Electrolytic Cells Lesson 9 Electrolytic Cells

Electrolytic CellsLesson 9Electrolytic Cells

Notes on Electrolytic Cells

An electrolytic cell is a system of two inert (nonreactive) electrodes (C or Pt) and an electrolyte connected to a power supply. It has the following characteristics 1.Nonspontaneous redox reaction 2.Produces chemicals from electricity 3.Forces electrolysis to occur

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

1. Draw and completely analyze a molten NaBr electrolytic cell. PtPtElectrons go from anode to cathode. Na+

Br-_reductioncathode2Na+ + 2e- 2Na(l)-2.71 v

+oxidationanode2Br- Br2(g)+ 2e--1.09 v

2Na+ + 2Br- Br2(g) + 2Na(l)E0 = -3.80 v MTV = +3.80 vThe negative is reductionThe positive is oxidationThe MTV is the minimum theoretical voltage required to start a reaction Molten or (l) means ions but no watercations to cathodeanions to anode

What do you do if there is water in the cell?

Reduction of waterWater will reduce before anything below this line. Nothing below this line can reduce in aqueoussolutionPb2+ reduces in waterWater reduces in a K+ solutionTreat as if it were here

Reduction

If you have water present (aq) 1.0 Msolution

Take the higher reaction The strongest oxidizing agent Consider the overpotential effect

Oxidation of waterTreat as if hereBr- (lower) oxidizes before water Water (lower) oxidizes before F-

Oxidation

If you have water present (aq) 1.0 Msolution

Take the lower reaction The strongest reducing agent Consider the overpotential effect

1. Draw and completely analyze an aqueous KI electrolytic cell. PtPtK+H2OI-The negative is reductionThe positive is oxidationCationor waterReductionCathodeConsonants

highestFor Reduction take the highest

1. Draw and completely analyze an aqueous KI electrolytic cell. PtPtK+H2OI-The negative is reductionThe positive is oxidationAnionor waterOxidationAnodeVowels

lowestFor Oxidation take the lowest

1. Draw and completely analyze a molten KI electrolytic cell. PtPtElectrons go from anode to cathode. K+H2OI-_reductioncathode2H2O+2e- H2(g) + 2OH--0.41 v

+oxidationanode2I- I2(s) + 2e--0.54 v

2H2O + 2I- H2 + I2(s) + 2OH- E0 = -0.95 vMTV = +0.95 vThe negative is reductionThe positive is oxidationcations to cathodeanions to anode

The overpotential effect is a higher than normal voltage required for the half reaction.

This is often due to extra voltage required to produce a gas bubble in solution.