C2.7 Electrolysis

Question 1

Define ‘electrolysis’

Answer 1

The breakdown of a substance containing ions by electricity.

Question 2

Define an ‘electrolyte’

Answer 2

A liquid, containing free moving ions, that is broken down by electricity during electrolysis.

Question 3

What 2 states must the electrolyte be in for electrolysis to work and why?

Answer 3

• Molten
• Dissolved in a solution
• So that the free ions can conduct the electricity

Question 4

What happens at the anode (positive electrode)?

Answer 4

Electrons are taken away from ions (oxidation)

Question 5

What happens at the cathode (negative electrode)?

Answer 5

Ions gain electrons that were taken away at the anode (reduction)

Question 6

What is oxidation?

Answer 6

When electrons are lost from a substance (or when oxygen is added)

Question 7

What is reduction?

Answer 7

When electrons are added to a substance (or when oxygen is removed)

Question 8

Where is the metal produced in electrolysis?

Answer 8

At the cathode

Question 9

Where is the non metal produced in electrolysis?

Answer 9

At the anode

Question 10

What happens at the cathode if a salt is dissolved in water ?

Answer 10

• If the metal is more reactive than hydrogen, the metal ions and OH- ions will stay in the solution and hydrogen is produced.
• If the metal is less reactive than hydrogen, the metal will be produced and the H+ ions and OH- ions will stay in the water.

Question 11

Why does the product formed at the cathode depend on its reactivity when a salt is dissolved in water?

Answer 11

If a metal is more reactive, it will be keener to stay as an ion to remain energetically stable.

Question 12

What are the three useful products produced when sodium chloride solution undergoes electrolysis and their uses?

Answer 12

• Hydrogen
• Chlorine - used for bleach and plastics
• Sodium hydroxide - used for soaps

Question 13

What type of solution is left behind when a salt is dissolved in water if the metal is less reactive than hydrogen?

Answer 13

A metal hydroxide solution

Question 14

What is the half equation for the anode?

Answer 14

2(non metal)- ===> (non metal)2 + 2e-

Question 15

What is the half equation for the cathode?

Answer 15

2(hydrogen/metal ion) + 2e- ===== (H2 or the metal)

Question 16

What is the half equation of the anode for the electrolysis of sodium chloride solution?

Answer 16

2Cl ===> Cl2 + 2e-

Question 17

What is the half equation of the cathode for the electrolysis of sodium chloride solution?

Answer 17

2H+ + 2e- ====> H2

Question 18

What is the half equation of the cathode for the electrolysis of molten lead bromide?

Answer 18

Pb2+ + 2e- ====> Pb

Question 19

How is aluminum manufactured?

Answer 19

By the electrolysis of a molten mixture of cryolite and aluminium oxide

Question 20

What is the ore of aluminium?

Answer 20

Bauxite

Question 21

What is produced at the anode during the electrolysis of aluminum?

Answer 21

Carbon dioxide - the oxygen formed at the anode reacts with the carbon the anode is made of.

Question 22

What is the usefulness of cryolite during electrolysis?

Answer 22

• Aluminium oxide has a high melting point so melting it would be very expensive
• Dissolving it in molten cryolite brings the temperature down so it is easier and cheaper

Question 23

Why do carbon electrodes need to be replaced often in the aluminum oxide electrolysis?

Answer 23

The carbon is used up when it reacts with the oxygen formed at the anode.

Question 24

What is electroplating?

Answer 24

Coating the surface of one metal with another metal (e.g coating copper with silver)

Question 25

What is the cathode in electroplating electrolysis?

Answer 25

The metal you want to plate

Question 26

What is the anode in electroplating electrolysis?

Answer 26

The metal you want to plate the other with

Question 27

What are two uses of electroplating?

Answer 27

• Decoration: silver is attractive but expensive, coating metals with it is cheaper than making things out of it.
• Conduction: metals like copper conduct well - they are used to plate metals for electronic circuits and computers