Electrolytic Cells

1. •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! Electrolytic Cells

2. Electrolytic 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 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 Cell + -

5. Downs Cell for Making Na and Cl2

6. Cell for Making NaOH and Cl2

7. Electrolytic 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 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 Electrolytic 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 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.

11. Electrical 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.

12. Electrical 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.

13. Units 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 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:

15. Electrical Work and Power • So now you can calculate electrical power and work! • Time for more math!

16. The Good and the Bad • We can use redox rxns to perform useful electrical work. • But redox rxns also have some bad aspects!