Students find difficulties in answering question related to this topic.
- which electrode is the negative terminal/positive terminal?
- mark the flow of electrons on the circuit
- which one is the oxidized substsance/releases electron/reducing agent?
- which one is the reduced substance/gain electron/oxizing agent.
- write half equation to show the reaction occur at each electrode
- write overall ionic equation for the reaction that occur in the U-tube
- Write observation.
Actually a student only need to remember some examples of oxidizing agent and reducing agent. Some examples of oxidizing agent and reducing agent are as follows
Lets take an examples when iron(II) sulphate solution and bromine water are used.
iron(II) sulphate solution is the reducing agent.
- iron(II) ion loses electron to form iron(III) ion.
- So, iron(II) ion is the oxidized substance.
- As electrons is produced, the electrode that dipped in the iron(II) sulphate solution is negative terminal
- electrons flow from this eletrode to another one through the external circuit.
- Observation: the green iron(II) sulphate solution turn yellow/brown
- Half equation: Fe2+ --> Fe3+ + e-
Bromine water is the oxizing agent.
- electrode that dipped in bromine receive electron from the external circuit.
- So, this electrode is the positive terminal.
- Bromine gain electron, so bromine is reduced to bromide ion.
- So bromine is the reduced substance.
- Half equation: Br2 + 2e- --> 2Br-
- Observation : the brown bromine water turn colourless.
As writing overall ionic equation, a student just need to add both half equation together.
Half equation 2Fe2+ + Br2 --> 2Fe3+ + 2 Br-
Try different questions to see how much you apply the tips given. All the best!
Photos shown my students were working in groups to set up apparatus to study transfer of electrons at at distance.