For a particular isomer of C 8 H 18, the combustion reaction produces 5099.5 kJ of heat per mole of C 8 H 18 (g) consumed, under standard conditions. C 8 H 18 (g) + 25 2 O 2 (g) ⟶ 8 CO 2 (g) + 9 H 2 O (g) Δ H ∘ rxn = - 5099.5 kJ / mol What is the standard enthalpy of formation of this isomer of C 8 H 18 (g) ?

the standard enthalpy of formation of this isomer of C₈H₁₈ (g) = - 375 kj/mol

Explanation:

The given combustion reaction

C₈H₁₈ + 25/2 O₂ → 8 CO₂ + 9 H₂O ... (1)

Heat of reaction or enthalpy of combustion = - 5099.5 kj/mol

from equation (1)

ΔH⁰reaction = (Enthalpy of formation of products - Enthalpy of formation of reactants)

Or, - 5099.5 = [8 x ΔH⁰f (CO₂) + 9 x ΔH⁰f (H₂O) ] - [ΔH⁰f (C₈H₁₈) + ΔH⁰f (O₂) ] ... (2)

Given ΔH⁰f (CO₂) = - 393.5 kj/mol & ΔH⁰f (H₂O) = - 285.8 kj/mol and ΔH⁰f (O₂) = 0

Using equation (2)

ΔH⁰f (C₈H₁₈) = - 621 kj/mol