Write out half reactions to show how many electrons are gained or lost by each species. Zn is oxidized and H+ is reduced
Oxidation half reaction:
Reduction half reaction:
Oxidation half reaction: Zn -> Zn2+ + 2e- (2 electrons are lost by Zn)
Reduction half reaction: 2H+ + 2e- -> H2 (2 electrons are gained by H+)