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 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

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

Answer 5

Positive electrode: Bubbles of gas (chlorine)

Negative electrode: Bubbles of gas rapidly produced (hydrogen)

Question 6

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

Answer 6

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

Question 7

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

Answer 7

Cathode: Cu2+ +2e- → Cu

Anode: 2Cl2- → Cl2 +2e-

Question 8

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

Answer 8

Cathode: 2H+ + 2e- → H2

Anode: 2Cl- → Cl2 +2e-

Question 9

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

Answer 9

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

Question 10

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

Answer 10

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

Question 11

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

Answer 11

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

Question 12

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

Answer 12

The gas produced will relight a glowing splint