Electrolysis

Question 1

What is electrolysis?

Answer 1

The process of electrically splitting an ionic compound into its (generally) component metal and non metal.

Question 2

What must be true in order for electrolysis to happen?

Answer 2

• The compound must contain ions. • The compound’s ions must be free to move - must be molten (liquid) or in solution (aqueous).

Question 3

In the electrolysis of CuCl₂, what would happen at the cathode?

Answer 3

The cathode is negative.

• Reduction happens at the cathode:

• Cu²⁺_(aq) + 2e^- → Cu_(s)

Question 4

In the electrolysis of CuCl₂, what would happen at the cathode?

Answer 4

The anode is positive.

• Oxidisation always happens at the cathode:

• 2Cl^-_(aq) → Cl_2(g) + 2e^-

Question 5

How can you tell, if the electrolysis is of a compound in solution, what will be discharged and produced the cathode?

Answer 5

If the metal cation is more reactive than hydrogen, then hydrogen will be produced.

Question 6

What is the equation for hydrogen being produced in electrolysis, and where would it take place?

Answer 6

At the cathode: 2H⁺ + 2e^- → H₂

Question 7

How can you tell, if the electrolysis is of a coumpound in solution, what will be discharged and produced at the anode?

Answer 7

If the anion is a halide, this will be discharged and a halogen will be produced. Any other negative ions will cause hydroxide to be discharged and oxygen will be produced.

Question 8

What is the equation for oxygen being produced in electrolysis and where would it take place?

Answer 8

At the anode: 4OH^- → O₂ + 2H₂O + 4e^-

Question 9

Which metals are below hydrogen in the reactivity series?

Answer 9

Copper, silver, gold, platinum.

Question 10

What are the steps you would need to take to work out the mass of lead deposided at the cathode from molten lead bromide (PbBr₂)?

Current of 10 amps flows through the molten lead bromide for 5 hours.

(n = number of moles of electrons; F = Faraday constant (96000C))

Answer 10

1. Write the half equation for the electrolysis: Pb²⁺ + 2e^- → Pb
2. Work out how much charge has passed: charge + current x time = 10 x (5 x 60 x 60) = 180000C
3. Work out how many moles of electrons this equals: n = charge ÷ F = 180000 ÷ 96000 = 1.875moles
4. Work out how many moles of lead are produced: moles of electrons:moles of lead = 2:1; 1.875 ÷ 2 = 0.9375moles of lead
5. Work out how much lead is produced: RFM of lead = 207; mass = moles x RFM = 207 x 0.9375 = 194.0625g

Question 11

What is an electric current?

Answer 11

A flow of electrons or ions.

Question 12

How would you distinguish between electrolytes and non-electrolytes?

Answer 12

Place a conductivity probe in a liquid and if there is a reading, it means that current is flowing through the circuit. If there is a curent flowing through the circuit, the liquid is an electrolyte,

Question 13

What does one faraday represent?

Answer 13

One mole of electrons.

Question 14

Can covalent compounds conduct electricity and why?

Answer 14

• No, because they make bonds by sharing electrons and therefore don’t contain ions.
• There is nothing which can carry charge which is free to move, so they can’t carry an electric current.

Question 15

Why might the mass of an electrode be less than expected from half-equations in the electrolysis of copper sulphate (copper is formed at the cathode)?

Answer 15

• Not all the copper may stick to the cathode.
• Some copper may be removed when the electrodes dry.
• The anode may be impure, so not all of its mass increase could be due to the copper.