4 FeSO2 + 5 O2 + 2 H2SO4 → 2 Fe2(SO4)3 + 2 H2O
This is an oxidation-reduction (redox) reaction:
4 SII - 16 e- → 4 SVI (oxidation)
4 FeII - 4 e- → 4 FeIII (oxidation)
10 O0 + 20 e- → 10 O-II (reduction)
FeSO2 is a reducing agent, FeSO2 is a reducing agent, O2 is an oxidizing agent.