- The standard enthalpy of formation (ΔH°f) for C₃H₈(g) is -103.8 kJ/mol.
- The standard enthalpy of formation (ΔH°f) for O₂(g) is 0 kJ/mol (by definition, for elements in their standard state).
- The standard enthalpy of formation (ΔH°f) for CO₂(g) is -393.5 kJ/mol.
- The standard enthalpy of formation (ΔH°f) for H₂O(g) is -241.8 kJ/mol.
Using Hess's Law and the formula:
ΔH°rxn = Σ n ΔH°f(products) - Σ m ΔH°f(reactants)
For the reaction: C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g)
ΔH°rxn = [3 × ΔH°f(CO₂(g)) + 4 × ΔH°f(H₂O(g))] - [1 × ΔH°f(C₃H₈(g)) + 5 × ΔH°f(O₂(g))]
Substitute the values:
ΔH°rxn = [3 × (-393.5) + 4 × (-241.8)] - [1 × (-103.8) + 5 × (0)]
ΔH°rxn = [-1180.5 + (-967.2)] - [-103.8 + 0]
ΔH°rxn = [-2147.7] - [-103.8]
ΔH°rxn = -2147.7 + 103.8
ΔH°rxn = -2043.9 kJ
The enthalpy change (ΔH°rxn) for the combustion of propane is -2043.9 kJ.
Parent Tip: Review the logic above to help your child master the concept of enthalpy worksheet.
