Required practical 3 - Electrolysis

Question 1

How do you set up a general electrolysis experiment?

Answer 1

• Place the positive and negative electrodes in a beaker containing a molten or dissolved ionic compound.
• Connect both electrodes to a power supply at 4V with wires.

Question 2

How could you investigate what happens when an aqueous solution of CuCl₂ is electrolysed?

Answer 2

• Half fill a beaker with aqueous CuCl₂.
• Place a lid on the beaker and insert the electrodes into the solution through holes in the lid (electrodes must not touch).
• Connect the electrodes to a low voltage power supply.
• Switch the power supply on to 4V.
• Turn off the power after a few minutes and record any observations.

Question 3

What forms at the cathode and the anode in electrolysis?

Answer 3

Cathode: metals or hydrogen.
Anode: non-metals.

Question 4

What would you observe at each electrode when copper chloride solution is electrolysed?

Answer 4

Positive electrode - bubbles of gas (chlorine).

Negative electrode - electrode coated with a brown solid (copper).

Question 5

Why must the positive electrode (anode) be regularly replaced?

Answer 5

The positive electrode is made of carbon and will react with oxygen to produce carbon dioxide.

Question 6

Write half equations for the reactions that occur at the electrodes when aqueous CuCl₂ is electrolysed

Answer 6

Cathode: Cu²⁺ + 2e⁻ → Cu
Anode: 2Cl⁻ → Cl₂ + 2e⁻

Question 7

Write half equations for the reactions that occur at the electrodes when aqueous NaCl is electrolysed

Answer 7

Cathode: 2H⁺ + 2e → H₂
Anode: 2Cl⁻ → Cl₂ + 2e⁻

Question 8

In the electrolysis of NaCl, hydrogen is produced at the cathode. Why is sodium not produced?

Answer 8

Hydrogen is produced because sodium is more reactive than hydrogen.
Sodium remains in the solution.

Question 9

How could you test that chlorine gas was produced at the anode?

Answer 9

The gas produced will bleach damp litmus paper - it will turn white.

Question 10

How could you test that hydrogen gas was produced at the anode?

Answer 10

The gas produced will make a squeaky pop with a lighted splint.

Question 11

How could you test that oxygen was produced at the anode?

Answer 11

The gas produced will relight a glowing splint.