Electrolysis (OCR Gateway GCSE Chemistry: Combined Science)

Electrolysis of Aqueous Sodium Chloride

Practical Skills Development

• Use of appropriate apparatus and techniques to draw, set up and use electrochemical cells for separation and production of elements and compounds
• Safe use of appropriate qualitative reagents and techniques to analyse and identify unknown samples or products including gas tests
• Presenting observations and other data using appropriate formats

Aim

• To electrolyse sodium chloride solution (brine) and identify the products at the electrodes

Health & Safety Aspects

Hazard symbols to show substances that are corrosive, toxic flammable and hazardous to the environment

• Safety glasses should be worn
• The products of this electrolysis are hazardous
  • Sodium hydroxide produced is a corrosive substance
  • The chlorine gas produced is toxic and harmful to the environment. The reaction should be carried out in a well ventilated area or fume cupboard
  • The hydrogen gas produced is a flammable gas is produced so you should ensure there are no naked flames in proximity to the equipment

Electrolysis of aqueous solutions

Materials

• Electrolysis cell
• Retort stand, clamp & boss
• 0.5 mol /dm³ sodium chloride solution
• DC Power pack or battery packs
• Wires & crocodile clips
• Micro test tubes to collect gases
• Blue litmus paper
• Universal indicator solution

Method

1. Set up the electrolysis cell and make sure it is secured to the stand with a clamp and boss
2. Fill the cell with the sodium chloride solution
3. Fill the micro tubes with sodium chloride solution and carefully invert them over each electrode to collect any gases given off
4. Connect the wires and crocodile clips to the electrodes and power pack
5. Turn on the current and run until the microtubes are filled with gas, then switch the power off
6. Test the gases in the microtubes with a) a lighted splint and b) damp blue litmus paper

Remember:

• The negative terminal is the cathode (where cations migrate to)
• The positive terminal is the anode (where anions migrate to)

Results

Observations from the Electrolysis Experiment

Conclusions 

• The gas produced at the cathode (negative electrode) burns with a squeaky pop, indicating hydrogen
• The gas at the anode initially turns the blue litmus red, indicating an acid, and is then bleached
  • The gas is chlorine which dissolves in the water forming hydrochloric acid
• The universal indicator turns blue/purple indicating a strong alkali has been formed
• The solution is sodium hydroxide
  • The indicator colour will often fade due to the bleaching action of the chlorine produced at the anode

Electrolysis of Aqueous Copper(II)Sulfate

Aim

• To electrolyse copper(II) sulfate solution and identify the products at the electrodes

Health & Safety Aspects

Hazard symbols to show substances that are corrosive, harmful to health and flammable

• Copper(II) sulfate solution is corrosive and harmful to health as it is a skin irritant and can cause serious eye damage 
• Propanone, which is often used to clean the electrodes, is flammable
• Avoid contact with the skin and use safety goggles when handling copper(II) sulfate solution
• Propanone should be kept away from naked flames, e.g. a Bunsen burner
• Safety glasses should be worn

Materials

• Electrolysis cells
• Retort stand, clamp & boss
• 0.5 mol / dm³ copper(II) sulfate solution
• DC Power pack or battery packs
• Wires & crocodile clips
• Micro test tubes to collect gases
• Blue litmus paper
• Universal indicator solution

Method

1. Set up the electrolysis cell and make sure it is secured to the stand with a clamp and boss
2. Fill the cell with the copper(II) sulfate solution
3. Fill the microtubes with copper(II) sulfate solution and invert them carefully over each electrode to collect any gases given off
4. Connect the wires and crocodile clips to the electrodes and power pack
5. Turn on the current and run until the microtubes are filled with gas
6. Test the gases in the microtubes with a glowing splint

Results

Observations from the Electrolysis Experiment

Conclusions 

• The gas produced at the anode (positive electrode) relights a glowing splint, indicating oxygen
• A pink deposit is seen at the cathode, indicating copper metal
• The colour of the remaining solution begins to fade indicating the concentration of the copper(II) sulfate solution has decreased