1. Electrolysis 3.7 Electrolysis…

2. Electrolysis Use of electrical energy to produce chemical change...forcing a current through a cell to produce a chemical change for which the cell potential is negative. – Running a galvanic cell backwards. – Put a voltage bigger than the potential and reverse the direction of the redox reaction. – Used for electroplating.

3. Electrolysis The use of electrical energy to bring about chemical change – A voltaic cell where you add energy so that it turns a product-favored reaction into a reactant favored reaction – Examples: Electrolysis of water Electroplating of metals – Object to be plated is the cathode

4. 1.0 M Zn +2 e-e- e-e- Anode Cathode 1.10 Zn Cu 1.0 M Cu +2

5. 1.0 M Zn +2 e-e- e-e- Anode Cathode A battery >1.10V Zn Cu 1.0 M Cu +2

6. Voltaic Cells and Electrolysis Cells Type of CellElectrodeFunctionPolarity VoltaicAnodeOxidation- CathodeReduction+ ElectrolysisAnodeOxidation+ CathodeReduction-

7. Electrolysis Electrolytic reactions typically occur in liquid (molten) or aqueous solutions – If there is no water present, you have a pure molten ionic compound thus The cation will be reduced The anion will be oxidized – If water is present, you have an aqueous solution of the ionic compound, thus Must decide which species is being oxidized and reduced; the salt ions or the water! No alkali or alkaline earth metal can be reduced in an aqueous solution – water is more easily reduced Polyatomic ions are typically NOT oxidized in an aqueous solution – water is more easily oxidized

8. Electrochemical Electrolytic Spontaneous Nonspontaneous Energy released Energy absorbed Cu 2+ (aq) + Mg (s) Cu (s) + Mg 2+ (aq) Electrochemical cell -- chemical energy to electrical energy. Electrolytic cell -- electrical energy to chemical energy.

10. Answer

11. More Calculate ∆G for the reaction. – ∆G = -nFE – ∆G = -(2)(96,500)(-1.36) – ∆G = 262kJ Is the reaction spontaneous or non- spontaneous? Justify your answer. – E⁰ is negative and ∆G is positive. Reaction is non- spontaneous

12. Counting Electrons Current flowing in an electrical circuit is the amount of charge (in unit of coulomb, c) per unit time (s). – Unit for current: A Current = electric charge / time A = c/s

13. Stoichiometry of Electrolysis -How much chemical change occurs with the flow of a given current for a specified time?     Current (A) x time (s) Charge (c) use Faraday’s Faradays (mol e - ) Refer to balanced equation Amount reactant (or product) Mass (g) reactant (or product)

14. Stoichiometry of Electrolysis iT = nFN i = current (c/s) T = time (sec) n = # of electrons per reduction F = Faraday’s constant N = number of moles of material reduced

15. Stoichiometry of Electrolysis Calculate the mass of aluminum metal which can be produced by the electrolysis of molten AlCl 3 if a current of 10.0 amp is used for 1 hour. (10.0 C / s )(3600s) = (3)(96500 c / mole e - )N N = 0.124 mols Al 0.124mol x 26.9815g/mol = 3.35g Al

16. Practice How many grams of copper can be plated out when a current of 10.0 amps is passed through a Cu 2+ solution for 30.0 minutes? (10.0 c / s )(1800s) = (2)(96500 c / mole e - )N N = 0.0933 mol Cu 0.0933mol x 63.456g/mol = 5.92 g

17. Practice A current of 2.40A is passed through a solution containing Cu +2 (aq) for 30.0 minutes, with copper metal being deposited at the cathode. What mass of copper, in grams, is deposited? – (2.40 C / s )(1800s) = (2)(96500 c / mole e - )N – N = 0.0224 mol Cu – 0.0224 mol x 63.456g/mol = 1.42g Cu