Tinh Chat Va Ung Dung Cua Crom

8/13/2019 Tinh Chat Va Ung Dung Cua Crom

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MT S TNH CHT V NG DNG CA CROM TS Vi Anh Tun Khoa Ha hc

Trng i hc KHTN i hc Quc gia H Ni

Crom l nguyn t kim loi chuyn tip thuc nhm VIB ca bng h thng tun hon. Crom c cuhnh electron lp v ha tr l 3d54s1. S oxi ha c trng ca crom l +3 v +6. Tn gi ca crom xut

pht t ting Hy Lp "chroma" c ngha l mu sc v cc hp cht ca crom u c mu.

Crom l kim loi mu trng bc. Do c s electron c thn ti a nn bn ca lin kt trong tinh th crom l ln nht, v vy crom c nhit si, nhit nng chy v cng rt cao (tonc= 1907

oC; tos=

2671oC, cng theo thang Mohs: 8,5).

1. iu ch crom

Khong vt chnh ca crom l st cromit (FeCr2O4). Nhng nc giu m qung crom l Nam Phi,

Kazakhstan, n , Nga, Th Nh K v Zimbabwe. Nc ta c mt m sa khong cromit kh ln Cnh, Thanh Ha.

Trong cng nghip lng ln crom c sn xut t qung di dng hp kim ferocrom cha 50 70%crom, c sn xut bng cch dng than cc kh qung cromit trong l in:

Fe(CrO2)2 + 4 Cto

Fe + 2 Cr + 4 CO

iu ch crom tinh khit ngi ta phi tch st ra khi qung cromit theo quy trnh sau: Qungcromit c un nng chy vi hn hp canxi cacbonat v natri cacbonat v c mt khng kh. Crom b oxiha thnh cromat v st chuyn thnh Fe2O3. Ha tan hn hp vo nc, tch phn oxit st khng tan thu

c dung dch cromat. Thm axit sunfuric chuyn cromat thnh dicromat:

4 FeCr2O4 + 8 Na2CO3 + 7 O2to 8 Na2CrO4 + 2 Fe2O3 + 8 CO2

2 Na2CrO4 + H2SO4 Na2Cr2O7 + Na2SO4 + H2O

Kh dicromat thnh crom(III) oxit bng cacbon. Tip theo kh crom (III) oxit bng nhm s thu ccrom kim loi:

Na2Cr2O7 + 2 Cto Cr2O3 + Na2CO3 + CO

Cr2O3 + 2 Alto

2 Cr + Al2O3

Cu 1.1. (a) Vit cc phng trnh ho hc t Na2Cr2O7, C (than ), Al (bt nhm) v cc iu kincn thit thuc Cr.

(b) CrO2Cl2 (cromyl clorua) l mt ho cht quan trng. Hy vit cc phng trnh ho hc to raCrO2Cl2t:

i. CrO3tc dng vi axit HCl.

ii. Cho K2Cr2O7tc dng vi KCl trong H2SO4c, nng.

(c) Thm cht thch hp v hon thnh cc phng trnh ho hc sau:

i. KNO2 + KNO3 + ? K2CrO4 + NO

ii. NaNO2 + ? + NaI I2 + NaHSO4 + NO + H2O

iii. HNO3 + P2O5 ? + N2O5


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(a. Na2Cr2O7 + 2 Cto

Na2CO3 + Cr2O3 + CO

Cr2O3 + 2 Alto

2 Cr + Al2O3

b. i. CrO3 + 2 HCl CrO2Cl2 + H2O

ii. K2Cr2O7 + 4 KCl + 3 H2SO4 3 K2SO4 + 2 CrO2Cl2 + 3 H2O

c. i. 3 KNO2 + KNO3 + Cr2O3 2 K2CrO4 + 4 NO

ii. 2 NaNO2 + 4 H2SO4 + 2 NaI I2 + 4 NaHSO4 + 2 NO + 2 H2O

iii. 2 HNO3 + P2O5 2 HPO3 + N2O5)

Cu 1.2. (a) Ti sao crom c kh nng th hin nhiu trng thi oxi ho? Cho bit nhng s oxi ho phbin ca crom?

(b) Nu v gii thch s bin i tnh cht axit baz trong dy oxit: CrO, Cr2O3, CrO3. Vit phngtrnh ho hc ca cc phn ng minh ho.

(c) Vit phng trnh ion ca cc phn ng iu ch Al2O3v Cr2O3t dung dch gm kali cromit vkali aluminat.

(a. Cu hnh electron ca crom l [Cr]3d54s1, do nguyn t crom c 6 electron ha tr nn crom thhin nhiu trng thi oxi ha; S oxi ha ph bin ca crom l +2, +3, +6.

b. i t CrO Cr2O3CrO3th tnh axit tng dn, tnh baz gim dn.

Gii thch: khi i tCrO Cr2O3CrO3th s oxi ha ca nguyn t crom tng do tnh cng hatr ca lin kt Cr O tng, lm cho tnh baz gim, tnh axit tng.

Trong thc t CrO l oxit axit, Cr2O3l oxit lng tnh v CrO3l oxit axit.

CrO + 2 H+ Cr2+ + H2O

Cr2O3 + 6 H+ 2 Cr3+ + 3 H2O

Cr2O3 + 2 OH(nng chy) 2 CrO2

+ H2O

CrO3 + 2 OH CrO4

2 + H2O

c. Cch 1: Thm H2O2v NaOH d vo dung dch:

2 Cr(OH)4 + 3 H2O2 + 2 OH

2 CrO42 + 8 H2O

Thm tip CO2n d vo dung dch thu c:

Al(OH)4 + CO2 Al(OH)3 + HCO3

Lc ly kt ta em nung thu c Al2O3tinh khit:

2 Al(OH)3 Al2O3 + 3 H2O

Thm H2SO4n d vo phn dung dch lc, c cn dung dch kt tinh K2Cr2O7.

2 CrO4 + 2 H+ Cr2O7

2 + H2O

Nung K2Cr2O7vi C thu c Cr2O3

K2Cr2O7 + 2 C K2CO3 + Cr2O3 + CO

Cch 2: Thm H2O2v KOH d vo dung dch:

2 Cr(OH)4 + 3 H2O2 + 2 OH

2 CrO42 + 8 H2O

Thm tip CO2n d vo dung dch thu c:Al(OH)4

+ CO2 Al(OH)3 + HCO3


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Lc ly kt ta em nung thu c Al2O3tinh khit:

2 Al(OH)3 Al2O3 + 3 H2O

Thm H2O2v H2SO4n d vo phn dung dch lc:

Cr2O72 + 8 H+ + 3 H2O2 2 Cr

3+ + 3 O2 + 7 H2O

Thm NH3long kt ta Cr3+:

Cr3+ + 3 NH3 + 3 H2O Cr(OH)3 + 3 NH4+

Lc ly kt ta em nung thu c Cr2O3tinh khit:

2 Cr(OH)3 Cr2O3 + 3 H2O)

Cu 1.3. Ha tan hon ton 0,1331 gam mt mnh hp kim cha st v crom bng dung dch axit sunfuriclong, d to thnh cc ion Fe2+v Cr3+. Chun dung dch thu c bng dung dch pemanganat nhit phng thy va ht 20,08 ml dung dch. Bit rng iu kin th nghim Cr3+khng tham gia phn ng.

Trong mt th nghim khc, ngi ta chun 10 ml dung dch axit oxalic 0,0500 M trong mi trngaxit sunfuric bng dung dch pemanganat trn thy va ht 9,75 ml dung dch.

(a) Hy vit cc phn ng xy ra dng ion rt gn.

(b) Hy tnh thnh phn % theo khi lng ca st v crom trong hp kim ban u. (Cr = 52,00; Fe =55,85).

Mt bnh in phn cha 100 lit dung dch CrO3nng 230 g/lit. Tin hnh in phn dung dch trongkhong thi gian 10 gi vi cng dng in 1500 A. Sau khi kt thc th nghim thy khi lng cacatot tng ln 670 gam, bit rng c kh thot ra c catot v anot.

(c) Hy cho bit kh thot ra anot v catot?

(d) Hy tnh hiu sut ca qu trnh in phn iu ch crom.

(e) Hy tnh th tch kh thot ra catot v anot 25C v 1,00 atm.

(a. Fe + 2 H+ Fe2+ + H2

Cr + 3 H+ Cr3+ + 3/2H2

5 Fe2+ + MnO4 + 8 H+ 5 Fe3+ + Mn2+ + 4 H2O

5 H2C2O4 + 2 MnO4 + 6 H+ 10 CO2 + 2 Mn

2+ + 8 H2O

b. MnO4: 0,02051 M;

Fe: 86,4%; Cr: 13,6%;

c. CrO3 + H2O 2 H+ + CrO42

CrO42 + 8 H+ + 6 e Cr + 4 H2O

Anot (+): 2 H2O O2 + 4 H+ + 4e

Catot (): 2 H+ + 2e H2

d. C: molF

Itne 7,559

96485

3600101500

S mol electron do cromat nhn l:

molne 3,77600,52

670'


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Hiu sut ca qu trnh in phn iu ch crom:

%8,131007,559

3,77'

e

e

n

nH

e. C: molnn

n eeH 2,2412

3,777,559

2

'

2

moln

n eO 9,1394

7,559

42

=> litVH310.89,5

1

298082,02,2412

litVO310.42,3

1

298082,09,1392

)

Cu 1.4.Ngun nguyn liu chnh iu ch crom lqung cromit (FeCr2O4), trong cromit st c hatr II.

(a) Hy xc nh s oxi ha ca crom trong cc cu t sau: Cr2O3, CrO42, Cr2O72, FeCr2O4.

(b) Hy vit phng trnh phn ng kh qung cromit bng cacbon, bit sn phm thu c l st, cromv cacbon dioxit.

Quy trnh iu ch mui crom(III) t qung cromit nh sau: u tin oxi ha cromit bng khng khtrong natri hidroxit nng chy to thnh natri cromat (Na2CrO4); Ha tan sn phm vo nc, axit ha dungdch bng dung dch H2SO4 thu c dungdch c mu da cam; C cn dung dch, ly cht rn em kh

bng cacbon thu c Cr2O3; Ha tan oxit ny bng dung dch axit sunfuric thu c dung dch mui crom(III).

(c) Hy vit cc phng trnh phn ng xy ra trong quy trnh trn.

xc nh hm lng crom trong mu qung cromit, ngi ta s dng phng php chun iot thiosunfat. Oxi ha 5,00 g mu qung cromit bng oxi trong dung dch natri hidroxit, chuyn ton b dungdch thu c (cha natri cromat) vo bnh nh mc 1,00 lit v nh mc n vch bng nc ct (dungdch A). Ly 25,00 ml dung dch A cho vo bnh nn c cha dung dch m. Thm khong 3 gam KI. ykn ming bnh v trong ti cho phn ng xy ra hon ton, sn phm to thnh l I 3

v Cr3+. Chun lng I3

sinh ra bng dung dch Na2S2O3 0,1000 M, sn phm to thnh l I v ion tetrathionat

(S4O62), thy va ht 23,10 ml dung dch.

(d) Hy vit cc phng trnh phn ng xy ra trong quy trnh phn tch trn.

(e) Hy tnh nng ca natri cromat trong dung dch A.(g) Hy tnh thnh phn % theo khi lng ca cromit trong mu qung.

(h) Hy tnh thnh phn % theo khi lng ca crom trong mu qung.

(a. +3, +6, +6, +3;

b. FeCr2O4 + 2 C Fe + 2 Cr + 2 CO2

c. 4 FeCr2O4 + 7 O2 + 16 NaOH 2 Fe2O3 + 8 Na2CrO4 + 8 H2O

2 Na2CrO4 + 2 H2SO4 2 NaHSO4 + Na2Cr2O7 + H2O

Na2Cr2O7 + 2 C

to

Na2CO3 + Cr2O3 + COCr2O3 + 3 H2SO4 Cr2(SO4)3 + 3 H2O


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d. 4 FeCr2O4 + 7 O2 + 16 NaOH 2 Fe2O3 + 8 Na2CrO4 + 8 H2O

2 CrO42 + 9 I + 16 H+ 2 Cr3+ + 3 I3

+ 8 H2O

I3 + 2 S2O3

2 3 I + S4O62

e. MCCrO

0308,000,25

3

2

2

11000,010,23

2

4

g. molnOFeCr

0154,08

40,10308,0

42

=> %94,6810000,5

85,2230154,0% 42

OFeCr

h. molnnOFeCrCr

0308,0242

=> %04,3210000,5

00,520308,0%

Cr )

2. Phc cht ca crom

2.1. Phc cht crom (0)

a)Phc hexacacbonyl crom(0), Cr(CO)6

2s2s

s

s*

C OCO

2p2p

z

z*

x y

x* y*

Gin MO ca phn t CO.

Trong phn t CO, cp electron trn MO zc nng lng cao hn nhng cp electron trn MO xv

ynn c kh nng to lin kt cho nhn vi cc obitan lai ha d2sp3trng ca nguyn t crom.


