3.5.4Movement and Interactions
Required Practical 21: Electrolysis of Solutions — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 7 parts Lesson 57 of 59
Part 1 of 5Aim and predictions
Aim
- Aim To find out which products form at each electrode when different aqueous solutions are electrolysed.
Prediction for copper salts
- Prediction for copper salts Copper is less reactive than hydrogen, so copper metal should deposit on the cathode.
Prediction for sodium salts
- Prediction for sodium salts Sodium is more reactive than hydrogen, so hydrogen gas should be released at the cathode.
Part 2 of 5Apparatus
The cell
- The cell A 100 cm3 beaker holds the electrolyte, with two graphite rods held by a stand and clamp.
- The rods are joined by leads to a low-voltage power supply.
Collecting gases
- Collecting gases Small test tubes can be placed over the electrodes to trap gases for testing.
- Blue litmus paper, a splint and a lighter are needed for the tests.
Part 3 of 5Method
1
Pour the solution into the beaker until it is about half full.
2
Place the two graphite electrodes in the solution so that they do not touch each other.
3
Connect the electrodes to the power supply and switch on, at a low voltage.
4
Watch each electrode and note any bubbles, colour or solid formed. Test any gas collected.
5
Switch off, remove the electrodes and check the cathode for a metal coating.
6
Repeat with each solution: copper chloride, copper sulfate, sodium chloride, sodium bromide and sodium nitrate, using clean electrodes each time.
Part 4 of 5Expected results
| Solution | Cathode | Anode |
|---|---|---|
| Copper chloride | Pink-brown copper coating | Bubbles of green-yellow chlorine |
| Copper sulfate | Pink-brown copper coating | Bubbles of colourless oxygen |
| Sodium chloride | Hydrogen bubbles | Chlorine bubbles |
| Sodium bromide | Hydrogen bubbles | Orange-brown bromine |
| Sodium nitrate | Hydrogen bubbles | Oxygen bubbles |
Part 5 of 5Gas tests and safety
Testing and safety
Hydrogen
- Hydrogen A lit splint pops with a squeak.
Oxygen
- Oxygen A glowing splint relights.
Chlorine
- Chlorine Damp blue litmus turns red, then bleaches.
Safety
- Safety Chlorine and bromine are toxic, so work in a fume cupboard or with small volumes in a ventilated room, and wear eye protection.
Key terms
| Term | Meaning |
|---|---|
| Electrolysis | Using an electric current to break down an ionic compound that is molten or in solution. |
| Inert | Does not react chemically; graphite electrodes are inert. |
| Effervescence | The formation of bubbles of gas in a liquid. |
| Deposit | A solid, such as copper metal, that forms on an electrode. |
| Reactivity series | A list of metals in order of how readily they form positive ions. |
Avoid these mistakes
1
Letting the electrodes touch, which causes a short circuit and no electrolysis.
2
Reusing dirty electrodes, so that copper from a previous run affects the next result.
3
Writing 'chlorine gas turns litmus red' without mentioning that it is then bleached white.
4
Failing to say the electrodes are inert, so that they do not take part in the reaction.
5
Describing hydrogen as relighting a glowing splint; that is the test for oxygen.
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