Question

Consider the following data:

S(s) + O2(g) → SO2(g)    ΔHrxn = -297 kJ

2SO3(g) → 2SO2(g) + O2(g)  ΔHrxn = 198 kJ

Use Hess's law to determine es002-1.jpgHrxn for this reaction:

2S(s) + 3O2(g) es002-2.jpg 2SO3(g)

es002-3.jpg =
kJ

Answer 1

Multiply the first equation by 2:
2S(s) + 2O2(g) → 2SO2(g) ΔH = 2(−297) = −594 kJ

Reverse the second equation:
2SO2(g) + O2(g) → 2SO3(g) ΔH = −198 kJ

Add them:
2S(s) + 3O2(g) → 2SO3(g) ΔH = −594 + (−198) = −792 kJ

ΔHrxn = −792 kJ.