Question

Use the ΔHrxn values of the following reactions:

2 SO2 (g) + O2 (g) → 2 SO3 (g) ΔHrxn = -196 kJ

1/4 S8 (s) + 3 O2 (g) → 2 SO3 (g) ΔHrxn = -790 kJ

to calculate the ΔHrxn value of this reaction: 1/8 S8 (s) + O2 (g) → SO2 (g)

Answer 1

2 SO2 (g) + O2 (g) → 2 SO3 (g) ΔHrxn = -196 kJ    ........ eqn 1

1/4 S8 (s) + 3 O2 (g) → 2 SO3 (g) ΔHrxn = -790 kJ ........ eqn 2

we need to find ΔHrxn for below reaction:
1/8 S8 (s) + O2 (g) → SO2 (g)   ....eqn 3

eqn 3 can be written as
0.5*eqn2 - 0.5*eqn 1

so,
ΔHrxn (3) = 0.5* ΔHrxn (2) -0.5* ΔHrxn (1)
                    = 0.5* (-790) - 0.5* (-196)
                    = -395 + 98
                    = -297 KJ
Answer: -297 KJ