C5 - Electrolysis

Question 1

Define electrolysis [1]

Answer 1

The breakdown of an ionic compound when molten or in aqueous solution by the passage of electricity

Question 2

Describe inert electrode [1]

Answer 2

Does not react with the electrolyte or products formed during electrolysis

Question 3

Describe electrolyte [1]

Answer 3

Ionic compounds that are in the molten state or dissolved in water

Question 4

Describe anode [1]

Answer 4

The positive electrode

Question 5

Describe cathode [1]

Answer 5

The negative electrode

Question 6

Explain what happens to anions in electrolysis [2]

Answer 6

Negative ions that are attracted to the anode where they give up electrons (oxidised)

Question 7

Explain what happens to cations in electrolysis [2]

Answer 7

Positive ions that are attracted to the cathode where they gain electrons (reduced)

Question 8

Describe the rules for what forms at the anode [1]

Answer 8

Hydrogen (from H+ in water) is produced unless the cations in the ionic compound from the metal are less reactive than hydrogen

Question 9

Describe the rules for what forms at the cathode [1]

Answer 9

Oxygen (from OH- in water) is produced unless the ionic compound contains halide ions (Cl-, Br-, I-)

Question 10

Molten lead bromide - describe the electrolysis [2]

Answer 10

Cathode = Pb²+ + 2e- → Pb Anode = 2Br- → Br₂ + 2e-

Question 11

Concentrated hydrochloric acid - describe the electrolysis [2]

Answer 11

Cathode = 2H+ + 2e- → H₂ Anode = 2Cl- → Cl₂ + 2e-

Question 12

Dilute sulfuric acid - describe the electrolysis [2]

Answer 12

Cathode = 2H+ + 2e- → H₂ Anode = 4OH- → 2H₂O + O₂ + 4e-

Question 13

Copper sulfate (using copper electrodes) - describe the electrolysis [2]

Answer 13

Cathode = Cu²+ + 2e- → Cu (cathode increases in mass as copper is plated here) Anode = Cu → Cu²+ + 2e- (anode wears away as copper ions dissolve into the solution to replace those formed at the cathode)

Question 14

Copper sulfate (using carbon electrodes) - describe the electrolysis [3]

Answer 14

Cathode = Cu²+ + 2e- → Cu Anode = 4OH- → 2H₂O + O₂ + 4e- H+ and SO₄²- are left in solution which form H₂SO₄

Question 15

Molten aluminium oxide - describe the electrolysis [4]

Answer 15

Cathode = Al₃+ + 3e- → Al Anode = O²- → O₂ + 4e- Molten cryolite is added to lower the melting point Oxygen formed at the graphite anode reacts with the carbon to form carbon dioxide so the anode needs to be replaced regularly

Question 16

Aqueous sodium chloride - describe the electrolysis [4]

Answer 16

Cathode = 2H+ + 2e- → H₂ Anode = 2Cl- → Cl₂ + 2e- Na+ and OH- are left in solution which forms NaOH Inert electrodes are used because the products are reactive

Question 17

Outline the uses of electroplating [3]

Answer 17

Coating a cheaper metal with a more expensive one e.g. silver coating on cutlery copper coating on saucepans