Question

Sulfur burns in oxygen in two steps:
i.S(s) + O2(g) ->SO2(g) change of Heat=-297kJ
ii. 2SO2(g) + O2(g)->2SO3(g) change of heat=-198kJ

Calculate the total heat released if this is the final reaction: 2S(s) + 3O2(g)->2SO3(g)

Answer 1

Hess' law. Multiply equation i by 2 (multiply H by 2 also) and add to equation ii. That will give you the equation you want so delta H for the final reaction is the sum of 2*i + ii = final rxn.