In writing equations for oxidation-reduction reactions, just as for other reactions, the compositions and formulas must be known for the substances that reacts and for the products that are formed. The oxidation No. method is now best illustrated in the following steps:
- Write the correct formula for each reactant and product.
- Identify atoms which undergo change in oxidation number in the reaction by assigning the oxidation number to all elements in the reaction.
- Calculate the increase or decrease in oxidation number per atom and for the entire molecule/ion in which it occurs. If these are not equal then multiply by suitable no. so that these become equal. (If you realise that two substances are reduced and nothing is oxidised or vice-versa, something is wrong. Either the formulas of reactants or products are wrong or the oxidation no.s have not been assigned properly).
- Ascertain the involvement of ions if the reaction is taking place in water, add H+ or OH- ions to the expression on the appropriate side so that the total ionic charges of reactants and products are equal. If the reaction is carried out in acidic solution, use H+ ions in the equation; if in basic solution, use OH- ions.
- Make the numbers of hydrogen atom in the expression on the two sides equal by adding H2O molecules to the reactants or products. Now, also check the number of oxygen atoms. If there are the same number of oxygen atoms in the reactants and products, the equation then represents the balanced redox reaction.
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