn

RTa1 + ln

1n

RT

iai

a

11 -kh

= o2 + ln2Fn

RT a2 +iai

a

Fn

RT

22

2

ln -2kh

Trong o :

i : he so ac trng cho kha nang xam nhap vao lp kep cua ion i.1kh, 2kh : qua the khi phong ien ong thi cua cac ion 1, 2 trong hethong ket hp.

So hang th 3 trong ca ve phai va ve trai bieu th s dch chuyen ien

the khi co cac ion i tham gia lp kep.

. Anh hng cua lp ien tch kep va trang thai cua ion trong dung dch:Nong o cua ion trong lp kep c tnh theo cong thc ;

[Men+]lk = [Men+]dd e- nF1/RT

Khi co cac ion khac cung phong ien th nong o phong ien cua motloai ion se nho hn bnh thng v b ion kia ay ra khoi lp kep. Do o khi

phong ien ong thi hai ion th thng mot ion hoac ca hai ion b giam toc

o.

. Anh hng cua ban chat lp nen :-Lp nen lam ha thap ien the phong ien cua ion. Tac dung kh phan

cc cua nen do chung tao thanh hp kim vi ion phong ien.

V du : Na+ phong ien tren ien cc Hg ien the 1,7v thay v

2,7v.- Lp nen lam tang ien the phong ien cua ion

Khi nghien cu s ket tua cua Ag ta thay, toc o ket tua cua no khac

nhau cac ni tren be mat ien cc. Nguyen nhan cua hien tng o la do

be mat ien cc khong ong nhat.


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V . Ly thuyet hoa tan ien hoa hoc kim loai :Neu kim loai hoa tan anot va tao thanh cac ion hydrat hoa n gian

th co bieu dien no bang phng trnh :

[M] + xH2O = MeZ+aq + ZeNeu tao thanh ion phc th :

[M] + xA- + yH2O = [MAx]aqZ-X + ZeKim loai hoa tan anot ien the dng hn ien the can bang co

ngha la hoa tan kem theo phan cc anot :

a = ia - CbCo phan cc anot la do co s cham chuyen van, cham phan huy pha

rn hay la cham chuyen ien tch ngha la ngc lai qua trnh catot tng

ng. gan trang thai can bang co mot s oi xng nao o gia qua trnhthoat kim loai catot va hoa tan kim loai anot. .

Tnh chat cua kim loai trong qua trnh hoa tan anot t c nghien cu, tuynhien co the dung ly thuyet cham phong ien cho kim loai nhom sat :

a = aa + bblogiaa, bb la cac hang so

Trong qua trnh hoa tan, tren be mat kim loai co the tao thanh cac

mang oxt do tac dung hoa hoc cua anot vi cac chat oxy hoa hoac do oxy

hoa anot. S tao thanh lp oxt be mat lam cho kim loai b thu ong qua trnh

hoa tan b can tr hay ngng han va ien the anot tang len. Khi ay cac ion

khac se tham gia vao phan ng ien cc. V du : ion OH- se phong ien giaiphong oxy.

VI.S HOA TAN ANOT CUA CAC HP KIM :Trong cac hp kim a pha th cac pha oc lap vi nhau ve phng

dien ien hoa hoc. Cac pha ch hoa tan anot khi ien the anot at ti ien the

ion hoa. Cac pha co ien the am nhat se hoa tan trc. ch sau khi chung hoa

tan hoan toan hoac ien the anot at ti ien the ion hoa cua cac pha dng

hn th nhng pha nay mi b hoa tan.

Neu pha co ien the am hn hoa tan de dang va ham lng cua notrong hp kim tng oi ln th ien the ien cc anot thng khong at ti

ien the hoa tan pha dng hn. Khi ay cac pha dng se ri xuong di

dang mun.

Nhng hp kim mot pha la nhng hp chat hoa hoc hay dung dch ran

cua cac kim loai khac nhau se hoat ong nh mot kim loai duy nhat khi ta

phan cc anot. Hoat o cua kim loai am trong hp kim nho hn trang thai


44/44

t do va ien the hoa tan cua hp kim nam gia ien the hoa tan cua 2 cau t

c s ( tuy nhien ien the hoa tan cua hp kim nay thng gan ien the hoa

tan cua cau t am hn).

VII.IEN PHAN:ien phan la s phan huy hoa hoc cua cac chat trang thai nong

chay hay trong dung dch khi co dong ien chay qua.

Khi tien hanh qua trnh ien phan Catot Cation nao o co ien the

dng hn se phong ien trc, con anot anion nao o co ien the am hn

se phong ien trc.

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