C6 - Electrolysis

Question 1

What is electrolysis? What is the electrolyte?

Answer 1

Electrolysis = the decomposition of a liquid by passing an electric current through it. 
Electrolyte = the liquid that is decomposed or broken down. It must be able to conduct electricity, and this can only happen if its ions are free to move.

Question 2

Define the terms cathode, cation, anode, anion,

Answer 2

• Cathode = negative electrode. Positively charged ions called cations move towards it and are discharged there.
• Anode = positive electrode. Negatively charged ions called anions move towards it and are discharged there.

Question 3

How is charge carried through the liquid?

Answer 3

The potential difference across the two electrodes causes the ions to move and so carry charge through the liquid.

Question 4

What are the ions in water? Where are they discharged and what do they produce?

Answer 4

• hydrogen ions, H+(aq) -> discharged at the cathode, producing hydrogen, H2(g)
• hydroxide ions, OH–(aq) -> discharged at the anode, producing oxygen, O2(g).

Question 5

Write down the half equations for water at each electrode.

Answer 5

• At the cathode: 2H+ + 2e– → H2(g)
• At the anode: 4OH– → O2 + 2H2O + 4e-

Question 6

When can ionic substances be electrolysed?

Answer 6

When ionic substances are solid, their ions are in fixed positions and cannot move from place to place. However, they can be electrolysed if they are heated until they melt, because the ions in a molten liquid can move.

Question 7

What happens to the metal ions or hydrogen ions and the non-metal ions during electrolysis? Give an example with aluminium oxide.

Answer 7

• metal ions or hydrogen ions move to the negative electrode, and non-metal ions (apart from hydrogen ions) move to the positive electrode.
• when molten aluminium oxide is electrolysed, aluminium forms at the negative electrode and oxygen at the positive electrode.

Question 8

Write down the half equations for the electrolysis of molten aluminium oxide.

Answer 8

• At the cathode: Al3+ + 3e– → Al
• At the anode: 2O2– → O2 + 4e-

Question 9

Describe the process and equipment for an electrolysis experiment.

Answer 9

The electrolyte is poured into a beaker. Two graphite rods are used as electrodes. Graphite is a form of carbon that conducts electricity and it does not usually react with the electrolysis products. A d.c. power supply / battery supplies the potential difference needed for a current to flow.

Question 10

Describe the electrolysis of copper sulfate solution.

Answer 10

copper is formed at the cathode, which becomes coated with copper. Oxygen is formed at the anode, and bubbles of this gas are released from the surface of the anode.

Question 11

What happens at the cathode during the electrolysis of an aqueous solution depending on how reactive the metal is?

Answer 11

If a metal is less reactive than hydrogen, it is produced at the cathode. However, if the metal in solution is more reactive than hydrogen, or there is no metal in solution, hydrogen gas is produced instead.

Question 12

What is produced at the anode during the electrolysis of an aqueous solution?

Answer 12

If the aqueous solution contains chloride ions, chlorine is produced at the anode. If not, oxygen gas is produced instead.

Question 13

What are the half equations for hydrogen at the cathode and oxygen at the anode ?

Answer 13

Hydrogen at cathode : 2H+ + 2e– → H2

Oxygen at anode : 4OH– → O2 + 2H2O + 4e-

Question 14

How do you work out the charge transferred during electrolysis?

Answer 14

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