C6 - Electrolysis

Question 1

When can ionic compounds carry charge and what are they known as

Answer 1

• When they are molten or aqueous
• This is because their ions can move and conduct electricity
• This substance is now known as the electrolytes

Question 2

What are the electrodes made of

Answer 2

• A conducting metal or graphite
• These are inert (unreactive) so they don’t interfere with the electrolysis

Question 3

What are the two different electrodes

Answer 3

• Negative electrode (cathode)
• Positive electrode (anode)

Question 4

What happens at the positive electrode

Answer 4

• Negative ions travel to the anode
• They discharge by losing electrons (oxidation) and froming an element

Question 5

What happens at the negative electrode

Answer 5

• Positive ions go to the cathode
• They discharge by gaining electrons (reduction) and forming an element

Question 6

How is aluminium extracted from aluminium oxide using electrolysis

Answer 6

• Aluminium oxide is mixed with cryolite to lower the melting point which saves money
• Electrodes are made of graphite
• Mixture is then heated until it is molten
• Aluminium is produced at cathode and is then removed
• Oxygen is produced at the anode

Question 7

Why is carbon(graphite) used as the electrodes in aluminium oxide electrolysis

Answer 7

• Cheap
• Good conductor

Question 8

Why does the anode need to be replaced often

Answer 8

• The anode is made from graphite (carbon)
• The carbon reacts with the oxygen made here during electrolysis to form CO2 (due to high temps)
• This slowly wears the anode away so needs to be repaced frequently

Question 9

Half equation at the cathode in the electrolysis of aluminium oxide

Answer 9

Al3+ + 3e- —> Al

The aluminium is reduced (gains electrons)

Question 10

Half equation at the anode in the electrolysis of aluminium oxide

Answer 10

2O2- —> O₂ + 4e-

Oxygen is oxidised (loses electrons)

Question 11

Why is electrolysis expenisve

Answer 11

• lots of energy required to melt the ionic compound
• lots of energy is needed to produced the electric current

Question 12

What reaction happens to water molecules during electrolysis

Answer 12

• They ionise forming hydroxide ions and hydrogen ions
• H₂O ⇌ H+ + OH-

Question 13

Rule for products at the cathode of the electrolysis of an aqueous solution

Answer 13

• Hydrogen is produced at the cathode if the metal is more reactive than hydrogen
• This is because the more reactive ion stays in the compound

Question 14

Rule for products at the anode of the electrolysis of an aqueous solution

Answer 14

• If the solution contains halide ions, the halogen will be produced at the electrode

Question 15

Required pratical for the electrolysis of copper (II) chloride solution

Answer 15

• Set up beaker with 50cm3 of the solution
• Add a petri dish with 2 holes and put graphite electrodes in them
• Attach them to a power pakc and make sure they don’t touch
• Once it is turned on, the cathode will be coated in copper
• The anode will have bubbles of chlorine around it and this can be proved as it turns damp blue litmus paper white

Question 16

Required pratical for the electrolysis of sodium chloride solution

Answer 16

• Same set up as before
• At the anode, there are bubbles of chlorine as it is a halogen
• At the cathode, there are bubbles of hydrogen

Question 17

Possible half equations at the cathode for the electrolysis of aqueous solutions

Answer 17

• 2H+ (aq) + 2e- —> H₂ (g)

Question 18

Possible half equations at the anode for the electrolysis of aqueous solutions

Answer 18

• 4OH- (aq) —> O₂(g) + 2H₂O(l) +4e-
• (If chlorine is used) - 2Cl- (aq) —> Cl₂(g) +2e-

Question 19

another name for positive ions and negative ions

Answer 19

Positive ions - cations
Negative ions - anions

Question 20

Full balanced equation for the breakdown of aluminium oxide by electrolysis

Answer 20

• Aluminium oxide —> aluminium + oxygen
• 2Al₂ O3 (l) —> 4Al (l) + 3O₂

Question 21

Explain the purpose of cryolite

Answer 21

• Al₂O3 must be molten to be electrolysed
• Cryolite reduces the melting point of the mixture
• This means less energy is required to melt it so reduces the costs