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3d 4s 4p

d2sp3

lin kt

lin kt

Cr(0):

Cr CO

COCr

CO CO CO CO CO CO

Ngoi lin kt cho nhn (CrCO), trong hexacacbonyl cn c lin kt

cho CrCO to nn bi nhng cp electron d ca nguyn t crom vi nhng MO * trng ca phn t

CO v nh lin kt ny, cc phn t cacbonyl kim loi c lm bn thm.

-cho

Cr C O

Vy phc Cr(CO)6c cu hnh bt din u v nghch t.

Cr

OC

OC CO

CO

CO

CO

Nh vy phc Cr(CO)6tun theo quy tc 18 electron: "Nguyn t kim loi trong cacbonyl kim loi c

khuynh hng nhn thm mt s electron ca phn t CO t c 18 electron v ha tr". Di y lmt s phc cacbonyl kim loi tun theo quy tc 18 electron.

Cacbonyl S electronha tr ca M

S electron COng gp

Tng Hnh hc phn t

Ti(CO)7 4 14 18

V2(CO)12 5 12+1 18

Cr(CO)6 6 12 18 Bt dinu

Mn2(CO)10 7 10+1 18 2 bt din chung 1nh

Fe(CO)5 8 10 18 Lng thp tam gicCo2(CO)8 9 8+1 18

Ni(CO)4 10 8 18 T din


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OC Mn

OC

CO

CO

CO

Mn

OC

CO

CO

CO

CO

3d 4s 4p

d2sp3

Mn (0):

Mn CO

COMn

CO CO CO CO CO

Mn-Mn

b)Phc crom dibenzen Cr(C6H6)2

Phn t crom dibenzen c hnh bnh kp (thuc loi hp cht c kim).

3d 4s 4pd

2

sp3

Cr (0):

Cr MO *

MO Cr

*

E

Gin nng lng cc MO ca benzen

Cr

Trong hp cht ny, quy tc 18 electron cng c tun theo.

Cu 2.1. Cui th k 18 Ludwig Mond pht hin ra rng niken dng bt mn phn ng vi cacbonmonoxit to thnh tetracacbonyl niken, Ni(CO)4, mt cht lng khng mu, d bay hi. Phc Ni(CO)4tuntheo quy tc 18 electron.

(a) S dng quy tc 18 electron, hy d on cng thc phc cacbonyl ca Fe(0) v Cr(0).

(b) S dng quy tc 18 electron hy d on cng thc phc nitrozyl ca Cr(0)?

(c) Hy gii thch ti sao Mn(0) v Co(0) khng th to ra cc phc cacbonyl n nhn kiu M(CO)x(Ml kim loi), m ch c th to cc phc cacbonyl c lin kt kim loi kim loi?

(d) Hy cho bit cu trc khng gian ca Ni(CO)4, Mn2(CO)10v Co2(CO)8.

(e) Hy cho bit phc V(CO)6v cc phc cho phn (a) v (d) l thun t hay nghch t?


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(g) Hy gii thch ti sao cacbon monoxit lin kt vi kim loi bn hn rt nhiu so vi khi lin kt vi

bo trong hp cht kiu R3BCO; R l gc ankyl?

(h) Quy tc 18 electron cng nghim ng i vi phc ca crom v benzen.

i/ Hy vit cng thc ca phc.

ii/ Hy cho bit cng thc ca phc c cu trc tng t c iu ch bng phn ng gia bt st

vi xiclopentadien? Hy vit phng trnh phn ng iu ch phc.(a. Fe(CO)5v Cr(CO)6;

b. Cr(NO)4

c. Do nguyn t Mn v Co c s l electron.

d.

CO

NiOC

CO

CO OC Mn

OC

CO

CO

CO

Mn

OC

CO

CO

CO

CO OC Co

COOC

OC

Co

OC

CO

CO

CO

e. Fe(CO)5, Cr(CO)6, Ni(CO)4, Mn2(CO)10, Co2(CO)10: nghch t;

V(CO)6thun t;

g. Do trong R3BCO khng c lin kt cho.

-cho

Cr C O

h. i/

Cr

ii/ Fe + 2 C5H6 Fe(C5H5)2 + H2

Fe

)

2.2. Phc cht crom (II)

Cr(II) axetat l kt ta mu v l hp cht bn nht ca Cr(II), c cu to ime

[Cr(CH3COO)2.H2O]2. Trong phn t dime lin kt Cr Cr l lin kt bn (1, 2, 1).


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3d 4s 4p

d2sp3

Cr (II):

Cr

Cr-CrCr-Cr Cr-Cr

Cr Cr

O

O

O

O

H2O OH2

O

O

O

O

Cr(II) axetat c iu ch theo phn ng:

2 CrCl2 + 4 NaCH3COO + 2 H2O [Cr(CH3COO)2.H2O]2 + 4 NaClCu 2.2. X l mt hp cht ca crom. Phn tch thnh phn nguyn t cho thy: Cr 27,7%; C: 25,5%; H

4,25% theo khi lng, phn cn li l oxi.

(a) Hy cho bit cng thc n gin nht ca X.

(b) Bit rng trong cng thc n gin nht c cha mt phn t nc, hy cho bit phi t cn li lg? Hy cho bit s oxi ha ca Cr trong X.

(c) Nghin cu t tnh cho thy X c tnh nghch t, hy gii thch tnh cht ny ca hp cht X v vcu trc khng gian ca X.

(a. 5:8:4:166,2:25,4:125,2:533,016

55,42:1

25,4:12

5,25:52

7,27::: OHCCr

=> Cng thc kinh nghim ca X l: CrC4H8O5.

b. Cr(H2O)(CH3COO)2, phi t cn li l axetat, s oxi ha ca crom: +2.

c. X l ime: [Cr(H2O)(CH3COO)2]2.

Cr Cr

O

O

O

O

H2O OH2

O

O

O

O

)

Cu 2.3. Kim loi A c mu trng bc v c nh kim. Trong cng nghip, ngi ta dng kim loi A m ln cc vt bng kim loi bo v cho kim loi khng b g, lp m thng ch dy khong 0,005

3d 4s 4p

d2sp3

Cr (II):

Cr

Cr-CrCr-Cr Cr-Cr


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mm. Hidroxit ca A l cht B c dng A(OH)2, B l cht kt ta mu vng nhng thng ln tp cht nn cmu hung. Khi tip xc vi khng kh, B chuyn thnh C l cht nhy c mu lc nht, khng tan trongnc nhng tan c trong c dung dch kim cng nh dung dch axit. Cht B tan trong dung dch HCl dcho dung dch D c mu xanh lam nhng khi c cn dung dch th li c mui rn khan mu trng, ht mmnh. Cho dung dch D tc dng vi dung dch NaCH 3COO c thu c kt ta t tan mu E. E dngdime l hp cht c cu to c bit v l mt trong nhng chtd iu ch v bn nht ca A(II).

(a) Tm cc cht t A n E v vit cc phng trnh phn ng xy ra.

(b) Nu bn cht lin kt ca A trong dime E.

(a. A: Cr;

4 Cr(OH)2 + O2 + 2 H2O 4 Cr(OH)3

B C

Cr(OH)3 + 3 H+ Cr3+ + 3 H2O

Cr(OH)3 + OH [Cr(OH)4]

Cr(OH)2 + 2 HCl CrCl2 + 2 H2OB D

2 CrCl2 + 4 NaCH3COO + 2 H2O [Cr(CH3COO)2(H2O)]2 + 4 NaCl

D E

b. Xem phn l thuyt)

2.3. Phc cht crom (III)

Do c kch thc b v in tch ln nn ion Cr3+c kh nng to phc rt mnh, cc phc bn thnggp l [CrX6]

3(X: F, Cl, SCN, CN), [Cr(C2O4)2], NH4[Cr(NH3)2(SCN)4] (mui Reinecke c mu m).

Trong dung dch nc, mui CrCl3tn ti 3 dng khc nhau:

[Cr(H2O)6]Cl3 [Cr(H2O)5Cl]Cl2 [Cr(H2O)4Cl2]Cl

tm xanh nht xanh thm

3 AgCl 2 AgCl 1 AgCl

Cu 2.4. Cc hp cht A, B, C c cng cng thc phn t CrCl3.6H2O. Trong dung dch nc tn ticn bng gia chng nh sau:

[Cr(H2O)6]Cl3 [Cr(H2O)5Cl]Cl2.H2O [Cr(H2O)4Cl2]Cl.2H2O.

(A) (B) (C)

Trong mt th nghim ngi ta cho dung dch cha 0,32 gam CrCl3.6H2O i qua mt lp nha trao ication di dng H+. Cn 28,8 ml dung dch NaOH 0,125M chun ht lng H+ chuyn vo dungdch.

(a) Gi tn cc phc cht A, B v C. Phc cht no c ng phn hnh hc?

(b) Xc nh cng thc ca phc trong dung dch.

(c) Vit cu hnh electron ca Cr trong ion phc xc nh c (b) v xc nh t tnh ca phc cht.

(a. A: Hexaaquacrom (III) clorua

B: Cloro pentaaqua crom (III) clorua hirat


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C: icloro tetraaqua crom (III) clorua hirat (C ng phn hnh hc)

b. [Cr(H2O)6nCln]Cl3n.nH2O [Cr(H2O)6nCln]3n+ (3n) Cl+ n H2O

1,20.103 1,20.103 mol

[Cr(H2O)6nCln]3n + (3n) RCOOH ... + (3 n) H+

1,20.103 1,20.103(3 n)mol

H+ + OH H2O

3,60.103mol

C: nH+= 1,20.103(3 n) = 3,60.103

=> n = 0;

Vy phc trong dung dch l [Cr(H2O)6]Cl3 (phc A)

c. Phc thun t; BM.87,3)23(3

4p3d 4s

[Cr(H2O)6]3+

sp3

d2

4d

6 H2O )

Cu 2.5. Thm t amoni dicromat vo mui amoni thioxianat nng chy s thu c mui Reinecke ccng thc NH4[Cr(SCN)x(NH3)y], mui ny c thnh phn % theo khi lng ca cc nguyn t nh sau: Cr15,5 %; S 38,15 % v N 29,2 %.

(a) Hy xc nh cc gi tr x v y trong cng thc trn.

(b) Hy xc nh s oxi ha ca crom trong phc.

(c) Hy cho bit hnh hc ca anionphc.

(d) Hy v cc ng phn lp th ca anion phc trn.

(a. NH4[Cr(SCN)4(NH3)2];

b. +3;

c. bt din;

d. ng phn hnh hc

Cr

NCS

NCS SCN

SCN

NH3

NH3

-

Cr

NCS

NCS SCN

NH3

NH3

SCN

-

)

Cu 2.6. Ion glyxinat, NH2CH2COO, l mt phi t hai cng, to phc trisglyxinatocrom(III). Lu

rng cc nguyn t oxi v nit ca cng mt phi t ch c th lin kt cc v tr cnh nhau trong cu hnhbt din.

(a) Hy cho bit phc trn c bao nhiu ng phn hnh hc?

(b) Hy cho bit ng phn hnh hc no l bt i?


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Mt phc cht khc ca crom c thnh phn % theo khi lng ca cc nguyn t nh sau: 19,5 % Cr;40,0 % Cl, 4,5 % H v 36,0 % O. Ha tan 0,533 g phc vo 100 ml nc v thm tip 10 ml dung dchHNO32 M. Thm lng d dung dch bc nitrat. Lc, ra kt ta v emsy kh thu c 0,287 g cht rn.Mt khc un nng nh 1,06 g phc 100 C thy thot ra 0,144 g nc.

Nhit ng c ca dung dch thu c khi ha tan 1,33 g phc vo 100ml nc l 0,18 C. (Hng

s nghim lnh ca nc l 1,82 K.kg.mol

1

).(c) Hy cho bit cng thc ca phc.

(d) Hy v tt c cc ng phn lp th ca phc trn.

(a. Hai ng phn hnh hc (fac v mer).

CrO

N O

O

N

N

bt i

CrO

N N

O

O

N

bt i

b. C hai ng phn hnh hc u bt i.

c. Gi CTPT ca phc l CrxClyHzOt. C:

6:12:3:125,2:46,4:128,1:375,00,16

0,36:

01,1

5,4:

45,35

0,40:

0,52

5,19::: tzyx

=> CTPT ca phc l CrCl3(H2O)6. (M = 266,47)

Th nghim 1:

[Cr(H2O)

6nCl

n]Cl

3n.nH

2O

[Cr(H2O)

6nCl

n]3n+ (3n) Cl + n H

2O

2,00.103 2,00.103(3n) mol

Ag+ + Cl AgCl

2,00.103 mol

C: nAgCl= 2,00.103(3n) = 2,00.103

=> n = 2

Th nghim 2:

[Cr(H2O)6nCln]Cl3n.nH2O [Cr(H2O)6nCln]Cl3n + n H2O

3,98.103 7,99.103 mol

C: nH2O= 3,98.103n = 7,99.103

=> n = 2

Th nghim 3:

[Cr(H2O)6nCln]Cl3n.nH2O [Cr(H2O)6nCln]3n + (3n) Cl + n H2O

4,99.103 4,99.103 4,99.103(3n) mol

C:dm

ion

m

nkkmt

(Coi lng nc do phc phn li ra khng ng k)


13/45

Trang 13

=> molmolkgK

kgK

k

tmn dmion

3

1 10.89,9

..82,1

1,018,0

C: nion= 4,99.103 + 4,99.103(3n) = 9,89.103

=> n = 2

Kt qu ca 3 th nghim ph hp vi nhau, vy cng thc ca phc l [Cr(H2O)4Cl2]Cl .2H2O;

d. 2 ng phn hnh hc)

Cu 2.7.(a) Hy v v gi tn cc ng phn lin kt v ng phn lp th ca mi hp cht phi trsau:

i. [Pt(NH3)2(SCN)2].

ii. [Co(en)(H2NCH2COO)2]Cl.

