Electrolysis PAPER 2

Question 1

What is electrolysis?

Answer 1

• electrolysis is the breaking down of a substance using electricity
• an electric current is passed through an electrolyte (a molten or dissolved ionic compound) causing it to decompose
• oxidation (loss of electrons) and reduction (gain of electrons) occurs

Question 2

Where do the ions go in electrolysis?

Answer 2

• the positive ions (cations) in the electrolyte move towards the cathode (negative electrode) and reduced (gain electrons)
• the negative ions (anions) in the electrolyte move towards the anode (positive electrode) and are oxidised (lose electrons)
• as ions gain or lose electrons they form the uncharged substances and are discharged

Question 3

What are the half equations for sodium and hydrogen ions

Answer 3

Na —> Na+ + e-
2H+ + 2e- —> H2

Question 4

What happens at the electrodes of the electrolysis of molten lead (II) Bromide?

Answer 4

CATHODE
- the positive Pb2+ ions are attracted to the negative cathode
- a lead ion accepts two electrons and is reduced to a lead atom
- Pb2+ + 2e- —> Pb

ANODE
- the negative Br- ions are attracted to the positive anode
- two bromide ions lose one electrons each and are oxidised to form a bromine molecule
- 2Br- —> Br2 + 2e-

• the electrodes are from an inert material so they are unreactive and don’t take part in the reaction

Question 5

What is produced with potassium chloride as the molten electrolyte

Answer 5

Cathode:
- potassium
- K+ + e- —> K

Anode:
- Chlorine
- 2Cl- —> Cl2 + 2e-

Question 6

What is produced with aluminium oxide as the electrolyte?

Answer 6

Cathode
- aluminium
- Al3+ + 3e- —> Al

Anode
- oxygen
- 2O2- —> O2 + 4e-

Question 7

What is different about electrolysis in an aqueous solution?

Answer 7

• in aqueous solutions there will be hydrogen ions (H+) and hydroxide ions (OH-) as well as the ions from the ionic compound
• at the cathode, if H+ ions and metal ions are present, hydrogen gas will be formed if the metal is more reactive then hydrogen - if it is less reactive then a solid layer of pure metal will be formed
• at the anode, if OH- ions and halide ions (Cl-, Br-, I-) are present, molecules of the halide will form - if no halide ions then oxygen will form

Question 8

Electrolysis of sodium chloride

Answer 8

• a solution of sodium chloride contains Na+, Cl-, OH-, H+ ions
• sodium metal is more reactive then hydrogen so hydrogen gas is formed at the cathode
• chloride ions are present in the solution so chlorine gas is produced at the anode

Question 9

What is formed when copper sulphate is electrolysised?

Answer 9

Cathode:
- copper
- Cu2+ + 2e- —> Cu

Anode
- oxygen
- 4OH —> O2 +2H20 + 4e-

Question 10

What is formed when dilute sulphuric acid is electrolysed?

Answer 10

Cathode
- hydrogen
- 2H+ + 2e- —> H2

Anode
- oxygen
- 4OH- —> O2 + 2H2O + 4e-

Question 11

Why do covalent compounds not conduct electricity?

Answer 11

When atoms bond covalently they share electrons. There are no free electrons to move and carry charge so they do not conduct electricity

Question 12

Why do ionic compounds conduct electricity only when molten or in aqueous solution?

Answer 12

• in solids the positive and negative ions vibrate but are in a fixed position and do not move.they are held in a giant lattice arrangement so ions are unable to move and carry charge
• in molten state the ions are free to move about in the liquid and conduct electricity
• dissolved in water the ions are free to move about in the solution and conduct electricity