C6 Electrolysis

Question 1

What is electrolysis?

Answer 1

Splitting up compounds using electricity

Question 2

What is an electrode?

Answer 2

Rod that conducts electricity, come in pairs, often made up of graphite, one negative and one positive

Question 3

What ions go to the anode?

Answer 3

Negative ions

Question 4

What ions go to the cathode?

Answer 4

Positive ions

Question 5

What is an electrolyte?

Answer 5

Ionic liquid or solution broken down by electrolysis

Question 6

What happens when the ionic compounds are dissolved in water?

Answer 6

• Water molecules break up into hydrogen ions, H+ and hydroxide ions, OH-
• So in the ionic solution (e.g. sodium chloride solution), there will be four types of ions present
• Two from the ionic compound and two from the water

Question 7

Rules for ionic solutions - Anode

Answer 7

If -ions are halogens, for example:
- chloride Cl-
- bromide Br-
- iodide I-
the halogen is produced

If -ions are not halogens, for example:
- sulphate
- nitrate
- carbonate
oxygen is produced

Question 8

Rules for ionic solutions - cathode

Answer 8

If +ions (metals) are more reactive than hydrogen:
- potassium, K
- sodium, Na
- calcium, Ca
- magnesium, Mg
- zinc, Zn
- iron, Fe
then hydrogen is produced

If +ions (metals) are less reactive than hydrogen:
- copper, Cu
- silver, Ag
- gold, Au
then the metal is produced

Question 9

Why is graphite used for the electrodes?

Answer 9

Because it has free electrons that can move through the graphite, and carry the charge

Question 10

Electrolysis of aluminium oxide - why is the aluminium oxide dissolved in cryolite?

Answer 10

To lower the melting point and make it molten. To save some energy

Question 11

Electrolysis of aluminium oxide - which ions go to which electrodes?

Answer 11

Anode - oxide
Cathode - aluminium

Question 12

Electrolysis of aluminium oxide - what happens at the cathode?

Answer 12

Reduction

Question 13

Electrolysis of aluminium oxide - what happens at the anode?

Answer 13

Oxidation

Question 14

Why is extracting aluminium so expensive?

Answer 14

The anodes need to be replaced frequently

Question 15

Why do the electrodes have to be replaced every few months?

Answer 15

They burn off, wears away

Question 16

Explain why aluminium oxide must be molten for electrolysis to take place?

Answer 16

So the ions can move to the electrodes

Question 17

Anode

Answer 17

Positive electrode

Question 18

Cathode

Answer 18

Negative electrode

Question 19

Cation

Answer 19

Positively charged ion

Question 20

Anion

Answer 20

Negatively charged ion

Question 21

Changes at the electrodes

Answer 21

• At the negative electrode (cathode), positive ions gain electrons, so are reduced
• At the positive electrode (anode), negative ions lose their extra electrons, so are oxidised

Question 22

Extraction of aluminium - at the cathode

Answer 22

• Each aluminium ion, Al(3+), gains three electrons
• The ions turn into aluminium atoms
• Al(3+) ions are reduced (gain electrons) to form Al atoms
  Al(3+) + 3e- –> Al

Question 23

Extraction of aluminium - at the anode

Answer 23

• Each oxide ion, O(2-), loses two electrons
• The ions turn into oxygen atoms
• The O(2-) ions are oxidised (lose electrons) to form oxygen atoms
• These bond in pairs to form molecules of oxygen gas, O2
  2O(2-) –> O2 + 4e-

Question 24

Explain the difference between the processes in electrolysis and in a chemical cell

Answer 24

• Electrolysis uses electricity to produce a chemical reaction
• Cells use a chemical reaction to produce electricity