Electrolysis

Question 1

What are electrolytes?

Answer 1

• Electrolytes are ionic. The charge moves through the molten liquid by the movement of ions.
• If the electrolyte solidifies then the ions are in fixed positions and cannot move and the current cannot flow.

Question 2

What is electrolysis?

Answer 2

Electrolysis is the flow of charge by moving ions. These moving ions discharge at electrodes.

Question 3

What happens to the ions and electrons in electrolyte?

Answer 3

• Positive ions move towards the cathode.
• Negative ions move towards the anode.
• Ions reach the electrodes and are discharged; they turn into atoms or molecules.

Question 4

Sodium Ion

Answer 4

Na⁺

Question 5

Chlorine Ion

Answer 5

Cl^-

Question 6

Potassium Ion

Answer 6

K⁺

Question 7

Lead Ion

Answer 7

Pb²⁺

Question 8

Aluminium Ion

Answer 8

Al³⁺

Question 9

Hydroxide Ion

Answer 9

OH^-

Question 10

Copper Ion

Answer 10

Cu²⁺

Question 11

Hydrogen Ion

Answer 11

H⁺

Question 12

Bromine Ion

Answer 12

Br^-

Question 13

Oxygen Ion

Answer 13

O^2-

Question 14

Potassium Chloride Symbol

Answer 14

KCl

Question 15

Lead(II) Bromide Symbol

Answer 15

PbBr₂

Question 16

Aluminium Trioxide Symbol

Answer 16

Al₂O₃

Question 17

The reaction of sodium chloride at the cathode can be written as:

Answer 17

• Na⁺ + e^- → Na
• The sodium chloride splits into ions, so the ions are free to move.
• The positive Na⁺ ions migrate towards the negative cathode.
• Each Na⁺ Ion gains one extra electron from the cathode.

Question 18

The reaction of sodium chloride at the anode can be written as:

Answer 18

• 2Cl^- - 2e^- → Cl₂
• The negative Cl^- ions migrate to the positive anode.
• The Cl^- ions are discharged as chlorine gas.
• Two Cl^- ions each gain one electron and combine to form a chlorine molecule.

Question 19

Half equation of potassium chloride at the anode and cathode.

Answer 19

• Cathode: 2K⁺ + 2e^- → 2K
• Anode: 2Cl^- - 2e^- → Cl₂

Question 20

Half equation of lead(II) bromide at the anode and cathode.

Answer 20

• Cathode: Pb²⁺ + 2e^- → Pb
• Anode: 2Br^- _ 2e^- → Br₂

Question 21

Half equation of aluminium trioxide at the anode and cathode.

Answer 21

• Cathode: 2Al³⁺ + 6e^- → 2Al
• Anode: 6O^2- - 12e^- → 3O₂

Question 22

What are the products of different electrolytes?

Answer 22

• If sodium hydroxide (NaOH) or sulfuric acid H₂SO₄ solution is electrolysed, hydrogen is formed at the cathode and oxygen is formed at the anode.
• If copper(II) sulfate, CuSO₄, solution is electrolysed with carbon electrodes, copper is formed at the cathode and oxygen is formed at the anode.

Question 23

What affects the amount of substance during electrolysis?

Answer 23

• The amount of substance formed during electrolysis varies with time and current - it increases with increasing time and current.
• The substances produced at the electrodes are discharged ions.
• The only thing that affects the number of ions discharged is the amount of charge transferred.

Question 24

The electrode reactions in the electrolysis of NaOH_(aq) or H₂SO_4(aq) are:

Answer 24

• At the cathode: 2H⁺ + 2e^- ⇒ H₂
• At the anode: 4OH - 4e^- ⇒ 2H₂O + O₂

Question 25

The electrode reactions in the electrolysis of CuSO_4(aq) with carbon electrodes are:

Answer 25

• At the cathode: Cu²⁺ + 2e^- ⇒ Cu
• At the anode: 4OH - 4e^- ⇒ 2H₂O + O₂

Question 26

In the electrolysis of sodium hydroxide

Answer 26

Hydrogen is made rather than sodium, as sodium is much higher up the reactivity series, so hydrogen is discharged in preference.