(b) Hy v gin phn tch mc nng lng ca cc obitan d trong trng tinh th ca mi phc sau.

i. [Cr(CN)6]3.

ii. [Cr(NH3)6]

2+

.iii. [Cr(CN)6]

4.

(a. i.

Pt

NH3

NH3

NCS SCN Pt

SCN

NH3

NCS NH3

Pt

NCS

NH3

SCN NH3Pt

NH3

NH3

SCN NCS

Pt

NH3

NH3

NCS NCS Pt

NCS

NH3

NCS NH3

ii.

N

Co

N

O N

O N

N

Co

N

ON

ON

O

Co

N

N N

O N

O

Co

N

NN

ON

O

Co

O

N N

N N

O

Co

O

NN

NN

b.

[Cr(CN)6]

3-

[Cr(NH3)6]

2+

[Cr(CN)6]

4-

)


14/45

Trang 14

Cu 2.8. Phc kali xiano K1ca nguyn t A c momen t = 3,8 B.M. Hp cht c cc i hp thtng ng vi bc chuyn d d ti 26700 cm 1. Phn ng kh ion A2O7

2 (c mu da cam) bng etanoltrong mi trng axit to ra ion trung tm ca phc xiano. Phc K1b kh bi kali trong amoniac lng cho

phc xiano K2 (c s phi tr khng thay i). Bng cch thay i tt c phi t trong K2bng amoniac,momen t ca phc K3thu c tng ln thnh = 4,9 B.M..

(a) Hy cho bit A l nguyn t no?(b) Hy cho bit cng thc v gi tn phc K1.

(c) Hy biu din s phn b ca electron trn cc obitan d ca nguyn t trung tm trong phc K1.

Cho NA= 6,0221023mol1; c = 3,0108m/s v h = 6,621034J.s.

(d) Hy tnh nng lng tch ca trng phi t (theo kJ/mol) ca phc K1.

(e) Cho cc phi t H2O, Clv Fcc gi tr nng lng tch: 158 kJ/mol,182 kJ/mol v 208 kJ/mol.

Hy cho bit nng lng tch tng ng vi miphi t.

(f) Hy cho bit cng thc v gi tn ca phc K2.(g) Hy biu din s phn b ca electron trn cc obitan d ca nguyn t trung tm trong phc K2v

K3, v cho bit phc l spin cao hay spin thp?

(h) Phc K2c cu trc bt din lch. Hy cho bit tn gi ca hiu ng gy ra iu ny.

(i) Hy cho bit hnh hc ca phc A(acac)2? Ch n hiung c cp phn (h).

(a. Cr;

b. K3[Cr(CN)6], kali hexaxiano cromat (III);

c.

; B 87,3)23(3

d. 34 8 1 1 231006,62.10 . 3.10 . 26700 6,022.10Acm

E hc N J s m s cmm

3319.10 / 319 /J mol kJ mol

e. H2O: 208 kj/mol; Cl: 158 kj/mol; F: 182 kj/mol.

f. K4[Cr(CN)6], kali hexaxiano cromat (II);

g.

spin thp spin cao

h. Hiu ng Jahn Teller;

i. Vung phng)

Cu 2.9. Trong cu hi ny cc phc ca Cr3+v Cr2+u c cu hnh bt din.


15/45

Trang 15

Thm amoniac vo dung dch Cr(III) to thnh Cr(OH)3, mt kt ta mu lc xm. Cr(OH)3l hidroxitlng tnh, phn ng vi axit to ion phc mu tm A, phn ng vi dung dch NaOH to ion phc muxanh thm B.

(a) Hy vit cng thc ca cc ion phc A v B.

(b) Hy vit phng trnh phn ng ca Cr(OH)3vi axit v baz.

(c) Hy gii thch ti sao cc ion phc A v B li c mu khc nhau?Ion phc A c tnh axit (pKa= 3,95). Baz lin hp ca A d dng b dime ha vi cu ni l ion hydroxo

to thnh ion phc X c cng thc [Cr2(OH)2(H2O)8]a

.

(d) Hy vit phn ng phn li axit ca A.

(e) Hy v cu trc khng gian v xc nh in tch aca ion phc X.

C l phc triammintriclorocrom(III). Khi thm ion oxalat (ox) vo C th mt phi t Cl v mt phi tNH3s c thay th bng mt phi t ox to thnh ion phc D.

(f) Hy vit cng thc ca cc ion phc C v D.

(g) Hy v cc ng phn lp th ca C v D (s dng k hiu O O cho phi t oxalat).

Ion phc A b kh bng hn hng km thy ngn to thnh phc E mu xanhda tri rt km bn caCr2+.

(h) Hy v hai gin phn tch nng lng ca cc obitan d trong trng tinh th ca phc E.

(i) Hy tnh momen t ca phc E ng vi hai gin trn.

(k) Hy thit lp biu thc tnh nng lng lm bn trong trng tinh th ca E ng vi hai gin trntheo nng lng tch (o) v nng lng ghp i electron (P).

(a. [Cr(H2O)6]3+; [Cr(OH)6]

3;

b. Cr(OH)3 + 3 H3O+ [Cr(H2O)6]3+

Cr(OH)3 + 3 OH [Cr(OH)6]

3

c. Do nng lng tch trong trng phi t ca hai phc khc nhau.

d. [Cr(H2O)6]3+ + H2O [Cr(OH)(H2O)5]

2++ H3O+;

e.

f. C: [CrCl3(NH3)3]; D: [CrCl2(NH3)2ox];

g.

h.


16/45

Trang 16

i. BM9,4)24(41

BM8,2)22(22

k. DqE ooolb 65

3

5

3

5

231 ;

ooolb PDqPE 165

242 )

Cu 2.10.Tinh th Ruby c mu m v c s dng lm trang sc. t ngi bit c rngtri tim ca my pht tia laze u tin, c ch to bi Maiman vo nm 1960, l mt tinh th ruby ln.Mu ca ruby l do s hp th nh sng ca ion Cr3+c mt trong tinh th oxit nhm (Al2O3) khngmu. Ion Cr3+c 3 electron phn lp 3d v c kh nng hp th nh sng do s chuyn dch ca electron

gia cc obitan ca phn lp 3d c mc nng lng khc nhau.

(a) Hy cho bit ph hp th no di y l ph hp th ca ruby?

ng ruby dng trong my pht laze l mt ng hnh tr c chiu di 15,2 cm v c ng knh 1,15 cm.

Hm lng ion Cr3+

trong ng l 0,050 % theo khi lng. Khi lng ring ca Al 2O3 l 4,05 g cm3

.Nguyn t khi ca Cr = 52u.

(b) Hy tnh s ion Cr3+c trong ng ruby.

Trong ruby ion Cr3+c phi tr bi 6 ion oxit c cu hnh bt din. Hnh dng ca nm obitan 3dc cho di y. Hnh tip theo l gin phn tch nng lng ca cc obian d thnh hai nhm, mtnhm gm ba obitan c mc nng lng thp hn (t2g) v mt nhm gm hai obitan c mc nng lng caohn (eg).

(c) Hy cho bit nhng obitan 3d no (dz2, dxy, dyz, dx2y2, dxz) thuc nhm t2gv nhng obian 3d nothuc nhm eg?


17/45

Trang 17

eg

t2g

(d) Hy v gin phn tch mc nng lng ca cc obitan d trong trng tinh th ca phc Cr3+trong ruby.

Mt vin ruby c t trn mt chic cn n. Khi cn thng bng, ngi ta a mt cc nam chmvo ngay pha di a cn ng vin ruby.

(e) Hy cho bit iu g s xy ra?

(A) Cc nam chm ko vin ruby xung.

(B) Cc nam chm khng lm vin ruby dch chuyn.

(C) Cc nam chm y vin ruby i ln.

(D) Cc nam chm lm cho vin ruby di chuyn ln xung.

(a. Ph s 4

b. ionmolionmolgam

cmgamcm

cm

CrN 201231

3

2

310.7,3.10.022,6

.52

100

05,0.05,4

2

15,114,32,15

)(

c. d.

eg

t2gdxy dxz dyz

dz dx - y2 2 2

e. A)

Cu 2.11. X l ion phc xianua ca crom c cu hnh bt din, tan trong nc v c tnh kh. Bit X c

momen t = 2,83 B.M.

(a) Hy xc nh s oxi ha ca crom trong X.

Thay th ton b phi t xianua trong X bng amoniac thu c phc Y. Bit XY.(b) Hy xc nh momen t (theo BM) ca Y.


18/45

Trang 18

(a. C: 83,2)2( nn

=> n = 2,

Vy crom c s oxi ha +2 hoc +4 trong phc X.

Do X c tnh kh nn crom c s oxi ha +2. X: [Cr(CN)6]4

[Cr(CN)6]4-

d

2

sp

3

6CN-

b. BMnnY 90,4)24(4)2(

[Cr(NH3)6]2+

sp3d2

6NH3 )

Cu 2.12. (a) Hy v cc ng phn ca cc phc sau:

i/ [Cr(NH3)4Cl2]Cl

ii/ [CoPy3Cl3], py l pyridin.

iii/ [Co(SCN)(H2O)5]Cl

iv/ [PtCl(PMe3)3]Br, Me l CH3.

v/ [Co(en)2Cl2]Cl.

(b) CrCl3phn ng vi NH3to thnh cc phc c cng thc chung l CrCl3(NH3)x(x = 3 6). Hyvit cng thc ca cc phc trn v so snh dn in ca chng trong cc dung dch c cng nng .

(a. i/ [Cr(NH3)4Cl2]Cl: 2 ng phn (cis, trans).

Tetraammin dicloro crom (III) clorua

ii/ [CoPy3Cl3]: 2 ng phn (mer, fac).

Tricloro tris(pyridin) coban(III)

iii/ [Co(SCN)(H2O)5]Cl: 2 ng phn (lin kt).

Pentaaqua thioxianato coban(II) cloruaPentaaqua isothioxianato coban(II) clorua

iv/ [PtCl(PMe3)3]Br: cu hnh vung phng, 2 ng phn (ng phn ion ha)

[PtCl(PMe3)3]Br: Cloro tris (trimetylphotphin)platin (II) bromua.

[PtBr(PMe3)3]Cl: Bromo tris (trimetylphotphin)platin (II) clorua.

v/ [Co(en)2Cl2]Cl: 3 ng phn (cis, trans).

Dicloro bis (etylendiamin)coban(III) clorua;

b. dn in: [Cr(NH3)6]Cl3> [Cr(NH3)5Cl]Cl2 > [Cr(NH3)4Cl2]Cl > [Cr(NH3)3Cl3])


19/45

Trang 19

3. Phng php chun cromat

Phng php chun dicromat da trn kh nng oxi ho ca ion dicromat trong mi trng axit:

Cr2O72 + 14 H+ + 6e 2 Cr3+ + 7 H2O ; E

0= 1,33V

Da cam lc

u im:

K2Cr2O7l cht gc.

khng phn ng vi Cl.

dung dch K2Cr2O7bn theo thi gian, t b nh hng bi cht hu c.

Nhc im:

khng t ch th (do cng mu ca Cr2O72

khng ln).

phn ng tng i chm.

ng dng:

Xc nh cht kh: H2O2, S2O32, Fe2+, SO32, I, ru etylic, RSH, vitamin C, COD.

Xc nh cht oxi ho (chun ngc): Fe3+, NO3, Cu2+, MnO4

, IO3.

Xc nh cht khng c tnh oxi ha kh: Pb2+, Ba2+(kt ta di dng MCrO4).

Cu 3.1. Ly 10 ml mu dung dch cha Cr(III) cho vo bnh nn, thm 5 ml dung dch H 2SO43M, 5ml H3PO42M, 5 git dung dch AgNO31% v 1 git dung dch MnSO41%. Pha long dung dch bng ncct ti khong 100 ml. un trn bp in, cho tng lng nh mui (NH4)2S2O8vo, lc dung dch lin tc,khi dung dch c mu hng (mu ca Cr2O7

2v MnO4) th dng cho pesunfat. Tip tc un si k cho

pesunphatphn hy ht. Thm tng git dung dch HCl 6M vo cho ti khi dung dch mt mu hng v

chyn sang mu vng da cam th ngng li. un tip 5 pht na ui ht Cl2ra khi dung dch. ngui,thm 5 ml H3PO42M, vi git cht ch th diphenyl amin, chun dung dch thu c bng dung dch Fe

2+

0,032M ti khi dung dch chuyn t mu tm sang mu xanh nht thy ht 6,75 ml.

