Electrolytic CellsLesson 9

Electrolytic 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 Cell The 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.

PtPt

DC Power Source

- +DC Power Source

- +

Electrons 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

e-

e-

MTV = +3.80 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 cathodeanions to anode

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

Reduction of water

Water will reduce before anything below this line. Nothing below this line can reduce in aqueoussolution

Pb2+ reduces in water

Water reduces in a K+ solution

Treat as if it were here

Reduction

If you have water present (aq) 1.0 M solution

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

Oxidation of water

Treat as if here

Br- (lower) oxidizes before water

Water (lower) oxidizes before F-

Oxidation

If you have water present (aq) 1.0 M solution

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

1. Draw and completely analyze an aqueous KI electrolytic cell.

PtPt

DC Power Source

- +DC Power Source

- +

K+

H2OI-

The negative is reduction

The positive is oxidation

Cation or water

Reduction

Cathode

Consonants

highest

For Reduction take the highest

1. Draw and completely analyze an aqueous KI electrolytic cell.

PtPt

DC Power Source

- +DC Power Source

- +

K+

H2OI-

The negative is reduction

The positive is oxidation

Anion or water

Oxidation

Anode

Vowels

lowest

For Oxidation take the lowest

1. Draw and completely analyze a molten KI electrolytic cell.

PtPt

DC Power Source

- +DC Power Source

- +

Electrons 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 v

e-

e-

MTV = +0.95 v

The negative is reduction

The positive is oxidation

cations to cathode

anions 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.