Hess’s Law Practice Problems Answers
Determine ∆Ho for each of the following problems. Use a separate piece of paper to show
your work. You can always check your answer using molar enthalpies of formation (∆Hof)
1. Find the standard molar enthalpy for the reaction C(s) + ½ O2(g) → CO(g)
Given that C(s) + O2(g) → CO2(g) ∆Ho = -394 kJ
and CO2(g) → CO(g) + ½ O2(g) ∆Ho = +283 kJ
1. Cancel C(s) + O2(g) → CO2(g) ∆Ho = -394 kJ
CO2(g) → CO(g) + ½ O2(g) ∆Ho = +283 kJ
2. Add
C(s) + ½ O2(g) → CO(g) ∆Ho = -111 kJ
2. The standard enthalpy changes for the formation of aluminium oxide and iron (III) oxide
are 2 Al(s) + 3/2 O2(g) → Al2O3(s) ∆Ho = -1676 kJ
2 Fe(s) + 3/2 O2(g) → Fe2O3(s) ∆Ho = -824 kJ
Calculate ∆Ho for the reaction: Fe2O3(s) + 2 Al(s) → Al2O3(s) + 2 Fe(s)
1. Inverse B 2 Al(s) + 3/2 O2(g) → Al2O3(s) ∆Ho = -1676 kJ
Fe2O3(s) → 2 Fe(s) + 3/2 O2(g) ∆Ho = +824 kJ
2. Cancel 2 Al(s) + 3/2 O2(g) → Al2O3(s) ∆Ho = -1676 kJ
Fe2O3(s) → 2 Fe(s) + 3/2 O2(g) ∆Ho = +824 kJ
3. Add Fe2O3(s) + 2 Al(s) → Al2O3(s) + 2 Fe(s) ∆Ho = -852 kJ
3. Coal gasification converts coal into a combustible mixture of carbon monoxide and
hydrogen, called coal gas, in a gasifier: H2O(l) + C(s) → CO(g) + H2(g) ∆Ho = ?
Calculate the standard enthalpy change for this reaction from the following chemical
equations: 2 C(s) + O2(g) → 2 CO(g) ∆Ho = -222 kJ
2 H2(g) + O2(g) → 2H2O(g) ∆Ho = -484 kJ
H2O(l) → H2O(g) ∆Ho = +44 kJ
1. Divide equation A and B by 2 and inverse equation B
C(s) + ½O2(g) → CO(g) ∆Ho = -111 kJ
H2O(g) → H2(g) + ½O2(g) ∆Ho = +242 kJ
H2O(l) → H2O(g) ∆Ho = +44 kJ
2. Cancel C(s) + ½O2(g) → CO(g) ∆Ho = -111 kJ
H2O(g) → H2(g) + ½O2(g) ∆Ho = +242 kJ
H2O(l) → H2O(g) ∆Ho = +44 kJ
3. Add H2O(l) + C(s) → CO(g) + H2(g) ∆Ho = +175 kJ