(a) Vit cc phng trnh phn ng xy ra trong th nghim.

(b) Cho bit vai tr ca AgNO3v MnSO4thm vo.

(c) Tnh nng ca Cr (III) trong dung dch mu.

(d) Nu S2O82khng b phn hu ht th phng php mc sai s dng hay sai s m.

(a. 2Cr3++ 3S2O82 + 7H2O Cr2O7

2 + 6SO42 + 14H+

2Mn2+ + 5S2O82 + 8H2O 2MnO4 + 10SO42 + 16H+

2S2O82 + 2H2O 4SO4

2 + O2 + 4H+

2MnO4 + 10Cl + 16H+ 2Mn2+ + 5Cl2 + 8H2O

Cr2O72 + 6Fe2+ + 14H+ 2Cr3+ + 6Fe3+ + 7H2O

b. AgNO3: xc tc; MnSO4: ch th

c. 0,0072M)

Cu 3.2.Dung dch Cr2O72trong mi trng axit c th oxi ho c etanol v etanal thnh axit etanoic

(axit axetic) v ion dicromat b kh thnh Cr(III). Dung dch AgNO3/ NH3ch oxi ho c etanal, khngoxi ho c etanol. Trong qu trnh ny, ion Ag+b kh thnh bc kim loi.


20/45

Trang 20

C 500,0 ml dung dch cha etanol v etanal. xc nh nng ca mi cht, u tin ngi ta tinhnh chun ho dung dch K2Cr2O7 trong mi trng axit bng cch chun vi dung dch FeSO4 (ionicromat oxi ho Fe2+thnh Fe3+). Chun b dung dchchun Fe2+bng cch ho tan 7,43 gam FeSO4.7H2O(MW= 287,0) vo bnh nh mc 100,0 ml v nh mc n vch bng nc ct deion. Chun 25,0 mldung dch ny thy ht 23,12 ml dung dch dicromat. Tip , chun 50,0 ml dung dch mu cha etanolv etanal thy ht 22,45 ml dung dch dicromat. Trong mt th nghim khc, thm lng d dung dchAgNO3/ NH3vo 50,0 ml dung dch mu thy c 0,234 gam Ag to thnh.

(a) Vit cc phng trnh phn ng xy ra trong quy trnh.

(b) Ti sao phi axit ho dung dch dicromat?

(c) Tnh nng mol ca dung dch dicromat?

(d) Tnh nng ca etanol v etanal trong dung dch mu.

(a. 2Cr2O72 + 3C2H5OH + 16H

+ 4Cr3+ + 3CH3COOH + 11H2O.

Cr2O72 + 3CH3CHO + 8H

+ 2Cr3+ + 3CH3COOH + 4H2O

Cr2O72

+ 6Fe2+

+ 14H+

2Cr3+

+ 6Fe3+

+ 7H2O

2Ag+ + CH3CHO + H2O 2Ag + CH3COOH + 2H+

b. Trong mi trng trung tnh hoc kim, tnh oxi ho ca Cr2O72yu hn trong mi trng axit. Mt

khc, trong mi trng trung tnh, sn phm kh lCr(OH)3khng tan

c. Fe2+: 0,259M; Cr2O72: 0,0467M

d. C2H5OH: 0,716M; CH3CHO: 0,0217M)

Cu 3.3.Hm lng Fe2O3trong qung hematit c xc nh bng phng php chun oxi ha kh. Trong mt th nghim, 4,00 g bt hematit c x l bng dung dch axit clohidric c. Phn khng tan

c lc b, kh ton b ion st trong phn dung dch thnh Fe2+

. Chun dung dch thu c ( pH = 0)bng dung dch dicromat 0,100 mol/lit). Th tch dung dch cn t ti im tng ng l 33,74 ml.

(a) Hy vit phng trnh phn ng ca st (III) oxit vi axit clohidric.

(b) Hy chn nhng cht kh thch hp trong s cc cht kh cho di y kh Fe(III) thnh Fe(II):bromua, nitrit, sunphit, hn hng km thy ngn v Sn2+.

Cho th kh chun ca mt s cp oxi ha kh lin hp nh sau:

Cp oxi ha/ kh E (V) Cp oxi ha/ kh E (V) Cp oxi ha/ kh E (V)

Zn+/Zn 0,76 Fe +/Fe + 0,77 BrO3

/Br

1,44

Sn

+

/Sn

+

0,15 Cr2O7

/Cr

+

1,33 Mn

+

/Mn

+

1,51SO4

/SO3

0,17 Cl2/Cl

1,36 MnO4

/Mn

+ 1,51

(c) Hy vit phng trnh phn ng chun v tnh hng s cn bng ca phn ng.

(d) Hy tnh thnh phn % theo khi lng ca Fe2O3c trong mu hematit.

(e) Hy tnh th ca dung dch ti im tng ng trong php chun trn.

(g) Hy chn nhng cht ch th oxi ha kh thch hp cho php chun trn t cc cht ch th chodi y.

st phenantrolin (xanh nht/ E = 1,14V).

ferroin (xanh/ da camE = 1,06V). indigocamin (xanh/ vngE = 0,29V).


21/45

Trang 21

metylen xanh (xanh/ khng mu E = 0,53V).

nilblue sunphat (tm/ khng mu E = 0,41V).

(h) Hy cho bit ion clorua c b oxi ha trong qu trnh chun khng?

Mt phng php khc nh lng st (II) l phng php chun kali pemanganat. trnh tothnh clo ngi ta thm vo dung dch st (II) 10,0 ml dung dch bo v. Dung dch ny cha MnSO4, axit

sunfuric v axit photphoric c.(i) Hy tnh gi tr pH ti thiu ti ion clorua b oxi ha thnh clo bi pemanganat. Bit nng ca

tt c cc ion khc u l 1 M.

(j) Hy cho bit th kh ca cp MnO4, H+/ Mn2+ thay i nh th no khi thm MnSO4vo dung

dch?

(k) Hy cho bit vai tr ca axit photphoric trong dung dch bo v?

(a. Fe2O3+ 6 HCl 2 FeCl3+ 3 H2O

b. Cht kh thch hp: Zn, Sn2+, SO32; do chng c E < 0,77 V;

c. 6 Fe2++ Cr2O72+ 14 H+ 6 Fe3++ 2 Cr3++ 7 H2O

560592,0

)77,033,1(6

10.7,510

K

d. 40,5%

e. Ti imtng ng

][6][

][3][

2

72

2

33

OCrFe

CrFe

C:][

][log0592,0

2

30

/ 23

Fe

FeEE

FeFe

23

142

720

2/ ][

]][[log

6

0592,032

72

Cr

HOCrEE

CrOCr

=>23

142

720

2/2

30

/ ][

]][[log0592,06

][

][log0592,07 32

7223

Cr

HOCrE

Fe

FeEE

CrOCrFeFe

][][

][]][[log0592,06

223

3142

720

2/

0

/ 32

7223

FeCr

FeHOCrEE

CrOCrFeFe

][6][

][3]][[log0592,06

2

72

23

3142

720

2/

0

/ 32

7223

OCrCr

CrHOCrEE

CrOCrFeFe

][2

][log0592,06

3

140

2/

0

/ 32

7223

Cr

HEE

CrOCrFeFe

=> VEECr

HEE

E CrOCrFeFe

CrOCrFeFe

25,17

6

7

][2

][log0592,06 0

2/

0

/3

140

2/

0

/ 3272

2332

7223

g. Cht ch th thch hp: st phenantrolin, ferroin; do E ca cht ch th nm trong khong bc nhyth ca ng cong chun .

h. Khng v E(Cl2/Cl) > E(Cr2O7

2/Cr3+);


22/45

Trang 22

i.1

][1log

5

0592,051,136,1

8

H;

pH = 1,58;

j. gim

k. to phc vi Fe3+; Fe(PO4)23)

Cu 3.4.Quy trnh phn tch crom trong mu thp khng g cha Fe, Cr v Mn.

Pha dung dch chun FeSO4: Ho tan 9,9823 gam mui Mohr (FeSO4.(NH4)2SO4.6H2O) vo bnh nhmc 250 ml c H2SO4v nh mc n vch bng nc ct.

(a) Tnh nng mol ca dung dch FeSO4thu c.

Chun ho dung dch KMnO4: Ly 25,0 ml dung dch FeSO4va pha ch trn cho vo bnh nn, thm1 ml dung dch H3PO4c ( to phc khng mu vi Fe

3+), chun dung dch thu c bng dung dchKMnO4thy va ht 24,64 ml.

(b) Tnh nng mol ca dung dch KMnO4.

Chun b mu: Ho tan 0,2800 gam mu thp trong dung dch hn hp H3PO4v H2SO4c, un nngcho n khi thu c dung dch trong sut mu xanh.

(Fe Fe3+, Cr Cr3+, Mn Mn2+). Lm lnh dung dch n nhit phng. Thm 5 ml dung dchAgNO31%, 20 ml dung dch (NH4)2S2O820%. Sau vi pht dung dch c mu hng (pesunfat oxi ho Mn

2+

thnh MnO4, Cr3+thnh Cr2O7

2 , Ag+ng vai tr lm cht xc tc). un si dung dch phn hu htion pesunfat cn d (sinh ra SO4

2v O2). Thm t t tng git HCl c n khi dung dch chuyn t muhng sang mu vng (HCl phn ng chn lc vi MnO4

). Sau khi kt thc qu trnh thy c kt ta trngxut hin y bnh.

(c) Vit phng trnh phn ng ca Cr3+, Mn2+vi S2O82trong mi trng axit.

(d) Vit phng trnh phn ng loi MnO4bng dung dch HCl c.

(e) Hy cho bit kt ta trng l cht g? c to thnh nh th no?

Tin hnh chun : Chuyn dung dch thu c trn vo bnh nh mc 250 ml ri nh mc nvch bng nc ct (dung dch A). Ly 50 ml dung dch A cho vo bnh nn, thm tip 25,0 ml dung dchFeSO4. Lng FeSO4d c chun bng dung dch KMnO4thy va ht 19,89 ml.

(g) Vit phng trnh phn ng ca Fe2+vi Cr2O72.

(h) Tnh % khi lng ca Cr trong mu thp (Cr = 52,0).

(a. 0,1019 M;b. 0,02068 M;

c. 2 Cr3+ + 3 S2O82 + 7 H2O Cr2O7

2 + 6 SO42 + 14 H+

2 Mn2+ + 5 S2O82 + 8 H2O 2 MnO4

+ 10 SO42 + 16 H+

d. 2 MnO4 + 10 Cl + 16 H+ 2 Mn2+ + 5 Cl2 + 8 H2O

e. AgCl;

h. 15,19%)

Cu 3.5.Crom c iu ch t qung FeCr2O4, st c tch ra khi qung theo hai bc:

4 FeCr2O4 + 8 Na2CO3 + 7 O2 8 Na2CrO4 + 2 Fe2O3 + 8 CO2


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Trang 23

2 Na2CrO4 + H2SO4 Na2Cr2O7 + Na2SO4 + H2O

Kh icromat bng cacbon s thu c crom(III) oxit, kh oxit to thnh bng phng php nhit nhms thu c crom.

Na2Cr2O7 + 2 C Cr2O3 + Na2CO3 + CO

Cr2O3 + 2 Al Al2O3 + 2 Cr

(a) Hy tnh khi lng crom c to thnh t 2,1 tnqung c cha 72,0% FeCr2O4(Cr = 52,00; Fe= 55,85; O = 16,00). Gi thit hiu sut ca c qu trnh l 100%.

Mt mu thp c cha Mn v Cr. Mn v Cr trong 5,00 gam mu thp c oxi ha thnh MnO 4v

Cr2O72 v pha long thnh 100,0 ml dung dch. Ly50,0 ml dung dch, thm BaCl2n d v iu chnh

pH n gi tr thch hp, cromat kt ta hon ton v thu c 5,82 g BaCrO4. 50,0 ml dung dch cn liphn ng va vi 43,5 ml dung dch Fe2+1,60 M trong mi trng axit.

MnO4 + Fe2+ + H+ Mn2+ + Fe3+

Cr2O72+ Fe2+ + H+ Cr3+ + Fe3+

(b) Hy tnh thnh phn % theo khi lng ca Mn v Cr trong mu thp (Mn = 54,94; Ba = 137,33).

(a. S hp thc: FeCr2O4 2 Cr; mCr= 702 kg;

b. MnO4+ 5 Fe2++ 8 H+ Mn2++ 5 Fe3+ + 4 H2O

Cr2O72+ 6 Fe2++ 14 H+ 2 Cr3++ 6 Fe3+ + 7 H2O

Cr: 47,8%; Mn: 0,3%)

Cu 3.6.Hm lng crom trong mt mu thp khng g (cha Fe Cr) c xc nh theo quy trnhsau:

1. Ha tan 0,1000 g mu thp bng 20 ml dung dch axit sunfuric 1,8M, un nng.

2. Thm tip 4 ml dung dch axit nitric c v un khong 10 pht cho n khi dung dch chuyn tmu xanh sang mu lc.

3. Thm 10 ml dung dch bc nitrat 0,5% v 6 g amoni pesunfat ((NH 4)2S2O8). un si dung dchkhong 20 pht amoni sunfat bphn hy hon ton, dung dch thu c c mu da cam.

