<p>How do you balance s8 + o2 --> so4^-2 using half reactions?</p>
<p>That cannot be balanced as a full reaction because there is a net charge change in the reaction (you can’t have two neutral reactants form net negatively charged products), but it can be balanced as a half-reaction itself as S8 + 16O2 + 16e- –> 8SO4^-2. This is the combination of the combustion S8 + 8O2 –> 8SO2 and SO2 + O2 + 2e- –> SO4^-2.
So there has to be another half reaction that consumes e- for the reaction to actually occur.</p>
<p>Er. My chemistry book apparently says it is balanceable, by combining the half reactions.</p>