RP3: Electrolysis

Question 1

What is the aim of this experiment?

Answer 1

• Investigate what happens when aqueous solutions are electrolysed using inert electrodes

Question 2

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

Answer 2

1) pour copper (II) chloride solution into beaker to about 50cm³
2) add lid and insert carbon rods through holes. rods must not touch each other
3) attach crocodile leads to rods. connect rods to dc terminals of low voltage power supply
4) select 4v on power supply and switch on
5) look at both electrodes and identify where there is bubbling
6) using tweezers, hold piece of blue litmus paper in solution next to anode (lift lid temporarily). write observations
7) after no more than 5 mins, switch off and examine cathode. write down observations (should be metal coating on it)
8) clean equipment and repeat with solutions of copper (II) sulphate, sodium chloride and sodium sulfate

Question 3

What are the potential risks?

Answer 3

• Safety goggles must be worn.
• Room should be well ventilated because large quantities of chlorine gas is toxic

Question 4

Describe copper (II) chloride at the anode

Answer 4

• bubbles of gas
• bleaches blue litmus white
• chlorine formed
• gas state

Question 5

Describe copper (II) chloride at the cathode

Answer 5

• brown/red solid coating on rod
• copper formed
• solid state

Question 6

Describe sodium chloride at the anode

Answer 6

• bubbles of gas
• bleaches blue litmus white
• chlorine formed
• gas state

Question 7

Describe sodium chloride at cathode

Answer 7

• bubbles of gas (more rapid production)
• hydrogen formed
• gas state

Question 8

What safety precautions should be taken?

Answer 8

• safety goggles must worn
• room should be well ventilated because large quantities of chlorine gas is toxic

Question 9

What forms at the cathode and the anode in electrolysis?

Answer 9

• Cathode: Metals or hydrogen
• Anode: Non-metals

Question 10

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

Answer 10

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

Question 11

State the half equations for the reactions that occur at the electrodes when aqueous CuCL2 is electrolysed

Answer 11

Cathode: Cu2+ + 2e- –> Cu
Anode: 2Cl2- —> Cl2 + 2e-

Question 12

State the half equations for the reactions that occur at the electrodes when aqueous NaCL is electrolysed

Answer 12

Cathode: 2H+ + 2e- —> H2
Anode: 2Cl- —> Cl2 +2e-

Question 13

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

Answer 13

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