4. Thm 10 ml dung dch NaCl 5%.

5. Thm chnh xc 20,00 ml dung dch FeSO4(NH4)2SO41,00101mol lit1.

6. Chun dung dch thu c bng dung dch KMnO4 2,00 102mol lit1.

(a) Hy vit cc phngtrnh phn ng xy ra bc 2.(b) Hy vit phng trnh phn ng xy ra bc 3.

(c) Hy vit phng trnh phn ng xy ra bc 4 v cho bit mc ch ca qu trnh.

(d) Hy vit phng trnh phn ng xy ra bc 5. Hy cho bit mu ca dung dch thu c.

(e) Hy vit phng trnh phn ng xy ra bc 6.

(f) Php chun bc 6 cn 12,00 ml dung dch pemanganat t ti im tng ng. Hy tnhthnh phn % theo khi lng ca crom c trong mu thp.

(a. Fe2+ + 2 H+ + NO3 Fe3+ + NO2 + H2O

Cr2+ + 2 H+ + NO3 Cr3+ + NO2 + H2O


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Trang 24

xanh lc

b. 2 Cr3+ + 3 S2O82 + 7 H2O Cr2O7

2 + 6 SO42 + 14 H+

c. Ag+ + Cl AgCl

Mc ch: loi Ag+ra khi dung dch, ngn cn qu trnh Cr3+b oxi ha tr li thnh Cr2O72 bc

sau trong trng hp amoni pesunfat khng b phn hy ht;

d. 6 Fe2+ + Cr2O72 + 14 H+ 6 Fe3+ + 2 Cr3+ + 7 H2O

0,8 0,8/6 mmol

dung dch chuyn t mu da cam sang mu lc ca Cr3+.

e. 5 Fe2+ + MnO4 + 8 H+ 5 Fe3+ + Mn2+ + 4 H2O

1,2 0,24 mmol

f. Tng s mmol Fe2+a vo dung dch l: 200,1 = 2 mmol;

=> nCr= 1,6/6 mmol;

=> %87,13100100

5266,1

% Cr )

Cu 3.7. xc nh hm lng ca crom v st trong mt mu gm Cr2O3 v Fe2O3, ngi ta unnng chy 1,98 gam mu vi Na2O2 oxi ha Cr2O3 thnh CrO4

2. Cho khi nung chy vo nc, unsi phn hu ht Na2O2. Thm H2SO4 long n d vo hn hp thu c v pha thnh 100,00 ml, cdung dch A c mu vng da cam. Cho dung dch KI (d) vo 10,00 ml dung dch A, lng I3

(sn phmca phn ng gia Iv I2) gii phng ra phn ng ht vi 10,50 ml dung dch Na2S2O3 0,40 M. Nu chodung dch NaF (d) vo 10,00 ml dung dch A ri nh tip dung dch KI n d th lng I3

gii phng ra

ch phn ng ht vi 7,50 ml dung dch Na2S2O3 0,40 M.(a) Vit cc phng trnh phn ng xy ra.

(b) Gii thch vai tr ca dung dch NaF.

(c) Tnh thnh phn % khi lng ca crom v st trong mu ban u. Cho: Fe = 56; Cr = 52.

(a. Cr2O3 + 3 Na2O2 2 Na2CrO4 + Na2O (1)

2 Na2O2 + 2 H2O O2 + 4 OH + 4 Na+ (2)

OH + H+ H2O (3)

2 CrO42 + 2 H+ Cr2O7

2 + H2O (4)

Fe2O3 + 6 H+ 2 Fe3+ + 3 H2O (5)

Cr2O72 + 9 I + 14 H+ 2 Cr3++ 3 I3

+ 7 H2O (6)

2 Fe3+ + 3 I 2 Fe2+ + I3 (7)

2 S2O32 + I3

S4O62 + 3 I (8)

Fe3+ + 3 F FeF3 (9)

b. Vai tr ca dung dch NaF: Fc mt trong dung dch to phc bn, khng mu vi Fe3+, dng che Fe3+.

c. Trong 10 ml dung dch A: Cr2O

7

2: x mol; Fe3+: y mol.

3x + y/2 = 4,2.103/2


25/45

Trang 25

3x = 3.103/2

x = 5.104; y = 1,2.103

%Fe = 33,94%; %Cr = 26,27%)

Cu 3.8.Ha tan hon ton 1500 mg mt mu hp kim cha bc, ng v crom thnh 500,00 ml dungdch cha Ag+, Cu2+ v Cr3+ (dung dch A). Ly

50,00 ml dung dch A tin thnh theo quy trnh sau:Sau khi tch bc v ng,ion crom (III) c oxi ha theo phn ng sau:

OH + Cr3+ + H2O2 CrO42 + H2O

Thm 25,00 ml dung dch Fe2+0,100 M. Phn ng xy ra nh sau:

H+ + Fe2+ + CrO42 Fe3+ + Cr3+ + H2O

phn ng vi lng Fe2+d cn dng va 17,20 ml dung dch KMnO40,020 M. Phn ng xy ranh sau:

H+ + Fe2+ + MnO4 Fe3+ + Mn2+ + H2O

Trong mt th nghim khc, ly 200 ml dung dch A em in phn. C ba kim loi b kt ta hon tonsau khi in phn 14,50 pht vi cng dng 2A. Bit hiu sut ca qu trnh in phn l 90%.

Hy vit cc phng trnh phn ng xy ra v tnh thnh phn % theo khi lng ca mi kim loitrong hp kim u. Cu = 63,55; Ag = 107,87; Cr = 52,00.

(t s mol Ag, Cu v Cr trong 1500 mg mu hp kim ban u ln lt l x, y v z. C:

107,87x + 63,55y + 52,00z = 1,5 (1)

10 OH + 2 Cr3+ + 3 H2O2 2 CrO42 + 8 H2O

z/10 z/10

8 H+ + 3 Fe2+ + CrO42 3 Fe3+ + Cr3+ + 4 H2O

3z/10 z/10

8 H+ + 5 Fe2+ + MnO4 5 Fe3+ + Mn2+ + 4 H2O

C: 1,0025,002,00172,0510

35

10

3

42

zn

zn

MnOFe (2)

Qu trnh in phn:

Ag+ + e Ag

2x/5 2x/5

Cu2+ + 2e Cu

2y/5 4y/5

Cr3+ + 3e Cr

2z/5 6z/5

C: 01623,096485

605,1429,0

5

642

zyxne (3)

T (1), (2) v (3) suy ra:

x = 4,25.103mol; y = 1,43.102mol; z = 2,6.103mol


26/45

Trang 26

=> %Ag = 30,56%; % Cu = 60,58%; % Cr = 8,86%)

Cu 3.9.Trong mt nh my ha cht iu ch fomandehit bng phn ng oxi ha metanol, dung dchcha metanol v fomandehit c em phn tch. kim tra phng php ngi ta tin hnh th nghimvi dung dch chun cha metanol v fomandehit. Ccdung dch c s dng nh sau:

Metanol, 5,00 g/lit

Fomandehit, 5,00 g/lit

Kali dicromat, 3,000.102mol/lit

Amoni st(II) sunphat, 0,2000 mol/lit

Iot, 0,1000 mol/ lit

Natri thiosunfat, 0,2000 mol/lit

I. Trn 10,00 ml dung dch metanol vi 100,00 ml dungdch kali dicromat, thm khong 100 ml dungdch axit sunfuric c v yn khong 30 pht. Lng dicromat d c chun bng dung dch Fe2+vicht ch th axit diphenylamin sunphonic (mu i t tm sang xanh nht). Th tch dung dch Fe2+cn

dng t ti im tng ng l 43,5 ml.

II. Trn 10,00 ml dung dch fomandehit vi 50,00 ml dung dch iot. Thm dung dch NaOH cho ti khidung dch c mi trng kim v yn khong 10 pht. Thm axit clohidric trung ha dung dch, lngiot d c chun bng dung dch thiosunphat, vi cht ch th h tinh bt. Th tch dung dch thiosunfat cndng t ti im tng ng l 33,3 ml.

(a) Hy tnh t l s mol metanol/icromat v fomandehit /iot tham gia phn ng vi nhau.

(b) Hy vit phng trnh phn ng ca metanol vi icromat trong mi trng axit v ca fomandehitvi iot trong mi trng kim.

III. Bit rng iot khng phn ng vi metanol. Ly hai mu dung dch cha metanol v fomandehit chabit nng , mi mu c th tch 10,00 ml.

Mt mu c trn vi 100,00 ml dung dch kali dicromat v axit sunfuric c nh trong th nghim I.Lng dicromat d phn ng va vi 4,8 ml dung dch Fe2+.

Mu cn li c trn vi 50,00 ml dung dch iot v c tin hnh nh th nghim II. Lng iot dphn ng va vi 16,50 ml dung dch thiosunfat.

(c) Hy vit cc phng trnh phn ng xy ra v tnh nng (theo g/lit) ca metanol v fomandehittrong dung dch.

(a. Th nghim I:CH3OH + K2Cr2O7 ...

6 Fe2+ + Cr2O72 + 14 H+ 6 Fe3+ + 2 Cr3+ + 7 H2O

S mol metanol c trong 10,00 ml dung dch l:

moln OHCH310.56,1

32

501,03

S mol K2Cr2O7phn ng vi metanol l:

molnOCrK

310.55,16

2,00435,003,01,0

722


27/45

Trang 27

=>1

1

10.55,1

10.56,13

3

722

3

OCrK

OHCH

n

n

Th nghim II:

HCHO + I2 ...

I2 + 2 S2O32 2 I + S4O6

2

S mol fomandehit c trong 10,00 ml dung dch l:

molnHCHO310.67,1

30

501,0

S mol I2phn ng vi fomandehit l:

molnI310.67,1

2

2,00333,01,005,0

2

=>

1

1

10.67,1

10.67,13

3

2

I

HCHO

n

n

b. CH3OH + Cr2O72 + 8 H+ CO2 + 2 Cr

3+ + 6 H2O

HCHO + I2 + 3 OH HCOO + 2 I + 2 H2O

c. t s mol ca CH3OH v HCHO c trong 10,00 ml dung dch mu ln lt l x v y.

CH3OH + Cr2O72 + 8 H+ CO2 + 2 Cr

3+ + 6 H2O

x x

3 HCHO + 2 Cr2O72 + 16 H+ 3 CO2 + 4 Cr

3+ + 11 H2O

y 2y/3

6 Fe2+ + Cr2O72 + 14 H+ 6 Fe3+ + 2 Cr3+ + 7 H2O

C: 03,01,010.6,13

2

63

2 42

722

y

xny

xn FeOCrK

(1)

HCHO + I2 + 3 OH HCOO + 2 I + 2 H2O

y y

I2 + 2 S2O32 2 I + S4O6

2

C: 05,01,010.65,12

3232

2

yn

yn OS

I (2)

T (1) v (2) suy ra:

x = 6,07.104mol; y = 3,35.103mol

=> litgamlit

molgamlmolC OHCH /94,1

01,0

.32.10.07,6 114

3

litgamlit

molgamlmolCHCHO /05,10

01,0

.30.10.35,3 113

)


28/45

Trang 28

Cu 3.10.Dung dch dicromat trong mi trng axit c th oxi ha etanol v etanal thnh axit etanoic,v ion dicromat b kh thnh ion Cr(III). Dung dch bc (I) trong amoniac c th oxi ha etanal, nhngkhng phn ng vi etanol.

Dung dch X cha etanol v etanal cha bit nng . xc nh nng ca hai cht trong X, utin ngi ta tin hnh xc nh nng ca dung dch K2Cr2O7 c axit ha. Dung dch st (II) ciu ch bng cch ha tan 7,43 g FeSO4.7H2O vo nc c 100,0 ml dung dch. 25,0 ml dung dchny phn ng va vi 23,12 ml dung dch dicromat t ti im tng ng.

Chun 50,0 ml dung dch X bng dung dch icromat trn thy va ht 22,45 ml dung dch tti im tng ng.

Thm lng d dung dch bc (I) nitrat trong amoniac vo 50,0 ml dung dch X thy to 0,234 g bc.

(a) Hy vit cc phng trnh ion rt gn ca cc phn ng xy ra.

(b) Hy gii thch ti sao li phi axit ha dung dch icromat?

(c) Hy tnh nng mol ca dung dch K2Cr2O7.

