Example: Equilibrate: $..Fe_2O_3$ $+$ $..CO$ $\longrightarrow$ $..Fe$ $+$ $..CO_2$ Method: - First the oxidation numbers of all atoms are determined:
| atom | o.n. | $Fe$$_2O_3$ | 3 | $Fe_2$$O$$_3$ | -2 | $C$$O$ | 2 | $C$$O$ | -2 | $Fe$ | 0 | $C$$O_2$ | 4 | $C$$O$$_2$ | -2 |
- For one atom $Fe$ the oxidation number decreases from 3 to 0 and thus by 3 units. - For one atom $C$ the oxidation number increases from 2 to 4 and thus by 2 units. - For one "molecule" $Fe_2O_3$ the oxidation number decreases therefore by $2\cdot3=6$ units. - For one molecule $CO$ the oxidation number increases therefore by 2 units - The "molecules" $Fe_2O_3$ and $CO$ must be in the ratio 1:3, and so we can write: $1$$Fe_2O_3$ $+$$3$$CO$ $\longrightarrow$ $..Fe$ $+$ $..CO_2$
The rest is easy guesswork: $1Fe_2O_3$ $+$ $3CO$ $\longrightarrow$ $2Fe$ $+$ $3CO_2$