Balancing redox reactions in acidic solution
Oxidants and reductants in redox reactions may be polyatomic species where only one of the atoms changes in oxidation number.
Equations for redox reactions involving such species are most easily balanced as two half equations that are then combined to give the overall equation. One way of doing this is shown below.
Identify the atom(s) in the reactants that are in a different oxidation state in the products. One of the reactant species is the reductant; the other is the oxidant. More
Construct two half equations.
One of these has the oxidant as a reactant giving its product; the other has the reductant as a reactant giving its product. More
Balance the half equations separately following the steps below:
Combine the half equations by addition so that electrons cancel. More
If necessary, each equation is multiplied by a factor before addition. More
Equations for redox reactions involving such species are most easily balanced as two half equations that are then combined to give the overall equation. One way of doing this is shown below.
Identify the atom(s) in the reactants that are in a different oxidation state in the products. One of the reactant species is the reductant; the other is the oxidant. More
Consider HOCl + Br– 
Cl– + Br2.
In this reaction Cl(+1) (the oxidant) gives Cl(-1) and Br(-1) (the reductant) gives Br(0).
Cl– + Br2. In this reaction Cl(+1) (the oxidant) gives Cl(-1) and Br(-1) (the reductant) gives Br(0).
Construct two half equations.
One of these has the oxidant as a reactant giving its product; the other has the reductant as a reactant giving its product. More
The two half equations for the reaction above would be
HOCl Cl– and 2Br– Br2
HOCl
Cl– and 2Br– Br2Balance the half equations separately following the steps below:
- Balance atoms other than oxygen and hydrogen.Moreno change for
HOCl Cl– and 2Br– Br2 - Balance oxygen by adding H2O molecules MoreH2O is an oxygen-rich species in aqueous solution that is acidic.
HOCl Cl– + H2O - Balance hydrogen by adding H+. MoreH+ (as H3O+) is the hydrogen-rich species in an aqueous solution that is acidic.
H+ + HOCl Cl– + H2O
The H+ added combines with the O(-2) in the compound or ion from the same side of the equation to produce the H2O at the right. - Calculate total charge on reactants and total charge on products. MoreMultiply the charges shown by the coefficient on the charged species. Then add. Positives and negatives cancel!!
The total charge in the HOCl equation is +1 at the left and -1 at the right.
The total charge for the bromine equation is -2 at the left and 0 at the right. - Balance charge by adding electrons. MoreElectrons are negative so they should be added to the more positive side.
For HOCl: Add electrons at the left. Two are required
H+ + HOCl + 2e– Cl– + H2O and 2Br– Br2 + 2e–)
For bromine: Add electrons at the right. Two are required.
Combine the half equations by addition so that electrons cancel. More
The result above is: H+ + HOCl + 2Br– Cl– + H2O + Br2
Cl– + H2O + Br2If necessary, each equation is multiplied by a factor before addition. More
This is not required for the example above, but consider the example below:
Al Al3+ + 3e– and Cl2 + 2e– 2Cl–
One equation must be multipled by 2 and the other by 3.
The resulting equation would be:
2Al + 3Cl2 2AlCl3 (where the two ions are combined to give the compound).
Al
Al3+ + 3e– and Cl2 + 2e– 2Cl–One equation must be multipled by 2 and the other by 3.
The resulting equation would be:
2Al + 3Cl2
2AlCl3 (where the two ions are combined to give the compound).