ELECTROCHEMICAL CELLS

K Warne

Electrochemical Cell.

Two metals can react indirectly.

Cu2+ + 2e- Cu

Zn2++ 2e- --> Zn

Cu2+ + 2e-

--> CuAttract elelctons strongly

Attract elelctons weakly

OXIDISING AGENT

REDUCING AGENT

Top right reacts

with bottom left.

_

_

_

_

_

Zn

_

_

Cu

V

_

KNO3(aq)

+

>

Zn Zn2+ + 2e-

CuSO4ZnSO4

Salt bridge

anodecathode

Zn + Cu2+ Zn2+ + Cu

oxidation reduction

REDUCING AGENT OXIDISING AGENT

e-

WORKED EXAMPLES

1. The redox couples Mn2+(aq) Mn(s) and U3+(aq) U(s) have

Eθ values of –1.18 and –1.79 respectively. Determine Eθcell

when they are combined and write an equation for the

reaction that takes place.

Eθcell = Eθ

O.A. - EθR.A. = -1.18 – (-1.79) = +0.61v

3Mn2+(aq) + 2U(s) 3Mn(s) + 2U3+(aq)

2. The redox couples Co2+(aq) Co(s) and Ni2+(aq) Ni(s)

have Eθ values of –0.28v and –0.23v respectively. Write

an equation for the spontaneous reaction and use the Eθ

values to justify your answer.

Co(s) + Ni2+(aq) Co2+(aq) + Ni(s)

Eθ = EθO.A - Eθ

R.A = -0.23 – (-0.28) = +0.05v

Ni2+(aq) + Co(s) Co2+(aq) + Ni(s)

Electrolytic cell: Converts electrical energy to chemical energy.

Copper chloride

CuCl2

+-

Cu2+

Cl-

Cu2+(aq)+ 2e-

Cu(s)

REDUCTION

CATHODE

2Cl-(aq) Cl2(g) + 2e-

OXIDATION

Anode

Electrolysis – Electrolytic Cell

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STANDARD HYDROGEN ELECTRODE

……….. electrode

H2 gas pressure of ……….

(………. kPa)

Solution of …..M H+ ions

(HCl)

…………K

Eθ = ……………v

S.H.E. is ………. to set up.

……………….. standard

electrodes used.

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DETERMINATION OF CELL POTENTIALS

Eθ values are determined using a ……………………………… electrode.

The S.H.E is always the …………………. electrode.

Standard Conditions : ………………………………………………….

A secondary standard electrode the ………………. electrode can be used.

Eθcell= …………………………………………

Eθcell= ………………. – ……….

N+(aq)

N+(s)

Unknown electrodeStandard electrode

High resistance voltmeter

Salt bridge

If a cell potential of

1.45v is obtained

with metal N. Then

the electrode

potential of N+ /N is

..................................

..................................

..................................

..................................

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Electrolysis in aqueous solutions

Na+ + e- Na -2.71

Mg2+ + 2e- Mg -2.37

2H2O + 2e- H2 + 2OH - -0.83

Cu2+ + 2e- Cu +0.34

H2 + 2e- 2H+

SO42- + 4H+ + 2e- SO2 +0.17

H2O + O2 + 2e- 4OH- +0.40

Cl2 + 2e- 2Cl- +1.36

+

-Cathode: Reduction

Cu2+ + 2e- Cu

Anode: Oxidation

Cl2 + 2e- 2Cl-

The most

positive system

present will

react at the

cathode

?Which ions will

react at which

electrodes?

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Cathode

( )

Copper

chloride

CuCl2

+-

Cu2+

Cl-

Cu2+(aq)+ 2e-

Cu(s)

REDUCTION

CATHODE

(-)

2Cl-(aq) Cl2(g) + 2e-

OXIDATION

Anode

(+)

_

_

_

_

_

Zn

_

_

Cu

V

KNO3(aq)

+

>

Zn Zn2+ + 2e-

CuSO4

ZnSO4

Salt

bridge

Anode

(-)

Zn + Cu2+ Zn2+ + Cu

oxidation reduction

REDUCING

AGENTOXIDISING

AGENT

e-

_

Cu2+ + 2e- Cu

Chemical Energy Electrical Energy

+e-

Electrochemical Cells

Voltaic (Galvanic) vs Electrolytic

Electrical

Energy

Chemical

Energy

Electrical Energy Chemical Energy

Electrical

Energy

Chemical

Energy

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Electrolysis of aqueous solutions (MX)

(H2O)

M+

(Metal ion)

X-

(Non-Metal

ion)

- +

If the metal is more positive than

hydrogen, its ions will be reduced.

M+ + e M

If halide ions (X-) are present (more

positive than OH- ) ions then the

halide ions (X-) are oxidised.

X- X2 + 2e-

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Electrolysis - Lead Bromide

PbBr2

PbBr2 (Heat) Pb2+ + Br-

Pb2+(l) + 2e-

Pb(l) 2Br - (l) Br2(g) + 2e-

Pb2+(l) + 2Br -(l) Pb(l) + Br2(g)

+-

electrons

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Electrolysis – Sodium chloride

2H2O + 2e- H2 + 2OH- 2Cl- Cl2 + 2e-

2H2O + 2Cl- + (2Na+) Cl2 + H2 + 2OH- + (2Na+)

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Hi…

This is a SAMPLE presentation only.

My FULL presentations, which contain a lot more slides and other resources, are freely available

on my resource sharing website:

www.warnescience.net(paste into your browser if link above does not work)

Have a look and enjoy!

Keith Warne

WarneScience