Electric energy Chemical energy

Electrolysis

Galvanic cell

Chapter 8 Electrochemistry

Electrochemistry• Study of chemical reactions that can produce

electricity or use electricity to produce desired product.

• Study of interchange of chemical and electrical energy

• Electrochemical reaction always involves oxidation-reduction reactions– Electron transfer reactions– Electrons transferred from one substance to another

• Also called redox reactions

2

Galvanic Cells

The Daniell Cell

Flow of Zn2+

Flow of SO42-

Half-cell

Half-cell reaction

5

• Needed to complete circuit• Tube filled with solution of an electrolyte– Salt composed of ions not involved in cell reaction– KNO3 and KCl often used

• Porous plugs at each end of tube – Prevent solution from pouring out

– Enable ions from salt bridge to migrate between half-cells to neutralize charges in cell compartments• Anions always migrate toward anode • Cations always migrate toward cathode

Salt Bridge Salt bridge

Anode compartment

Half-cell

Half-reaction:

Oxidation

Cathode compartment

Half-cell

Half-reaction:

Reduction

Zn(s) → Zn2+(aq) +2e- Cu2+

(aq) + 2e- → Cu(s)

Cu2+(aq) + 2e- → Cu(s)

Cu2+(aq) + Zn(s) → Cu(s) + Zn2+

(aq) total cell reaction

2e-

7

Cell Notation

Cu(s)|Cu2+(aq)||Ag+(aq)|Ag(s)

• Single slash = boundary between phases (solid electrode and aqueous solution of ions)

• Double slash represents salt bridge– Separates cell reactions

• In each half (half-cell)– Electrodes appear at outsides– Reaction electrolytes in inner section– Species in same state separated with ;– Concentrations shown in ( )

anode cathode

anodeelectrode

anodeelectrolyte

cathodeelectrolyte

cathodeelectrode

Salt Bridge

8

Learning Check• Write the standard cell notation for the

following electrochemical cells:• Fe (s) + Cd2+ (aq) Cd(s) + Fe2+(aq) Anode = ox = Fe(s) Cathode = red = Cd2+(aq) Fe(s)|Fe2+(aq)||Cd2+(aq)|Cd(s) • Al(s) + Au3+(aq) Al3+(aq) + Au(s) Anode = ox = Al(s) Cathode = red = Au3+(aq) Al(s)|Al3+(aq)||Au3+(aq)|Au(s)

Your Turn!Write the standard cell notation (Pt electrodes) for the following reaction:

2Mn3+(aq) + 2I-(aq) → Mn2+(aq) + I2(s)

A. Pt(s)|Mn3+(aq); Mn2+(aq)||I-(aq)|I2(s)|Pt(s)

B. Pt(s)|I-(aq)|I2(s)||Mn3+(aq); Mn2+(aq)|Pt(s)

C. Mn3+(aq)|Pt(s); Mn2+(aq)||I-(aq)|I2(s)|Pt(s)

D. Pt(s)|Mn3+(aq); I-(aq)||Mn2+(aq)|I2(s)|Pt(s)

Oxidation reaction is on the right and reduction reaction is on the left of

the salt bridge (||).

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Reaction can be performed without

harnessing electricity!

DG of reaction: maximum work over and above volume work (electricity) that can be harnessed from the chemical reaction.

Cu wire is dipped into Zn2+ solution: nothing happens.

Cu wire is dipped into Ag+ solution:

Ag+ has higher tendency to be reduced than Cu2+.Cu2+ has higher tendency to be reduced than Zn2+.

2Ag+(aq) + Cu(s) → 2Ag(s) + Cu2+

(aq)

Work harnessed!!

Ag+ has higher tendency to be reduced than Cu2+.Cu2+ has higher tendency to be reduced than Zn2+.

How can we know?

Electrode PotentialReflects tendency towards reduction

Problem: Only potential difference can be measured between two half-cells.

Hydrogen Standard Electrode

This electrode used as standard.EMF of all other electrodes measured with reference to this electrode.

H2(g) → 2 H+(aq) +2e-

Standard Reduction PotentialSHE as anode (Oxidation).The other electrode cathode (Reduction).

H2(g) → 2 H+(aq) +2e- Cu2+

(aq) + 2e- → Cu(s)

Voltage: Potential difference

Measured voltage =

Potential of reduction electrode

- Potential of anode electrode

H2(g) → 2 H+(aq) +2e-

Cu2+(aq) + 2e- → Cu(s)

Cu2+(aq) + H2(g) → Cu(s) + 2H+

(aq)

Spontaneousat

Standard conditions

ReductionPotential

Cu2+(aq) + 2e- → Cu(s) Eo

red =0.34 V

H2(g) → 2 H+(aq) +2e- Zn2+

(aq) + 2e- → Zn(s)

Zn2+(aq) + H2(g) → Zn(s) + 2H+

(aq)

Zn(s) → Zn2+(aq) +2e-

2 H+(aq) +2e- → H2(g)

Zn(s) + 2H+(aq) → Zn2+

(aq) + H2(g)

ReductionPotential

Zn2+(aq) + H2(g) → Zn(s) + 2H+

(aq) nonspontaneous

Electricity must be applied to force this process to take place!

Zn2+(aq) + 2e- → Zn(s) Eo

red = -0.76 V

Cu2+(aq) + 2e- → Cu(s)

Zn(s) → Zn2+(aq) +2e-

Cu2+(aq) + Zn(s) → Cu(s) + Zn2+

(aq)

Electrochemical thermodynamics

DG of reaction: maximum work over and above volume work (available work) (electricity) that can be harnessed from the chemical reaction.

maxmax

electricalVp

non wworkG

Electrical heater Heating elementsresistive

EFzwG

EV

FzVw

FzQ

CueCu

reactionsmicalelectrochefor

QVtIVw

el

el

el

..

..

.

2

...

2

Work done

c

o Kz

E log0591.0

0591.0log

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c

EzK

Concentration Cells

• Consider the cell presented on the left.

• The 1/2 cell reactions are the same, it is just the concentrations that differ.

• Will there be electron flow?

Concentration Cells (cont.)

Ag Ag+ + e- -E1/2 Anode:

Ag+ + e- Ag E1/2 Cathode:

Q Ag

anode

Ag cathode

0.1

10.1

Ecell = E°cell - (0.0591/n)log(Q)0 V

Ecell = - (0.0591)log(0.1) = 0.0591 V

1

Concentration Cells (cont.)

Another Example:

What is Ecell?

Concentration Cells (cont.)

Ecell = E°cell - (0.0591/n)log(Q)0

Fe2+ + 2e- Fe2 e- transferred…n = 2

2

Q Fe2

anode

Fe2 cathode

0.01

.10.1

Ecell = -(0.0296)log(.1) = 0.0296 V

anode cathode

e-