1

Electrolytic CellsElectrolytic Cells•Voltaic cells are spontaneous with E > 0.•What’s going on in a voltaic cell?•A voltaic cell converts chemical energy into electrical energy and electrical work.•A current will be generated until the cell reaches equilibrium at E = 0.•For batteries, when E=0, the battery has died!

2

Electrolytic CellsElectrolytic Cells

• But what if we want a nonspontaneous redox rxn to occur?

• Can we force a cell to run backwards?• We can if we run an electrical current through

the cell!• So we are doing work on the cell.• Now we are converting electrical energy into

chemical energy.

3

Electrolytic CellsElectrolytic Cells

• When we run a current through a cell to force it to run in reverse, we have made an electrolytic cell.

• These redox rxns are called electrolysis.• There are a few differences between galvanic

cell notation and electrolytic cell notation.• Let’s look at an electrolytic cell for the

following rxn:NaCl(l) --> Na(l) + Cl2(g)

4

An Electrolytic CellAn Electrolytic Cell

-+

5

Downs Cell for Making Na and ClDowns Cell for Making Na and Cl22

6

Cell for Making NaOH and ClCell for Making NaOH and Cl22

7

Electrolytic CellsElectrolytic Cells• What did you notice that looks different than a

voltaic cell?• Are there 2 compartments?

– Although electrolytic cells may have 2 compartments, they don’t always need to have the anode and cathode half-cells separated.

• What about the anode and cathode signs? Now the cathode is - and the anode is +.

• Electrons are being forced from the anode to the cathode. So the anode is +.

8

Electrolytic Cells and StoichiometryElectrolytic Cells and Stoichiometry

• Time for math!!!• We can calculate the electrical charge (in C) or

current (amp) required to produce a desired amount of product.

• We can also calculate how much product can be made from a given amount of charge or current!

9

Electrical Work and Voltaic and Electrical Work and Voltaic and Electrolytic CellsElectrolytic Cells

• In a galvanic cell, work is produced, while in an electrolytic cell, work is required.

• We can calculate the amount of work or power required or produced.

• The work is directly related to the free energy ΔG; and the power is just energy per time.

10

Electrical Work in Voltaic CellsElectrical Work in Voltaic Cells

• In a galvanic cell, work is defined as:

• Where wmax is the maximum amount of work which can be produced (assumes 100% efficiency).

• The sign of wmax is negative as E > 0, so work is produced.

wmax = ΔG = -nFE

11

Electrical Work in Electrolytic CellsElectrical Work in Electrolytic Cells

• Now we apply an external electric potential, Eext, to force the redox rxn to occur.

• Therefore, the external potential must be greater than the cell potential, Ecell. Why?

• So work is performed on the cell by the surroundings.

Eext > Ecell

12

Electrical Work in Electrolytic CellsElectrical Work in Electrolytic Cells

• Now work is flowing into the cell, so the work is positive.

• Note that we use the external cell potential, Eext, to calculate the work.

• After all, this is the external work put into the cell.

w =wext= nFEext

13

Units for Electrical WorkUnits for Electrical Work

• Now you have the equations to solve for work, with units of J or kJ.

• But we usually like to express electrical work in terms of power or watts.

• Remember:

1 W = 1J/s

OR

1J = 1W•s

14

Units for Electrical WorkUnits for Electrical Work

• How is your electric bill stated?

kW•hr or just kWh• How many J are in a kWh?• Since a W = 1J/s or 1J = 1W•s, we can do this

conversion easily:

1kW • hr( )1000W1kW

⎛⎝⎜

⎞⎠⎟

3600s1hr

⎛⎝⎜

⎞⎠⎟= 3.6 ×106 W• s

= 3.6 ×106 J = 3600 kJ

15

Electrical Work and PowerElectrical Work and Power

• So now you can calculate electrical power and work!

• Time for more math!

16

The Good and the BadThe Good and the Bad

• We can use redox rxns to perform useful electrical work.

• But redox rxns also have some bad aspects!

17

18

19

20

21

22

23