For a particular isomer of C 8 H 18, C8H18, the combustion reaction produces 5104.1 kJ 5104.1 kJ of heat per mole of C 8 H 18 (g) C8H18 (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 = - 5104.1 kJ / mol C8H18 (g) + 252O2 (g) ⟶8CO2 (g) + 9H2O (g) ΔHrxn°=-5104.1 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 octane is - 220.1 kJ/mol

Explanation:

Step 1: Data given

The combustion reaction of octane produces 5104.1 kJ per mol octane

Step 2: The balanced equation

C8H18 (g) + 12.5 O2 ⟶ 8CO2 (g) + 9 H2O (g) ∆H°rxn = - 5104.1 kJ/mol

Step 3:

∆H°rxn = ∆H°f of products minus the ∆H° of reactants

∆H°rxn = ∆H°f products - [∆H°f reactants]

-5104.1 kJ/mol = (8*∆H°fCO2 + 9*∆H°fH20) - (∆H°fC8H18 + 12.5∆H°fO2)

∆H°f C8H18 = ∆H°f 8CO2 + ∆H°f 9H2O + 5104.1 kJ/mol

∆H°f C8H18 = 8 * (-393.5 kJ) / mol + 9 * (-241.8 kJ/mol) ] + 5104.1 kJ / mol

∆H°f C8H18 = - 220.1 kJ/mol

The standard enthalpy of formation of this isomer of octane is - 220.1 kJ/mol