Topic 4 - Electrolysis

Question 1

What is electrolysis?

Answer 1

• electric current is passed through an electrolyte (a molten or dissolved ionic compound)
• ions move towards electrodes - where they react - and the compound decomposes

Question 2

Difference between chemical cells and electrolysis?

Answer 2

• electrolysis uses electricity to produce a chemical reaction
• cells use a chemical reaction to produce electricity

Question 3

Why is graphite used for the electrodes?

Answer 3

• graphite conducts electricity
• graphite is inert

Question 4

Explain why graphite conducts electricity?

Answer 4

• each carbon atom forms 3 covalent bonds
• one electron per carbon atom is delocalised
• electrons carry charge through the graphite

Question 5

Why does the positive electrode have to be continuously replaced?

Answer 5

• electrode reacts with oxygen
• electrode is carbon / graphite
• carbon dioxide is produced - gradually consuming the electrode

Question 6

Where do positive ions go in electrolysis?

Answer 6

positive ions in the electrolyte move towards the cathode and gain electrons (they are reduced)

Question 7

Where do negative ions go in electrolysis?

Answer 7

move toward the anode and lose electrons (they are oxidised)

Question 8

From what and how is aluminium extracted?

Answer 8

• aluminium is extracted from the ore bauxite by electrolysis
• auxite contains aluminium oxide

Question 9

Why is aluminium oxide mixed with cryolite?

Answer 9

• aluminium oxide has very high melting point - mixed with cryolite to lower melting point
• cryolite is an aluminium based compound with lower melting point than aluminium oxide

Question 10

What happens at the negative electrode in the electrolysis of aluminium oxide?

Answer 10

• positive Al3+ ions attracted to cathode - each pick up 3 electrons and turn into neutral aluminium atoms
• these then sink to the bottom of the electrolysis tank

Question 11

What happens at the positive electrode in the electrolysis of aluminium oxide?

Answer 11

• negative O2- ions attracted to anode where they each lose two electrons
• neutral oxygen atoms combine forming O2 molecules

Question 12

Half equation at anode for electrolysis of aluminium oxide?

Answer 12

2O2- –> O2 + 4e-

Question 13

Half equation at cathode for electrolysis of aluminium oxide?

Answer 13

Al3+ + 3e- –> Al

Question 14

What is brine?

Answer 14

Brine is a solution of sodium chloride (NaCl) and water (H2O)

Question 15

What is formed at the cathode in electrolysis of aqeous solutions?

Answer 15

• hydrogen gas produced if metal ions form an elemental metal more reactive than hydrogen
• if metal ions form an elemental metal less reactive than hydrogen - solid layer of pure metal produced instead

Question 16

What is formed at the anode in electrolysis of aqeous solutions?

Answer 16

• if halide ions are present, molecules of chlorine, bromine or iodine will be formed
• if halide ions arent present, then the OH- ions are discharged and oxygen will be formed

Question 17

Half equation at anode when halide ions aren’t present?

Answer 17

4OH- —> O2 + 2H2O + 4e-