(d) Hy tnh nng mol ca etanol v etanal trong X.(a. 3 C2H5OH + 2 Cr2O7

2+ 16 H+ 3 CH3COOH + 4 Cr3+ + 11 H2O

x 2x/3

3 CH3CHO + Cr2O72 + 8 H+ 3 CH3COOH + 2 Cr

3+ + 4 H2O

y y/3

CH3CHO + 2 Ag(NH3)2++ H2O CH3COO

+ 3 NH4+ + NH3 + 2 Ag

y 2y

6 Fe2+ + Cr2O72 + 14 H+ 6 Fe3+ + 2 Cr3+ + 7 H2O

b. Axit ha phn ng xy ra hon ton, sn phm ca phn ng l Cr3+ch khng phi kt taCr(OH)3.

c. C: MCFe

2672,01,005,278

43,72

=> MCOCrK

04815,012,23

6

1252672,0

722

d. t s mol ca etanol v etanal trong 50,0 ml dung dch X ln lt l x v y. C:

molyx

nOCrK

310.081,102245,004815,033

2

722

(1)

molynAg310.169,2

87,107

234,02 (2)

T (1) v (2) suy ra:

x = 1,079.103mol; y = 1,085.103mol;

=> lmolC OHHC /0216,005,0

10.079,1 3

52


29/45

Trang 29

lmolC CHOCH /0217,005,0

10.085,1 3

3

)

Cu 3.11. Oxi trong khng kh oxi ha chm Fe(II) trong dung dch axit thnh st (III).

(a) Hy vit phng trnh phn ng oxi ha Fe2+bng oxi.

(b) Hy chobit pH ca dung dch nh hng th no n phn ng (a).

Kesan, mt nh ha hc tr, quyt nh nghin cu nh hng ca oxi khng kh n dung dch st (II)peclorat. Kesan cn 13,4 gam mu Fe(ClO4)2.xH2O v ha tan vo nc to thnh 200,0 ml dung dch A.

Thuc th H3L, mt phi t hu c, phn ng hon ton vi ion Fe(III) theo phng trnh: Fe3+ + 2

H3L (aq) [FeL2]3 + 6 H+

Bit H3L c mu lc thm, phc [FeL2]3c mu vng nht v H3L khng to phc vi ion st (II). Do

c th s dng H3L chun nh lng ion Fe(III) khi c mt ion Fe(II). Ti im tng ng dungdch chuyn t mu vng nht sang mu lc nht.

Ly 20,0 ml dung dch A cho vo bnh nn. Tin hnh oxi ha hon ton st (II) thnh st (III) bng

lng d dung dch kali dicromat trong mi trng axit, ion Cr(III) sinh ra cng phn ng vi H 3L gingnh ion Fe(III). Chun dung dch thu c bng dung dch H3L 0,3182 M thy cn dng 30,20 ml dungdch t ti im tng ng.

(c) Hy cho bit s oxi ha ca crom trong kali icromat.

(d) Hy vit phng trnh ion rt gn ca phn ng oxi ha Fe(II) thnh st (III) bng icromat.

(e) Hy tnh nng mol ca Fe(II) trong dung dch A. Gi thit rng trong dung dch A khng c mtFe(III).

(g) Hy tnh gi tr cax.

xc nh nh hng ca oxi khng kh n dung dch st (II) peclorat, Kesan ly hai mu dung dch Au c th tch 20,0 ml cho vo hai bnh nn sch, k hiu l 1 v 2. Tin hnh chun ngay bnh nn 1 bngdung dch H3L 0,3182 M,kt qu cho thy mu lc bn xut hin ngay sau khi thm mt git dung dch thuc th. Bnh nn 2 c ngoi khng kh trong vng 1 tun, sau tin hnh chun thy ht 11,35 ml dung dch H3L 0,3182 M t ti im tng ng.

(h) Hy gii thch kt qu khi tin hnh th nghim vi bnh nn 1.

(i) Hy tnh nng mol ca Fe(III) trong bnh nn 2.

(k) Hy tnh % lng ion Fe(II) cn li trong bnh nn 2 so vi lng Fe(II) ban u.

(l) Nu bnh nn 2 c ngoi khng kh hai tun thay v mt tun, hy cho bit th tch dung dchH3L cn dng t ti im tng ng s ln hn, nh hn hay bng 22,70 ml?

(a. O2 + 4 Fe2+ + 4 H+ 4 Fe3+ + 2 H2O

b. Vi iu kin Fe3+v Fe2+cha b thy phn th khi pH tng th hiu sut phn ng gim.

c. +6

d. 6 Fe2+ + Cr2O72 + 14 H+ 6 Fe3+ + 2 Cr3+ + 7 H2O

x x x/3

e. t s mol Fe2+c trong 20,0 ml dung dch A l x.Fe2+ + 2 H3L (aq) [FeL2]

3 + 6 H+


30/45

Trang 30

x 2x

Cr3+ + 2 H3L (aq) [CrL2]3 + 6 H+

x/3 2x/3

C: molx

xn LH 310.610,93182,003020,0

3

22

3

=> x = 3,604.103mol

=> MCFe

1802,00200,0

10.604,3 3

2

g. C: Mx

CFe

2180,02,0

02,1875,254

4,13

2

=> x = 6,52

h. Kt qu thc nghim chng t trong bnh nn 1 khng c mt Fe(III).

i. MCFe

0903,00,20

2

13182,035,11

3

k. %Fe(II) cn li 0 1802 0 0903

100 49 890 1802

, ,, %

,

l. Khi ton b lng Fe2+b chuyn thnh Fe3+th th tch dung dch H3L cn dng phn ng va vi lng Fe3+sinh ra l:

mlVLH

65,223182,0

20,201802,0

max,3

Vy sau hai tun th th tch dung dch H3L cn dng s nh hn 22,70 ml)

Cu 3.12. Cho gin Latimer ca crom trong mi trng axit (pH = 0)nh sau:

Cr2O72- Cr(V) Cr(IV) [Cr(H2O)6]

3+ [Cr(H2O)6]2+ Cr

+0,293

+0,55 +2,1 -0,912

da cam lctmxanh

-0,744

X Y

(a) Hy tnh cc gi tr X

v Y trong gin trn.

(b) Hy cho bit Cr (IV) c b phn hy d ly thnh Cr (III) v Cr (VI) khng?

(c) Cp Cr2O72/Cr3+c s dng lm cht oxi ha. Hy vit bn phn ng ca cp trn.

(d) Hy tnh s thay i th ca cp trn nu pH ca dung dch tng t 1 n 3 (T = 298 K, E(Cr2O72

/Cr3+) = 1,33 V). Bit trong khong pH ny nng ca Cr2O72v Cr3+coi nh khng thay i.

(e) Chun 100 ml dung dch kali dicromat 0,01667 M bng dung dch Fe(II) 0,1 M. Hy tnh th khca dung dch thu c sau khi thm 100 ml dung dch Fe(II) 0,1 M. Bit trng thi cn bng c pH=1; E(Fe3+/Fe2+) = 0,77V.

Trong mt th nghim, mt bnh in phn cha 150 lit dung dch axit cromit H 2CrO4. Qu trnh inphn c thc hin trong khong thi gian 8 gi vi cng dng in 2000A. Vt cn m ng vai tr


31/45

Trang 31

l catot c ph 350g crom sau khi th nghim kt thc. Ti catot bn cnh crom kt ta cn xy ra phnng gii phng kh. Do hiu sut ca qu trnh in phn to crom nh hn 100%.

(g) Hy vit bn phn ng to crom catot v tnh hiu sut ca qu trnh in phn to crom.

(h) Hy vit cc bn phn ng to kh catot v anot v tnh th tch kh thot ra anot v catot 298K v 1,013.105N/m2.

(i) Mt loi thuc tr su cha ng (II) oleat Cu(C18H33O2)2(M = 626). Cho 9,9 g mu thuc tr suny vo hn hp axit sunfuric v axit nitric c un nng ph hy ht phn hu c. Thm lng d kalicromat vo dung dch thu c kt ta ng di dng mui cromat baz CuCrO4.2CuO.2H2O. Cht ktta c lc, ra v ha tan trong dung dch axit:

2 (CuCrO4.2CuO.2H2O) (r) + 10 H+(aq.) 6 Cu2+(aq.) + Cr2O7

2(aq.) + 9 H2O

Chun lng icromat sinh ra cn 15,7 ml dung dch Fe2+0,232 M. Hy tnh thnh phn % theo khilng ca ng (II) oleat c trong mu.

(a. C: 6 0,293 = 0,55 + x + 2,1 + 3 (0,744)

=> x = 1,34 VC: 3 (0,744) = y + 2 (0,912)

=> y = 0,408 V;

b. 3 Cr(IV) 2 Cr(III) + Cr(VI)

C: VEIVCrOCr

945,02

34,155,00)(/272

Cch 1: 390592,0)945,01,2(2

10.05,110

K

Cch 2: G0= nFE0pin= 2 96485 (2,1 0,945) = 222,88 kJ/mol < 0.Vy Cr(IV) t phn hy d ly to Cr(III) v Cr(VI).

c. Cr2O72 + 14 H+ + 6 e 2 Cr3+ + 7 H2O

d. pH = 1:

VCr

HOCrEE

CrOCrCrOCr 19,1)10lg(

6

0592,033,1

][

]][[lg

6

0592,0 14123

142

720

// 32

7232

72

pH = 3:

Vy khi pH tng t 1 n 3 th th kh ca h gim 0,27V;

e. Cr2O72 + 6 Fe2+ + 14 H+ 2 Cr3+ + 6 Fe3+ + 7 H2O

560592,0

)77,033,1(6

10.71,510

K

b: 8,335 50 mM

p 8,335 50 16,67 50 mM

cb 0,1 16,67 50 mM

C: 56

1462

72

2

72

6323

10.71,51,0])[6(][

10.00,5010.67,16

OCrOCrK

VCr

HOCr

EE CrOCrCrOCr 92,0)10lg(6

0592,0

33,1][

]][[

lg6

0592,0 14323

142

720

// 32723272


32/45

Trang 32

=> [Cr2O72] = 3,997.109M

=> VCr

HOCrEEE

CrOCrCrOCrdd 14,1

)10.67,16(

)10(10.997,3lg

6

0592,033,1

][

]][[lg

6

0592,023

1419

23

142

720

// 32

7232

72

Hoc:

[Fe2+] = 6[Cr2O72] = 2,398.108M

V

Fe

FeEEE

FeFeFeFedd 14,1

10.398,2

10.50lg0592,077,0

][

][lg0592,0

8

3

2

30

// 2323

g. Catot: CrO42 + 8 H+ + 6 e Cr + 4 H2O

molne 59796485

360082000

=> %76,610000,52

6

1597

350

h. Catot: 2 H+ + 2 e H2

Hoc: 2 H2O + 2e H2 + 2 OH

S mol e dng in phn H2CrO4.

moln 4,40600,52

3501

=> S mol e dng in phn nc to H2:

moln 6,5564,405972

=> 3807,66807013,1

29808314,02

6,556

2mlit

BarV

H

Anot: 2 H2O O2 + 4 H+ + 4 e

365,33650013,1

29808314,04

597

2mlit

BarV

O

i. %03,231009,9

626326

1232,00157,0

)(% 223318

OHCCu )

4. Hp cht CrO5

Cr O

O O-1

+6

-1

O O

H2CrO4 + 2 H2O2H2SO4 CrO5(ete) + 3 H2O

xanh

Cu 4.1. phn tch nh tch crom ngi ta cho dung dch dicromat trong mi trng axit tc dngvi H2O2 to ra hp cht CrO5km bn, mu xanh. Hp cht ny c bn ha bng cch chit vo etehoc thm phi t mt cng nh pyridin (py) to ra phc thp ng gic CrO5(py).

(a) Hy vit phng trnh phn ng to thnh CrO5.


33/45

Trang 33

(b) Hy cho bit s oxi ha ca crom trong CrO5v trong CrO5(py)? Hy v cu trc khng gian caCrO5(py).

(a. Cr2O72 + 4 H2O2 + 2 H

+ H2SO4 2 CrO5(ete) + 5 H2O

b. +6 v +6;

)

Cu 4.2. (a) D on hin tng, vit phng trnh ion ca cc phn ng xy ra trong cc th nghimsau:

i/ Thm t K2Cr2O7ln lt vo mi dung dch: Ba(CH3COO)2; (NH4)2CO3.

ii/ Thm t KNO2ln lt vo mi dung dch: KMnO4+ H2SO4; KI + H2SO4.

Nu vai tr ca K2Cr2O7v KNO2trong cc phn ng trn.

(b) Thm H2O2vo dung dch K2Cr2O7+ H2SO4sinh ra sn phm trung gian Cr3(O2)3+t bn mu xanh

ve d b phn hu cho ion Cr3+. Vit phng trnh ion ca cc phn ng.

(c) Tnh Eoca cp Cr2O72/Cr(OH)3.

Cho: EoCr2O72/ 2 Cr3+= 1,33V; EoMnO4

/ Mn2+= 1,51 V;

EoHNO2/ NO= 1,0 V; EoNO3

/ HNO2= 0,94 V; EoI3

/ 3I= 0,55 V.

CH3COOH c pKa= 4,76; H2CO3c pKa1= 6,35; pKa2= 10,33;

H2CrO4c pKa1= 0,61, pKa2= 6,50.

Cr2O72 + H2O 2 HCrO4

K = 101,36

Tch s tan ca Cr(OH)3bng 1030,8; ca BaCrO4bng 10

9,9.

