C6. Electrolysis

Question 1

What is an electrolyte

Answer 1

The compound that is broken down by electrolysis

Question 2

What is an Anode?

Answer 2

The positive electrode

Question 3

What is a cathode?

Answer 3

The negative electrode

Question 4

What happens during electrolysis

Answer 4

During electrolysis, cations (positively charged ions) move to the cathode (negative electrode). At the same time, anions (negatively charged ions) move to the anode (positive electrode), as opposite charges attract

Question 5

Why can electrolysis only occur when an ionic compound is molten or in a aqueous solution?

Answer 5

In order for electrolysis to occur, the compound must be dissolved in water, or melted to form a liquid. This means that the ions are free to move around and carry their charge to the electrodes

Question 6

What is electrolysis?

Answer 6

The breakdown of a substance containing ions by electricity

Question 7

What is the meaning of the mnemonic
O
I
L
R
I
G

Answer 7

Oxidation
Is
Loss
Reduction
Is
Gain

Question 8

How can hydrogen ions be present in electrolysis?

Answer 8

Water ionises to form H+ ions and OH- ions. The less reactive element will usually be formed. In aqueous solutions, there will usually be positively charged metal ions and H+ ions from the water which are attracted to the cathode.

Question 9

What is the half equation for hydrogen at the cathode?

Answer 9

2H+ (aq) + 2e- –> H₂ (g)

Question 10

What happens at the anode in the electrolysis of aqueous solutions?

Answer 10

Hydroxide ions (OH-) from water are often discharged. When hydroxide ions are discharged, you see oxygen gas given off at the anode

Question 11

What happens if the solution contains a highly concentration of a halide ion, such as Cl-?

Answer 11

The halide ions is discharged and the halogen is formed

Question 11

What is the half equation for hydroxide at the anode?

Answer 11

4OH- (aq) –> 2H₂O (l) + O₂ (g) + 4e-

Question 12

What is the half equation for Chloride at the anode?

Answer 12

2Cl- –> Cl₂ + 2e-

Question 13

Why is electrolysis used to extract aluminium?

Answer 13

Aluminium has a great affinity towards oxygen and so cannot be reduced by carbon or carbon monoxide

Question 14

How does electrolysis work for aluminium?

Answer 14

Bauxite is purified to form aluminium oxide
Aluminium oxide must be melted to enable the ions to move to the electrode however aluminium oxide has a very high melting point of 2050°C
Aluminium oxide is therefore mixed with cryolite, so the molten mixture can be electrolysed at 850°C

Question 15

Where does aluminium form in the electrolysis of aluminium oxide?

Answer 15

At the negative electrode (cathode)

Question 16

Where does oxygen form in the electrolysis of aluminium oxide?

Answer 16

At the positive electrode (Anode)

Question 17

What happens to aluminium at the cathode?

Answer 17

Each aluminium ions gain three electrons and turn into aluminium atoms. The aluminium ions are reduced (as they gain electrons) to form Al atoms

Question 18

What is the half equation for aluminium at the cathode?

Answer 18

Al³+ (l) + 3e- ➡ Al (l)

Question 19

What happens to oxygen at the anode?

Answer 19

Each oxide ions loses 2 electrons and turn into oxygen atoms. The oxide ions are oxidised form oxygen atoms which bond in pairs to form molecules of O₂

Question 20

What is the half equation for oxygen at the anode?

Answer 20

2O²- ➡ O₂ (g) + 4e-

Question 21

What products do you get when brine (concentrated sodium chloride solution) is electrolysed?

Answer 21

Chlorine gas at the anode (positive electrode)
Hydrogen gas at the cathode (negative electrode)
Sodium hydroxide

Question 22

What happens to chlorine at the anode?

Answer 22

The negative chloride ions are attracted to the Anode and each lose one electron. The chlorine ions are oxidised and bond together in pairs to give off as chlorine gas

Question 23

What happens to hydrogen at the cathode during the electrolysis of brine?

Answer 23

The positive hydrogen ions are attracted to the negative electrode. (the sodium ions are also attracted but the less reactive element gets discharged). Hydrogen ions each gain one electron. The hydrogen ions are reduced and bond together in pairs and are given off as the hydrogen gas

Question 24

How is sodium hydroxide formed in the electrolysis of brine?

Answer 24

The Chloride and hydrogen ions are removed during electrolysis. So this leaves a solution contain Na+ and OH- ions which is a solution of sodium hydroxide.

Question 25

When are hydrogen and oxygen formed in electrolysis

Answer 25

In AQUEOUS solutions (NOT WHEN THE COMPOUND IS MOLTEN CAUSE THERE IS NO WATER!!!!!)