Hess's Law Problem

Question

Given:

SO2 + 1/2O2 ---> SO3 = Delta H of +89.5kJ

S + 3/2O2 -----> SO3 = Delta H of -204.2kj

Calculate the Delta H for the reaction:

S + O2 ---> SO2

Answer 1

S + O2 ==> SO2 .. TARGET EQUATION

Given:

eq. 1: SO2 + 1/2 O2 ==> SO3 .. ∆H = 89.5 kJ

eq 2: S + 3/2 O2 ==> SO3 .. ∆H = -204.2 kJ

Procedure:

reverse eq. 1: SO3 ==> SO2 + 1/2 O2 .. ∆H = -89.5 kJ

copy eq. 2: S + 3/2 O2 ==> SO3 .. ∆H = -204.2 kJ

Add the two...SO3 + S + 3/2 O2 ==> SO2 + 1/2 O2 + SO3

Combine/cancel like terms to end up with...

S + O2 ==> SO2 .. TARGET EQUATION

∆H = -89.5 + -204.2 = -293.7 kJ