2(i) Electrolysis

Question 1

Electrolysis of Molten Lead Bromide

Answer 1

Positive Electrode:
2Br- -> Br2 + 2e- OXIDATION

Negative Electrode:
Pb2+ + 2e- -> Pb REDUCTION

Question 2

Electrolysis of Sodium Chloride

Answer 2

Positive Electrode:
2Cl- -> Cl2

Negative Electrode:
2H+ -> H2

• Leaves NaOH

Question 3

Electrolysis of Copper (II) Sulfate

Answer 3

Positive Electrode:
2O2- -> O2 + 4e-

Negative Electrode:
Cu2+ + 2e- -> Cu

Question 4

Electrolysis of Dilute Sulfuric Acid

Answer 4

Positive Electrode:
4OH- -> O2 + H2O + 4e-

Negative Electrode:
2H+ + 2e- -> H2

• Ratio of H2: O2 = 2:1

Question 5

Electrolysis Rules

Answer 5

At positive electrode:
* Halide ions are discharged, otherwise O2

At negative electrode:
* H+ ions are discharged, unless metal is less reactive than hydrogen

The products of an electrolysed aqueous solution are not always the same as the products of electrolysed molten salt

Question 6

Quantitive electrolysis via mass change/volume of gas produced: Copper Sulfate

Answer 6

With graphite (inert) electrodes: Cu + O2

With copper electrodes:
1. Electrodes are rubbed with emery cloth to remove corrosion (for a fresh layer)

2. At positive electrode: Cu -> Cu2+ + 2e-
   At negative electrode: Cu2+ + 2e- -> Cu
3. Dip electrodes in ethanol to help the electrodes faster

Question 7

Quantitive electrolysis via mass change/volume of gas produced: Gases

Answer 7

Use gas syringe

Question 8

Quantitive electrolysis via mass change/volume of gas produced: Gases

Answer 8

Use gas syringe

Question 9

Quantitive Electrolysis: Current and Mass made

Answer 9

Directly proportional

Question 10

Extracting aluminium through electrolysis: Electrodes

Answer 10

Positive Electrode (top) : Graphite
Negative Electrode (bottom) : Graphite

Question 11

Cryolite

Answer 11

Used to lower boiling point of aluminium oxide (Al2O3)

Question 12

Extracting aluminium through electrolysis: Electrodes reactions

Answer 12

Positive Electrode:
2O2- -> O2 -> 4e-
O2 reacts with carbon electrodes to form CO2 (electrodes have to be eventually replaced)

Negative Electrode:
Al3+ + 3e- -> Al (l)

Question 13

Disadvantage of extracting aluminium

Answer 13

Expensive - requires lots of electricity to make it molten to extract it