Half-reaction balancing method
Consider the reaction below:
Cl2 + 2Fe2+ → 2Cl− + 2Fe3+
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The two elements involved, iron and chlorine, each change oxidation state; iron from +2 to +3, chlorine from 0 to -1. There are then effectively two half-reactions occurring. These changes can be represented in formulas by inserting appropriate electrons into each half-reaction:
Fe2+ → Fe3+ + e− Cl2 + 2e− → 2Cl−
Given two half-reactions it is possible, with knowledge of appropriate electrode potentials, to arrive at the full (original) reaction the same way. The decomposition of a reaction into half-reactions is key to understanding a variety of chemical processes. For example, in the above reaction, it can be shown that this is a redox reaction in which Fe is oxidised, and Cl is reduced. Note the transfer of electrons from Fe to Cl. Decomposition is also a way to simplify the balancing of a chemical equation. A chemist can atom balance and charge balance one piece of an equation at a time.
For example:
- Fe2+ → Fe3+ + e- becomes 2Fe2+ → 2Fe3+ + 2e-
- is added to Cl2 + 2e- → 2Cl-
- and finally becomes Cl2 + 2Fe2+ → 2Cl- + 2Fe3+
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It is also possible and sometimes necessary to consider a half-reaction in either basic or acidic conditions, as there may be an acidic or basic electrolyte in the redox reaction. Due to this electrolyte it may be more difficult to satisfy the balance of both the atoms and charges. This is done by adding H2O, OH-, e-, and or H+ to either side of the reaction until both atoms and charges are balanced.
Consider the half-reaction below:
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PbO2 → PbO
OH-, H2O, and e- can be used to balance the charges and atoms in basic conditions.
2e- + H2O + PbO2 → PbO + 2OH-
Again Consider the half-reaction below:
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PbO2 → PbO
H+, H2O, and e- can be used to balance the charges and atoms in acidic conditions.
2e- + 2H+ + PbO2 → PbO + H2O
Notice that both sides are both charge balanced and atom balanced.
Often there will be both H+ and OH- present in acidic and basic conditions but that the resulting reaction of the two ions will yield water H2O (shown below):
H+ + OH- → H2O
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