C.6

Question 1

What is electrolysis?

Answer 1

Breaking down an ionic compound using electricity.

Question 2

What conditions do there need to be for ionic compounds to be electrolysed?

Answer 2

The compound must either be molten or dissolved in water, i.e. in a liquid form

Question 3

What is the electrolyte?

Answer 3

The electrolyte is the original ionic compound which is being broken down in electrolysis.

Question 4

What is the set up for electrolysis?

Answer 4

Two electrodes (thin pieces of metal) connecting to a power supply which dip into the electrolyte. They must not touch one another.

Question 5

What is the anode?

Answer 5

The positive electrode. It is attached to the power supply’s positive terminal and therefore attracts the negative ions (anions).

Question 6

What is the cathode?

Answer 6

The negative electrode. It is attached to the power supply’s negative terminal and therefore attracts the positive ions (cations).

Question 7

What are the charge carriers in electrolysis?

Answer 7

In the metal wire and electrodes = electrons
In the solution / molten compound = ions

Question 8

Why is electrolysis needed to extract aluminium?

Answer 8

Carbon is not reactive enough to displace aluminium from aluminium oxide, meaning it needs to be electrolysed.

• Aluminium oxide (Al2O3) comes from an ore called bauxite, and aluminium is a useful resource needed for pans, foil, drink cans etc.

Question 9

What is the equation for electrolysing Al2O3?

Answer 9

aluminium oxide –> aluminium + oxygen
2Al2O3 (l) –> 4Al (l) + 3O2 (g)

Question 10

What happens to each element in electrolysis of aluminium oxide?

Answer 10

Aluminium goes to the cathode, oxygen goes to the anode.

This is because the positive metal ions are attracted to the negative electrode, and the negative non metal ions are attracted to the positive electrode.

Question 11

How is electrolysis of aluminium oxide made more efficient?

Answer 11

By mixing it with cryolite. This lowers the melting point from 2050°C to 850°C which is cheaper but still very expensive.

Question 12

What else happens at the electrodes when electrolysing Al2O3?

Answer 12

As the electrodes are made of graphite, the oxygen at the anode reacts with the carbon electrode to produce carbon dioxide gas. This means the anodes gradually burn away.

Question 13

Why can only certain metals be electrolysed?

Answer 13

Electrolysis can only work for metals less reactive than oxygen. This is because the water produces H+ and OH- ions, and if a metal more reactive than hydrogen is electrolysed, hydrogen gas is produced.

This is because ions of low reactivity metals are discharged more easily than the hydrogen ions. If the metal is more reactive than hydrogenl the H+ ions are discharged as they are more reactive.

Question 14

What is the equation for producing hydrogen gas?

Answer 14

2H+ + 2e- –> H2

Question 15

What are halide ions?

Answer 15

Halide ions are ions produced by halogens (Chlorine, Bromine, Iodine etc)

Question 16

What happens if a compound contains halide ions?

Answer 16

The halide ions are discharged rather than the OH- produced by the water. Solutions containing any negative ions that are not halide ions discharge the OH- ions instead.

Question 17

What is the equation for producing oxygen?

Answer 17

4OH- + 4e- –> O2 + 2H2O

Question 18

Describe how to carry out the electrolysis RP.

Answer 18

1. Pour 50cm3 of copper II chloride solution into a beaker. Place a plastic petri dish with 2 holes in it over the beaker.
2. Insert a carbon graphite rod into each hole. The carbon electrodes are inert as carbon graphite is unreactive meaning they will not react. The electrodes must not touch.
3. Connect the electrodes to a low voltage power supply. Select 4V and switch it on. The copper will be discharged at the cathode.
4. Bubbles of chlorine gas will be released at the anode because halide ions are discharged over the hydroxide ions. Hold damp blue litmus paper at the anode to test for the presence of chlorine (should get bleached white).
5. Repeat using 50cm3 sodium chloride solution. Chlorine gas bubbles should be discharged at the anode (test again w damp litmus paper). Hydrogen gas bubbles should be discharged at the cathode as hydrogen is less reactive than sodium. Collect and test using the lighted splint test.