Cu 4.3. Cho gin qu trnh kh th kh: qu trnh kh din ra theo chiu mi tn, th kh chunc ghi trn cc mi tn v o pH = 0.

Cr2O72- Cr(V) Cr(IV) Cr

+0,293

+0,55 +1,34 E0

x -0,408 E0

y

-0,744

Cr3+ Cr2+

(a) Tnh E0xv E0

y.

(b) Da vo tnh ton, cho bit Cr(IV) c th d phn thnh Cr3+v Cr(VI) c khng?

(c) Vit qu trnh xy ra vi h oxi ha kh Cr2O72/ Cr3+v tnh bin thin th ca h nhit

298 K, khi pH tng 1 n v.

(d) Phn ng gia K2Cr2O7vi H2O2trong mi trng axit (long) c dng nhn bit crom v snphm to thnh c mu xanh. Vit phng trnh ion ca phn ng xy ra v cho bit phn ng ny c thucloi phn ng oxi ha kh hay khng? Ghi s oxi ha tng ng trn mi nguyn t.

Cho: VEoCrOCr 33,132

72 / ; R = 8,3145 J.K1.mol1; F = 96485 C.mol1.


34/45

Trang 34

(a. C: 6 0,293 = 0,55 + 1,34 + E0x+ 3 (0,744)

=> E0x= 2,1 V

C: 3 (0,744) = 0,408 + 2E0y

=> E0y= 0,912 V;

b. 3 Cr(IV) 2 Cr(III) + Cr(VI)

C: VEIVCrOCr

945,02

34,155,00)(/

272

Cch 1: 390592,0)945,01,2(2

10.05,110

K

Cch 2: G0= nFE0pin= 2 96485 (2,1 0,945) = 222,88 kJ/mol < 0.

Vy Cr(IV) t phn hy d ly to Cr(III) v Cr(VI).

c. Cr2O72 + 14 H+ + 6 e 2 Cr3+ + 7 H2O

d. pH = 0: 33,10// 3272

3272

CrOCrCrOCr EE

pH = 1:

VCr

HOCrEE


6

0592,033,1

][

]][[lg

6

0592,0 14123

142

720

// 32

7232

72

Vy khi pH tng 1 n v th th kh ca hgim 0,14V;

d. Cr2O72 + 4 H2O2 + 2 H

+ 2 CrO5 + 5 H2O

Khng phi phn ng oxi ha kh do khng c nguyn t no thay i soxi ha

Cr O

O O-1

+6

-1

O O )

Cu 4.4. Cp dicromat /crom(III) ( VECrOCr

380,10

/ 3272 ) thng c s dng trong ha phn tch.

(a) Hy vit bn phn ng oxi ha kh ca cp trn v cho bit dung dch crom(III)/ dicromat(c(Cr3+)=c(Cr2O7

2)= 1 mol/lit) c oxi ha c ioua thnh iot t dung dch trung ha c E(I2/I) = E(I2/I

)

= +0,54 V khng.

Mu vng m ca ion cromat v mu da cam m ca ion dicromat thng c s dng pht hin

ra crom. V d crom(III) oxit c un nng chy vi kali nitrat v natri cacbonat, dung dch Cr(III) phn

ng vi hydro peoxit, natri peoxo isunphat, brom l nhng th nghim c dng pht hin Cr(III).

(b) Hy vit phng trnh ca bn phn ng trn. Hy cho bit vai tr ca natri nitrat v natri cacbonat

trong phn ng u tin.

Cn c vo th kh chun ca cp brom/ bromua (E (Br2/Br) = +1,065 V) c th thy phn ng gia

Cr3+vi brom l khng xy ra iu kin chun.

(c) Hy tnh gi tr pH ti thiu cn thit lp phn ng xy ra. Bit nng ca cc cu t khc u

bng 1 M.


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Mt s cu t c mt trong dung dch c th gy cn tr n php phn tch nh tnh Cr(III). V d nu

s dng hidro peoxit th cc ion bromua v ioua s gy cn tr, cn nu s dng brom th Mn 2+s gy cn

tr.

(d) Hy gii thch ti sao cc cu t trn li gy cn tr n php phn tch.

Mt phn ng khc pht hin crom l phn ng ca dicromat vi hidro peoxit. Sn phm to thnh

c mu xanh thm:Cr2O7

2 + 4 H2O2 + 2 H+ 2 CrO5 + 5 H2O

(e) Hy cho bit cht oxi ha v cht kh trong phn ng trn.

(a. Cr2O72 + 14 H+ + 6 e 2 Cr3+ + 7 H2O

C: VCr

HOCrEE


6

0592,038,1

][

]][[lg

6

0592,0 14723

142

720

// 32

7232

72

=> IICrOCr

EE// 2

3272

, tc phn ng oxi ha ioua khng xy ra.

b. Cr2O3 + 3 KNO3 + 2 K2CO3 2 K2CrO4 + 3 KNO2 + 2 CO2KNO3: cht oxi ha

K2CO3: lm gim nhit nng chy ca hn hp

2 Cr(OH)3 + 3 H2O2 + 4 OH 2 CrO4

2 + 8 H2O

Hoc: 2 Cr3+ + 3 H2O2 + H2O Cr2O72 + 8 H+

2 Cr3+ + 3 S2O82 + 7 H2O Cr2O7

2 + 6 SO42 + 14 H+

Hoc:2 Cr(OH)3 + 3 S2O82 + 10 OH 2 CrO4

2 + 6 SO42 + 8 H2O

2 Cr(OH)3 + 3 Br2 + 10 OH

2 CrO42

+ 6 Br

+ 8 H2OHoc:2 Cr3+ + 3 Br2 + 7 H2O Cr2O7

2 + 6 Br + 14 H+

c. phn ng xy ra c th BrBrCrOCr

EE// 2

3272

=> VHECrOCr

065,1]lg[6

0592,038,1 14

/ 3272

=> [H+] < 5,24.103 (pH > 2,28)

Vy pH > 2,28 th Br2oxi ha c Cr3+.

d. H2O2 + 2 Br Br2 + 2 OH

H2O2 + 2 I I2 + 2 OH

Br2v I2sinh ra c mu lm cn tr s quan st mu ca cromat.

5 Br2 + 2 Mn(OH)2 + 12 OH 2 MnO4

+ 10 Br + 8 H2O

Mu tm ca pemanganat lm cn tr s quan st mu ca cromat.

e. Khng c cht oxi ha v khng c cht kh do khng c nguyn t no thay i s oxi ha

Cr O

O O-1

+6

-1

O O )


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Trang 36

5. ng dng ca crom

Vic a crom vo thp lm tng cao cng, bn nhit, bn n mn v bn ha cht ca ccloi thp. V d thp khng g (inox) cha 18 25% Cr.

Crom kim loi c s dng rng ri m ln b mt kim loi lm lp bo v chng n mn cho ccdng c,my mc, thit b, gia dng...

Nhiu hp cht ca crom c s dng lm cht to mu, v d ch cromat PbCrO4c s dng lm chtto mu vng cho sn, ch cromat baz (PbCrO4Pb(OH)2) c s dng lm cht to mu . Cr2O3c sdng lm cht to mu xanh trong ngnh cng nghip knh v gm. Cr(III) c s dng lm cht to mu cho ruby nhn to.

Mui cromat c s dng bo qun g khi mi, mt. Mui crom(III) c s dng trong ngythuc da. Cromit v crom(III) oxit c s dng lm vt liu chu la. Mt vi hp cht ca crom c sdng lm cht xc tc. Axit cromic l cht oxi ha mnh thng c s dng lm cht ty ra dng cthy tinh trong phng th nghim.

Cu 5.1. Qu trnh m crom c thc hin bng cch in phn dung dch axit cromic. Vt cn mng vai tr catot v anot l in cc tr. Mt bnh in phn cha 100,0 lit dung dch CrO 3c nng 0,230 kg/ l.

in phn dung dch vi cng dng 1500 A trong khong thi gian 10,0 gi. Sau qu trnh inphn khi lng catot tng 0,679 kg.

(a) Hy cho bit hiu sut ca qu trnh in phn iu ch crom.

(b) Hy tnh t l th tch kh thot ra catot v anot ( cng iu kin).

Thc nghim cho thy t l th tch kh thot ra catot v anot ( cng iu kin) l:

603,1A

C

V

V

(c) Hy gii thch s khc nhau gia gi tr tnh c (b) v gi tr thc nghim.

(d) Hy tnh th tch kh thc tsinh ra anot v catot ( ktc).

(a. C: molF

Itne 7,559

96485

3600101500

CrO42 + 8 H+ + 6 e Cr + 4 H2O

S mol electron do cromat nhn l:

molne 35,78600,52

679'

Hiu sut ca qu trnh in phn iu ch crom:

%0,141007,559

35,78'

e

e

n

nH

b. Gi thit rng ch c kh hidro thot ra catot v oxi thot ra anot.

Anot (+): 2 H2O O2 + 4 H+ + 4e

Catot (): 2 H+ + 2e H2

C: molnnn eeH 7,240

235,787,559

2

'

2


37/45

Trang 37

moln

n eO

9,1394

7,559

42

=> 721,12

2 O

H

A

C

n

n

V

V

c. Thc t c nhng qu trnh phkhc xy ra, v d:

CrO42 + 8 H+ + 3e Cr3+ + 4 H2O

d. CrO42 + 8 H+ + 6 e Cr + 4 H2O

78,35 mol

2 H+ + 2e H2

2x x

2 H2O O2 + 4 H+ + 4e

y 4y

C: 78,35 + 2x = 4y

603,1

2

2 y

x

n

n

V

V

O

H

A

C

=> x = 158,2; y = 98,7

=> VH2= 158,222,4 = 3,54.103lit

VO2= 98,722,4 = 2,21.103lit)

Cu 5.2. Ch cromat c s dng rng ri lm cht mu, tuy nhin c hai thnh phn c mt trong chtny u c c tnh i vi ngi.

(a) Mt mu nc ngm c bo ha PbCrO4 (r) v c pH = 6,00. Hy tnh nng cn bng ca Pb2+,CrO4

2, HCrO4v Cr2O7

2. Cho cc hng s cn bng:132

4

210.82,2]][[ CrOPbKsp

7

4

2

42 10.34,3

][

]][[

HCrO

CrOHKa

14

22

4

2

2

72 10.13,3][][

][

CrOH

OCrKD

1410.00,1]][[ OHHKW

(b) Bit trong d dy ca mt ngi b nhim c crom c nng cn bng ca HCrO 4v Cr2O7

2

bng nhau. Gi thit dch d dy c pH = 3,0. Hy tnh nng tng cng ca crom ha tan c trong d dyca ngi ny.

(a. Cc cn bng xy ra:

PbCrO4(r) Pb2+ + CrO4

2 1324

2 10.82,2]][[ CrOPbKsp (1)

CrO42 + H+ HCrO4

7

4

2

42 10.34,3

][

]][[

HCrO

CrOHK

a (2)


38/45

Trang 38

2 CrO42+ 2 H+ Cr2O7

2 + H2O14

22

4

2

2

72 10.13,3][][

][

CrOH

OCrK

D (3)

C: S = [Pb2+] = [CrO42] + [HCrO4

] + 2 [Cr2O72] (4)

(2) => ][994,210.34,3

][10]][[][ 247

2

4

6

2

2

44

CrO

CrO

K

CrOHHCrO

a

(3) => 224

22

4

261422

4

22

72 ][313][)10(10.13,3][][][ CrOCrOCrOHKOCr D

(4) => 2-24

-2

4

-2

4-2

4

13

][CrO1332]2,994[CrO][CrO][CrO

10.82,2

=> 010.82,2][CrO994,3][CrO626 132-243-2

4

=> [CrO42] = 2,66.107M

MPb 67-

132 10.06,1

2,66.10

10.82,2][

MHCrO 77

4 10.96,710.66,2994,2][

MOCr 1127272 10.21,2)10.66,2(313][

b. C: ][299410.34,3

][10]][[][ 2

47

2

4

3

2

2

44

CrO

CrO

K

CrOHHCrO

a

22

4

822

4

231422

4

22

72 ][10.13,3][)10(10.13,3][][][ CrOCrOCrOHKOCr

D

=> 224

82

4 ][10.13,3][2994 CrOCrO

=> [CrO42] = 9,57.106M

MHCrOOCr 02864,010.57,92994][][

6

4

2

72

=> CCr= [CrO4

2] + [HCrO4] + 2 [Cr2O7

2]

= 9,57.106 + 30,02864 = 0,0860 M)

Cu 5.3. Cc hp cht ca crom (VI), c bit l cc hidroxit c tnh oxi ha rt mnh. Trong phng thnghim ngi ta li dng tnh oxi ha ca crom(VI) loi tr kh hidro sunfua. Trong mt th nghimngi ta sc kh CO2c ln H2S vo dung dch kali dicromat c mt axit sunfuric, cht khng tan mu vngc to thnh v dung dch chuyn sang mu xanh.

(a) Hy vit cng thc ca cc hidroxit crom(VI) m bn bit.

