C6: Electrolysis

Question 1

What happens in electrolysis?

Answer 1

An electric current is used to break down a molten or solution ionic compound into its elements. The compound being broken down is the electrolyte.

Question 2

What are the electrodes?

Answer 2

• The positive electrode is the anode and is where negatively charged ions move.
• The negative electrode is the cathode and is where positively charged ions move.
• The electrodes are often inert so that they don’t react with electrolyte or products made.

Question 3

What happens when ions reach the electrodes?

Answer 3

They lose their charge and become elements.

Question 4

What is produced at each anode for aqueous solutions?

Answer 4

Cathode - Hydrogen is produced unless metal is less reactive than hydrogen.
Anode - Halide formed if present, if not hydroxide ions are discharged and oxygen gas is produced.

Question 5

What is the process in the electrolysis of aluminium.

Answer 5

• Compound electrolysed is aluminium oxide. It has a very high melting point so it is mixed with molten cryolite. It would be too expensive to melt it otherwise.
• The molten aluminium oxide undergoes electrolysis.
• The anode is made from carbon so needs to be constantly replaced as it reacts with oxygen produced, forming carbon dioxide making it burn away.

Question 6

What is formed at the electrodes in the electrolysis of aluminium?

Answer 6

Cathode: Aluminium is formed, Al3+(l) + 3e- —> Al(l)
Anode: Oxygen formed, 2O2- —> O2 + 4e-

Question 7

How do you carry out electrolysis for copper chloride and sodium chloride solutions?

Answer 7

1. Add around 50cm3 of copper chloride solution to a beaker.
2. Add lid with holes and insert electrodes making sure they don’t touch.
3. Add crocodile leads to electrode and connect rods to DC terminals with low voltage power supply.
4. Set power supply to 4V and switch it on.
5. Using forceps hold litmus paper at the anode.
6. After a few minutes turn power supply off and record observations at electrodes. Repeate with sodium chloride.

Question 8

What are the results at each electrode for the electrolysis of copper chloride solution?

Answer 8

Anode: Chlorine gas formed, bubbles of gas bleach blue litmus paper white.
Cathode: Brown solid coating of gas.

Question 9

What are the results at each electrode for the electrolysis of sodium chloride solution?

Answer 9

Anode: Bubbles of chlorine gas bleach blue litmus paper white.
Cathode: Bubbles of hydrogen gas.