Calculate ΔH°f of octane, C₈H₁₈(l), given the enthalpy of combustion of octane to CO₂(g) and H₂O(l), -5471 kJ/mol, and the standard enthalpies of formation of CO₂(g) and H₂O(l), -393.5 kJ/mol and -285.8 kJ/mol, respectively.

Question

Quizplus · Accepted Answer

The enthalpy change for the combustion of octane, ΔH°rxn, can be calculated using the enthalpies of combustion of octane, ΔH°comb, and the enthalpies of formation of products, ΔH°f:

ΔH°rxn = ΣΔH°f(products) - ΣΔH°f(reactants)
ΔH°rxn = [ΔH°f(CO2) + ΔH°f(H2O)] - [ΔH°f(C8H18)]
ΔH°rxn = [-393.5 kJ/mol + (-285.8 kJ/mol)] - [-5471 kJ/mol]
ΔH°rxn = +490.3 kJ/mol

Since the enthalpy of combustion is an exothermic reaction (heat is released), the enthalpy of formation for octane must be positive (heat is required to form it). Thus, ΔH°f for octane can be calculated by rearranging the equation:

ΔH°f(C8H18) = ΣΔH°f(products) - ΣΔH°f(reactants)
ΔH°f(C8H18) = [ΔH°f(CO2) + ΔH°f(H2O)] - ΔH°rxn
ΔH°f(C8H18) = [-393.5 kJ/mol + (-285.8 kJ/mol)] - (+490.3 kJ/mol)
ΔH°f(C8H18) = -249.2 kJ/mol

Therefore, the answer is A, -249.2 kJ/mol.

Calculate ΔH°f of Octane, C8H18(l), Given the Enthalpy of Combustion

Calculate ΔH°f of Octane, C₈H₁₈(l), Given the Enthalpy of Combustion