(b) Hy vit phn ng xy ra trong qu trnh tinh ch kh CO2.(c) Hy cho bit c th dng phng php trn loi kh SO 2 ln trong kh CO2c khng? Nu

c hy vit phng trnh phn ng xy ra.

(a. H2CrO4; H2Cr2O7; H2Cr3O10; H2Cr4O13;... nCrO3.CrO2(OH)2;

b. 3 H2S + K2Cr2O7+ 4 H2SO4 Cr2(SO4)3 + K2SO4 + 3 S + 7 H2O

c. 3 SO2 + K2Cr2O7 + H2SO4 Cr2(SO4)3 + K2SO4 + H2O

Cu 5.4.(a) Trong mi trng axit, ion cromat mu vng (CrO42) chuyn ha thnh ion dicromat mu

da cam (Cr2O72). Hy vit phng trnh phn ng.

(b) S oxi ha ca nguyn t kim loi trong ion cromat v ion dicromat bng bao nhiu?(c) Phn ng vit (a) cphi l phn ng oxi ha kh khng? Gii thch.


39/45

Trang 39

(d) Hy cho bit yu t chnh nh hng n v tr ca cn bng trn.

(e) Hy v cu trc hnh hc ca CrO42v Cr2O7

2.

Vt cn m crom c nhng vo dung dch Cr2O72trong mi trng axit, vt ny ng vai tr l catot

trong bnh in phn. (Cr = 51,996; 1 F = 96485 C)

(f) Bit rng ti anot xy ra qu trnh oxi ha nc, hy vit cc bn phn ng xy ra mi in cc v

phn ng tng cng xy ra trong qu trnh in phn.(g) C bao nhiu mol kh oxi thot ra khi c 52,0 g crom kt ta ti in cc?

(h) Nu cng dng in l 10,0 A th phi mt bao lu thu c 52,0 g crom?

(i) Xt v mt ha hc th ti sao crom li thng c s dng m trn b mt cc kim loi?

(a. 2 CrO42 + 2 H+ Cr2O7

2 + H2O

b. +6, +6;

c. khng, khng c nguyn t no thay i s oxi ha.

d. pH;

e.

O

CrO

O

O

O

CrO

OO

O

CrO

O

f. Anot (+) H2O 1/2 O2 + 2H+ + 2e

Catot () Cr2O72 + 14 H+ + 12e 2 Cr + 7 H2O

Cr2O72 + 2 H+ 2 Cr + H2O + 3 O2

1 1,5

g. 1,5 mol;

h. 6F

Itne

=> hsI

Ft 1,1657891

10

9648566

i. Cromphn ng vi oxi to ra lp mng oxit Cr2O3rt mng, bn v trong sut, gip bo v b mt

kim loi khi b n mn)Cu 5.5. Cho quy trnh iu ch 4 cht mu v c nh sau:

1) un nng hn hp dng bt mn cha 5,0 g natri dicromat v 0,62 g lu hunh 800C. Ha tan snphm vo nc.

2) un nng hn hp dng bt mn cha 3,0 g ch (II) oxit v 1,0 g thic(IV) oxit 650C.

3) un nng ch (II) oxit trong khng kh.

4) un nng hn hp dng bt mn cha 2,0 g silic dioxit, 0,66 g ng (II) oxit, 0,83g canxi cacbonatv 0,75 g borat 900C trong vi ngy.

(a) Hy vit cc phng trnh phn ng xy ra.(b) Hy cho bit vai tr ca borat trong th nghim 4?


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Trang 40

(c) Hy nu thm mt cht mu lc (cha st), mt cht mu vng (cha ch), mt cht mu xanh (chang) v mt cht mu (cha thy ngn) v nu phng php iu ch cc cht mu trn.

Trong mt xng v tranh c mt ci can mt nhn cha cht bt mu xanh. xc nh thnh phnca cht bt c trong can, ngi ta cn 1,818 g mu, ha tan vo nc v nh mc n vch trong bnh nhmc 100,00 ml bng axit sunfuric. Ly 10 ml dung dch ri thm vo lng d dung dch KI, chun

hn hp thu c bng dung dch natri thiosunfat 0,100 M thy va ht 16,45 ml.(d) Hy cho bit kim loi no c phn tch? Hy vit cc phn ng xy ra v tnh thnh phn % theokhi lng ca kim loi trong mu.

(e) Hy cho bit cht bt mu xanh ny c th l cht mu c iu ch phn (a) khng? Gii thch.

Php phn tch khc s cho bit thnh phn chnh xc ca cht bt mu xanh. Ly 2,000 g mu v unnng trong iu kin khng c khng kh. Thu c 1,439 g cht rn mu en, hi nc v cht kh lm cnc vi trong.

(f) Hy xc nh thnh phn ca cht bt mu xanh.

(a. Na2Cr2O7 + S Na2SO4 + Cr2O32 PbO + SnO2 Pb2SnO4

6 PbO + O2 2 Pb3O4

4 SiO2 + CuO + CaCO3 CaCuSi4O10 + CO2

b. Vai tr ca Na2B4O7.10H2O l lm cht chy, lm gim nhit nng chy ca hn hp.

c. Cht mu lc cha st:K[FeIIIFeII(CN)6] hoc Fe4III[FeII(CN)6]3

FeCl3 + K4[Fe(CN)6] K[FeIIIFeII(CN)6] + 3 KCl

4 FeCl3 + 3 K4[Fe(CN)6] Fe4III[FeII(CN)6]3+ 12 KCl

Cht mu vng cha ch: PbCrO4.

Pb(NO3)2 + K2CrO4 PbCrO4 + 2 KNO3

Cht mu xanh cha ng: Cu(CH3COO)2 3Cu(OH)2.

CuO + 2 CH3COOH + 4 H2O Cu(CH3COO)2 3Cu(OH)2 2H2O

Cht mu cha thy ngn: HgS (xinabar dng mu )

Hg(NO3)2 + H2S HgS + 2 HNO3

d. ng2 Cu2+ + 4 I 2 CuI + I2

I2 + 2 S2O32 2 I + S4O6

2

%5,57100818,1

54,631022

110.45,161,0

%

3

Cu

e. %Cu (trong CaCuSi4O10) = 16,9%, khng phi cht mu c iu ch phn (a);

f. CuCO3Cu(OH)2)

Cu 5.6.Nguyn tcrom c bn knh l 126 pm v khi lng ring ca crom l 7,14 g/ml.Bit cromkt tinh dng tinh thlp phng.


41/45


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Trang 42

(g) Hy cho bit c phn ng no xy ra khi pH ca dung dch cha cromat, Cr(III) v hidropeoxit c gi trbng 0 v bng 14? Tnh sut in ng chun ca cc phn ng .

(a. TH1: mng lp phng n gin:

cmpmRa 810.52,225212622

=> 33823 /40,5)10.52,2(10.022,6

0,521 cmgamd

=> khng hp l

TH2: mng lp phng tm mt:

cmpmR

a 810.564,34,356

2

1264

2

4

=> 33823

/63,7)10.564,3(10.022,6

0,524 cmgamd

=> khng hp l

TH3: mng lp phng tm khi:

cmpmR

a 810.91,22913

1264

3

4

=> 33823 /01,7)10.91,2(10.022,6

0,522 cmgamd

=> hp l;

Vy crom kt tinh theo kiu lp phng tm khi.

b. CrO2Cl2, cromyl clorua;

c. 2 CrO42+ 2 H+ Cr2O7

2 + H2O

3 CrO42+ 4 H+ Cr3O10

2 + 2 H2O

4 CrO42+ 6 H+ Cr4O13

2 + 3 H2O

CrO42+ 2 H+ CrO3 + H2O

Cu trc ca cc ion:


43/45

Trang 43

d. Cr2+ Cr Eo= 0,9V;

Cr(V) Cr(IV) Eo= 1,35V;

Cr2O72 Cr(III) Eo= 1,33V;

e.

iu kin: E2> E1.

CCr(V) v Cr(IV) u tphn hy dly.

3 Cr2+ 2 Cr3+ + Cr 170592,0)42,09,0(2

10.08,610

K

f. Cr(OH)3+ 3e Cr + 3 OHEo= 1,33V

Cr3+ + 3e Cr Eo= 0,74V

Cr(OH)3 Cr

3+

+ 3 OH

T = 1,26.10

30

Cr(OH)4+ 3e Cr + 4 OH Eo= 1,33V

Cr3+ + 3e Cr Eo= 0,74V

Cr3+ + 4 OH Cr(OH)4

= 7,92.1029

g. pH = 0:

Cr2O72 + 3 H2O2 + 8 H

+ 2 Cr3+ + 3 O2 + 7 H2O

Eo=1,330,695 = 0,635 V

pH = 14:

2 Cr(OH)4 + 3 HO2

2 CrO42 + 5 H2O + OH

Eo= 0,87(0,72) = 1,59 V)

Cu 5.7. Ngi ta tin hnh tch ion bari v srontri ra khi nhau da vo tan khc nhau ca muicromat. Nng cromat trong dung dch c th iu chnh bng cch thay i pH.

2 H+ + 2 CrO42 Cr2O7

2 + H2O K = 1,50 1015

Cho cc gi tr tch s tan: TBaCrO4

= 8,5 1011; TSrCrO4

= 3,6 105.

(a) Hy tnh tan ca BaCrO4v SrCrO4trong mi trng baz mnh.


44/45

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(b) pH ca dung dch K2Cr2O70,10 mol/lit c iu chnh n 3,00 bng cch s dng m axetat.Hy tnh nng Cr2O7

2v CrO42trong dung dch thu c.

Thm vo hy tnh gi tr nng nh nht ca Ba2+v Sr2+trong dung dch ny kt ta cromattng ng bt u xut hin.

(c) iu ch dung dch m ngi ta s dng dung dch axit axetic

0,10 mol/lit. Hy tnh khi lng natri axetat cn thm vo 1 lt dung dch trn thu c dung dch m cpH = 3,00. Bit hng s phn li ca axit axeticKa = 1,78 10

5.

Di y l gin Latimer ca crom trong mi trng axit (pH = 0).

Cr2O72- Cr(V) Cr(IV) [Cr(H2O)6]

3+ [Cr(H2O)6]2+ Cr

+0,293

+0,55 +1,34 x -0,408 y

da cam lctmxanh

-0,744

(d) Hy xc nh cc gi tr th x v y.(e) Hy cho bit Cr(IV) c b phn hy d ly to ra Cr(III) v Cr(VI) khng? Gii thch.

(f) Hy vit bn phn ng kh ca cp Cr2O72 /Cr3+. Hy tnh s gim th kh ca cp oxi ha kh

ny khi pH tng thm 1 n v.

Xt phc mu tm [CrCl2(ox)2]3.

(g) Hy cho bit s phi tr ca phc trn.

(h) Hy cho bit hnh hc phn t ca phc.

(i) Hy gi tn ca ion phc.

(k) Hy vit cc ng phn lpth ca phc trn.(a. Trong mi trng kim th CrO4

2b thy phn khng ng k:

BaCrO4 Ba2+ + CrO4

2

S S

=> MTS BaCrO611 10.22,910.5,8

4

=> MTS SrCrO35 10.610.6,3'

4

b. 2 H+ + 2 CrO42 Cr2O7

2 + H2O K = 1,50 1015

C: [Cr2O72] + 0,5 [CrO42]= 0,1

15

22

4

2

2

72 10.50,1][][

][

CrOH

OCrK

=> [Cr2O72] = 0,1 M

[CrO42] = 8,16.106M

kt ta BaCrO4bt u xut hin:

MCrO

TBa

BaCrO 5

6

11

2

4

2 10.04,110.16,8

10.5,8

][][ 4


45/45

kt ta SrCrO4bt u xut hin: MCrO

TBa

BaCrO41,4

10.16,8

10.6,3

][][

6

5

2

4

2 4

c.][][

][][][

OHHC

OHHCKH

B

Aa

C: [OH] CB= 7,8.104M

=> mCH3COONa= 7,8.10482 = 0,064 gam

d. x = 2,1 V

y = 0,912V

e. C: VEIVCrOCr 945,00

)(/272

=> 0)(/)(

0

)(/272 IIICrIVCrIVCrOCr

EE

=> phn ng phn hy d li t xy ra.

f. Cr2O72 + 6e + 14 H+ + 5 H2O 2 Cr(H2O)6

3+

VECrOCr 33,10

/ 3272

C:

][

]][[lg

6

0592,03

142

720

// 32

7232

72

Cr

HOCrEE

CrOCrCrOCr

Xt: [Cr2O72] = [Cr3+] = 1M; [H+] = 0,1 M

=> VECrOCr

19,11,0lg6

0592,033,1 14

/ 3272

=> Epin= 1,19 1,33 = 0,14 V

Vy khi pH tng thm 1 n v th th ca cp gim 0,14V;

g. 6;

h. bt din;

i. icloro bisoxalato cromat(III)k.

O

O O

Cl

Cl

O

CrO

O O

O

Cl

Cl

Cr

3- 3-

O

OO

Cl

Cl

O

Cr

3-

)

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