ELECTROLYSIS

molten OUTCOME QUESTION(S): C12-6-09 CELLS AND POTENTIALS Explain the operation of a voltaic (galvanic) cell at the visual, particulate and symbolic level. Include: writing half-cell reactions and overall reaction Calculate standard cell potentials and predict spontaneity of reactions. Compare and contrast voltaic (galvanic) and electrolytic cells. Vocabulary & Concepts molten

Process of using electricity to force a nonspontaneous reaction Electrolysis: “electro” – electricity “lysis” – break Electrolytic cells Process of using electricity to force a nonspontaneous reaction Different: it is nonspontaneous so the opposite metals are going to be reduced and oxidized from a voltaic cell A battery is needed to add energy and it must be attached to inert electrodes Different: inert electrodes do not take part in reaction – don’t “plate out” or “lose mass”

Anions migrate to (+) anode and are oxidized Inert electrodes must be charged to attract the appropriate ions to the appropriate side Different: electrodes oppositely charged Same: e- still flow from anode to cathode Same: anode oxidized, cathode reduced Anions migrate to (+) anode and are oxidized Cations migrate toward (-) cathode to be reduced

1. Molten Electrolytic Cells most metals occur naturally as compounds NaCl , Cu2O, TiO2 , CuF2 Unwanted elements of a compound are called “impurities” forcing the non-spontaneous redox is a common process for obtaining pure elements Need very high temperatures to make salts molten - “melt”

Na+(l) + Cl–(l) → Na (s) + Cl2(g) Down's Cell: Na+(l) + Cl–(l) → Na (s) + Cl2(g) 2 Cl-(l) → Cl2(g) + 2e- 2 Na+(l) + 2e-→ 2 Na(s) EoO = -1.36 V Notice: stronger element – Cl is oxidized EoR = -2.71 V Notice: weaker element – Na is reduced Eocell = - 4.07 V PURE PURE A battery must supply 4.07 V of power to make this electrolytic cell work

Electrolysis of water: Water can be both reduced and oxidized Oxidation: 2 H2O(l) → 4 H+(aq) + O2(g) + 4e– Reduction: 4 H2O(l) + 4e– → 4 OH–(aq) + 2 H2(g) Net: 6 H2O(l) → 2 H2(g)+ O2(g) + 4 H+(aq) + 4 OH–(aq) 2 These ions recombine to form 4 H2O Overall: 2 H2O(l) → 2 H2(g)+ O2(g)

2. Aqueous Electrolytic Cells: 3 possible reactants that will compete for electrons - cations, anions and water Because water is present – and it can oxidize and reduce – you might not get what you expected in the reaction… Must predict which substance is most likely to be oxidized and which is reduced This is done by comparing the oxidation and reduction potentials of all possible reactants

Possible reactants: K+ I– H2O What are the products formed at each electrode during the electrolysis of aqueous KI? Possible reactants: K+ I– H2O K+ cannot lose more electrons: Only I – and H2O can be oxidized Oxidization: 2 I–(aq) → I2(s) + 2e– E°o = – 0.54V H2O(l) → 2 H+(aq) + ½ O2(g) + 2e– E°o = –1.23V I- has most positive oxidation potential solid iodine is formed at the anode

Overall: 2 H2O(l) + 2 I–(aq) → H2(g) + I2(s) Possible reactants: K+ I– H2O I- cannot gain more electrons: Only K+ and H2O could be reduced Reduction: K+(aq) + 1e– → K(s) E°R = – 2.93V 2 H2O(l) + 2e– → 2 OH-(aq) + H2(g) E°R = – 0.83V H2O has most positive reduction potential hydrogen gas is formed at the cathode Overall: 2 H2O(l) + 2 I–(aq) → H2(g) + I2(s) Spectator ions created in the reaction would be K+ and OH-

object to be plated – cathode desired metal – anode Electroplating: Battery used to plate layer of metal onto another surface by reducing its own ions. object to be plated – cathode desired metal – anode immersed in a solution of same metal ions Only one element – the metal is oxidized at the anode, then reduces itself at the cathode item

reaction of metals with oxygen - spontaneous Corrosion: reaction of metals with oxygen - spontaneous metal – oxidized oxygen – reduced Most metals have reduction potentials below oxygen – except gold Sacrificial anode – second “weaker” element that sacrifices its electrons and oxidizes to save metal

molten CAN YOU / HAVE YOU? C12-6-09 CELLS AND POTENTIALS Explain the operation of a voltaic (galvanic) cell at the visual, particulate and symbolic level. Include: writing half-cell reactions and overall reaction Calculate standard cell potentials and predict spontaneity of reactions. Compare and contrast voltaic (galvanic) and electrolytic cells. Vocabulary & Concepts molten