Redox reaction with tin and sulfuric acid solution
The sulfuric acid dissociates in water: $$\ce{H2SO4 -&gt; H+ + HSO4-}$$$$\ce{HSO4- -&gt; H+ + SO4^{2-}}$$

When writing redox equations the spectator ions are left out because they do not participate in the redox reaction. 

In this case it is the hydrogen ions which are oxidising the tin and the sulfate and hydrogensulfate ions play no part in the reaction so they are left out. Including them in the hydrogen half equation would give (with sulfate ions): $$\ce{2H+ + SO4^{2-} +2e- -&gt; H2 + SO4^{2-}}$$

Clearly the sulfate ions are unnecessary and so they are omitted from the equation.