4 HNO2 + S → H2 + 4 NO + H2SO4
This is an oxidation-reduction (redox) reaction:
S0 - 6 e- → SVI (oxidation)
2 HI + 2 e- → 2 H0 (reduction)
4 NIII + 4 e- → 4 NII (reduction)
S is a reducing agent, HNO2 is an oxidizing agent, HNO2 is an oxidizing agent.