1 Electrolytic 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 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) + Cl 2 (g)

4 An Electrolytic Cell An Electrolytic Cell -+

5 Downs Cell for Making Na and Cl 2 Downs Cell for Making Na and Cl 2

6 Cell for Making NaOH and Cl 2 Cell for Making NaOH and Cl 2

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 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 Δ G; and the power is just energy per time.

10 Electrical Work in Voltaic Cells In a galvanic cell, work is defined as: Where w max is the maximum amount of work which can be produced (assumes 100% efficiency). The sign of w max is negative as E > 0, so work is produced.

11 Electrical Work in Electrolytic Cells Now we apply an external electric potential, E ext, to force the redox rxn to occur. Therefore, the external potential must be greater than the cell potential, E cell. 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, E ext, 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 = 1Ws

14 Units for Electrical Work How is your electric bill stated? kWhr or just kWh How many J are in a kWh? Since a W = 1J/s or 1J = 1Ws, 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!

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