Electrolytic Cells Lesson 8 Electrolytic Cells

ELECTROLYSIS Electrolysis is a method of using a direct electric current (DC) to drive an otherwise non- spontaneous chemical reaction. Electrolysis is commercially highly important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell

Electrolytic Cells Characteristics … 1.Nonspontaneous redox reaction 2.Produces chemicals from electricity 3.Forces electrolysis to occur An electrolytic cell is a system of two inert (nonreactive) electrodes (C or Pt) and an electrolyte connected to a power supply. It is just ONE cell…

Electrolytic Cell Oxidation always occurs at the anode and reduction at the cathode Electrons flow through the wire and go from anode to cathode Anions (- ions) migrate to the anode and cations (+ions) migrate towards the cathode. The electrode that is connected to the -ve terminal of the power supply will gain electrons and therefore be the site of reduction. Oxidation is connected to +ve terminal.

For electrolysis to work, you need a liquid sample: 1) molten cells Melt the crystals (produce a "molten" sample ) This is the only way to electrolyse insoluble salts, and is the only way to produce, by electrolysis, pure metals. 2) aqueous cells Dissolve the ionic substance in water (this is the most common)

Type 1 electrolytic cell Inert electrodes immersed in a molten ionic compound

1. Draw and completely analyze a molten NaBr electrolytic cell. Pt DC Power Source - + DC Power Source - + Electrons go from anode to cathode. Na + Br - _ reduction cathode 2Na + + 2e - → 2Na (l) v + oxidation anode 2Br - → Br 2(g) + 2e v 2Na + + 2Br - → Br 2(g) + 2Na (l) E 0 = v e-e- e-e- MTV = v The negative is reduction The positive is oxidation The MTV is the minimum theoretical voltage required to start a reaction Molten or (l) means ions but no water cations to cathode anions to anode

Type 1 electrolytic cell Inert electrodes immersed in a molten ionic compound Things are reversed…meaning: The oxidation half reaction is ABOVE the reduction half-reaction! Non-spontaneous…

Type 2 electrolytic cell Inert electrodes immersed in an aqueous ionic compound Things are different now, you have water to consider! Soooo…what do you do if there is water in the cell?

Reduction of water Water will undergo reduction above anything that is below this line. Below this line, they will undergo oxidation Pb 2+ undergoes reduction in water Water will undergo reduction in a K + solution Treat as if it were here

Reduction If you have water present (aq) 1.0 Msolution Consider the overpotential effect Take the higher reaction on the left The strongest oxidizing agent

Oxidation of water Treat as if here Br - (lower) will undergo oxidation before water Water (lower) undergoes oxidation before F -

Oxidation If you have water present (aq) 1.0 Msolution Consider the overpotential effect Take the lower reaction on the right side of the table The strongest reducing agent

1. Draw and completely analyze an aqueous KI electrolytic cell. Pt DC Power Source - + DC Power Source - + K+H2OI-K+H2OI- The negative is reduction The positive is oxidation Cationor water Reduction Cathode Consonants

highest For Reduction take the highest

1. Draw and completely analyze an aqueous KI electrolytic cell. Pt DC Power Source - + DC Power Source - + K+H2OI-K+H2OI- The negative is reduction The positive is oxidation Anionor water Oxidation Anode Vowels

lowest For Oxidation take the lowest

1. Draw and completely analyze an aqeuous KI electrolytic cell. Pt DC Power Source - + DC Power Source - + Electrons go from anode to cathode. K+H2OI-K+H2OI- _ reduction cathode 2H 2 O+2e - → H 2(g) + 2OH v + oxidation anode 2I - → I 2(s) + 2e v 2H 2 O + 2I - → H 2 + I 2(s) + 2OH - E 0 = v e-e- e-e- MTV = v The negative is reduction The positive is oxidation cations to cathode anions to anode

Overpotential: voltage actually required to drive electrolytic cells. Overpotential effect : a higher than normal voltage required for the half reaction. It can be caused by different reasons. In aqeous solutions, it is due to extra voltage required to produce a gas bubble such as (O 2 ) Final comments on electrolytic cells: Just like an electrochemical cell, they depend on the thermodynamic data of the reaction but ALSO…. They depend on the kinetic energy, activation energy and localized concentrations of the reactions involved and different designs of cells.

Homework Page ( a and c),66, 67, 70