Electrolytic CellsLesson 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 cathodeElectrons flow through the wire and go from anode to cathodeAnions (- 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 cellsMelt 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 cellsDissolve 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.

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

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

Pb2+ 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 M solution

Consider the overpotential effectTake the higher reaction on the leftThe 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 M solution

Consider the overpotential effectTake the lower reaction on the right side of the tableThe strongest reducing agent

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 an aqeuous 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

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 (O2)

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

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65 ( a and c) ,66, 67, 70