Half-equations are components that make up an overall reaction equation. Two half-equations (usually written as empirical equations) can be written as a pair in which overall reaction equation is form (must be balanced according to electron charge). The two that can form a pair must each be an oxidation and reduction half-reactions. Usually students taking SPM level chemistry will see this for redox reactions with ions or in acqueous solutions. Berry Berry Easy would like to clear all doubts from Berry Readers regarding this half-equations through Part 8 of the summarised notes for SPM Chemistry Form 4 on Electrochemistry. Also in this notes is the application of what you have learnt through the comparison of electrolytic cells and voltaic cells.
[Tips: Students occasionally forget to use minimum stoichiometric values when combining two half-equations as a pair. Although questions like this occur rarely in SPM level, but they do exist. The reason behind the requirement to write in minimum stoichiometric values is due to the half-equations being empirical in nature.]
SPM Chemistry Form 4 Notes – Electrochemistry (Part 8 )
Voltaic Cell
Berry Important Half Equations
Anode:
- 2Cl - –> Cl2 + 2e
- 2Br - –> Br2 + 2e
- 2I - –> I2 + 2e
- 4OH- –> 2H2O + O2 + 4e
Cathode:
- 2H+ + 2e –> H2
- Zn2+ + 2e –> Zn
- Fe2+ + 2e –> Fe
- Pb2+ + 2e –> Pb
- Cu2+ + 2e –> Cu
- Ag+ + e –> Ag
How to write overall reaction equation?
Example 1:
- Anode: 2I - –> I2 + 2e
- Cathode: 2H+ + 2e –> H2
- Overall: 2I - + 2H+ –> I2 + H2
Example 2:
- Anode: 2Br - –> Br2 + 2e
- Cathode: Pb2+ + 2e –> Pb
- Overall: 2Br - + Pb2+ –> Br2 + Pb
Example 3:
- Anode: 4OH- –> 2H2O + O2 + 4e
- Cathode: 2H+ + 2e –> H2 (x2)
- Overall: 4OH- + 4H+ –> 2H2O + O2 + 2H2
Example 4:
- Anode: 4OH- –> 2H2O + O2 + 4e
- Cathode: Cu2+ + 2e –> Cu (x2)
- Overall: 2Cu2+ + 4OH- –> 2Cu + 2H2O + O2
.
Comparison of Electrolytic Cells and Voltaic Cells
Similarities:
- Two electrodes involves in the reaction
- Electrons flow through the external circuit (connecting wires)
- Anode (oxidation): loss of electrons
- Cathode (reduction): gain of electrons
Differences:
| Electrolytic Cell | Voltaic Cell | |
| Main basic structures | A battery is needed to supply electrical energy | Battery is not needed. |
| Energy conversion | Electrical energy –> chemical energy | Chemical energy –> electrical energy |
| Transfer of electrons at the positive terminal | Anode (positive terminal): Oxidation – anions lose electrons at the anode | Cathode (positive terminal): Reduction |
| Transfer of electrons at the negative terminal | Cathode (negative terminal): Reduction – cations accept electrons from the cathode | Anode (negative terminal): Oxidation |
In the next part, Part 9 of short notes from Berry Berry Easy on Electrochemistry for SPM Chemistry Form 4 students, the main focus will be on the Electrochemical Series which all students love to hate. 
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