Electrons must cancel on addition
The overall reaction equation for a redox process does not have electrons as either a product or a reactant because in a redox process the amounts in moles of electrons donated by the reductant must equal the amount in moles of electrons accepted by the oxidant.
Thus if the number of electrons accepted by the oxidant in the half equation is not equal to the number of electrons donated by the reductant, the half equations must be multiplied by factors so that the electrons in these equations cancel when the two half equations are added. An example where this must be done is shown below.
Thus if the number of electrons accepted by the oxidant in the half equation is not equal to the number of electrons donated by the reductant, the half equations must be multiplied by factors so that the electrons in these equations cancel when the two half equations are added. An example where this must be done is shown below.
For the reaction of aluminium metal with chlorine gas, the two half equations are below.
Half equation 1:
Al
Al3+ + 3e–
Half equation 2:
Cl2 + 2e– 2Cl–
The electrons will cancel if, before combining these half equation, Half equation 1 is multipled by 2 and half equation 2 must be multiplied by 3.
Half equation 1 times 2:
2Al 2Al3+ + 6e–
Half equation 2 times 3:
3Cl2 + 6e– 6Cl–
Overall equation:
2Al + 3 Cl2 2Al3+ + 6Cl–
Note that the product is equivalent to 2AlCl3
Half equation 1:
Al
Al3+ + 3e–Half equation 2:
Cl2 + 2e–
2Cl–The electrons will cancel if, before combining these half equation, Half equation 1 is multipled by 2 and half equation 2 must be multiplied by 3.
Half equation 1 times 2:
2Al
2Al3+ + 6e–Half equation 2 times 3:
3Cl2 + 6e–
6Cl–Overall equation:
2Al + 3 Cl2
2Al3+ + 6Cl– Note that the product is equivalent to 2AlCl3