Redox Reactions

Oxidation Numbers Rules for calculating oxidation numbers (in this order)

1. Free or uncombined elements have an oxidation number of 0 \(\hskip{1cm}\ce{Na, Be, K, Pb, H2, O2, P4}\)
2. In monatomic ions, the oxidation number = the ion’s charge

\(\hskip{1cm}\ce{Li+} = +1 \hskip{1cm} \ce{Fe^{3+}} = +3 \hskip{1cm} \ce{O^{2−}} = −2\)

1. The oxidation number of oxygen is usually \(-2\) except in \(\ce{H2O2}\) and \(\ce{O2^{2−}}\) where it′ s \(-1\)
2. The oxidation number of hydrogen is \(+1\) except when it’s bonded to metals in binary compounds - hydrides. In these cases, its oxidation number is \(-1\) \(\ce{LiH, CaH2, LiAlH4 = −1}\)
3. Group I metals are \(+1\), group II metals are \(+2\) and fluorine is always \(-1\)
4. The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on that molecule or ion.

Using Oxidation Numbers

If an oxidation number increases over a reaction, it has undergone oxidation If an oxidation number decreases over a reaction, it has undergone reduction

E.g. Identify which element is undergoing reduction and which is undergoing oxidation in the following equations.