Question

Why do we need sulfuric acid in reducing nitric acid?

Answer 1

Concentrated nitric acid acts as an oxidizing agent for both sulfur & carbon. In the case of sulfur, S will get the oxygen atoms & the water needed for the reaction:S + 6HNO3 ----> H2SO4 + 2H2O + 6NO2 so if sulfur is converted to SO2 first, it will proceed into SO3 & eventually to H2SO4 . In the case of carbon,  C will get the oxygen atoms converting into CO2 "which is an end product". There are neither CO3 molecules nor H2CO3 (that can form under pressure from the product carbon dioxide).