The standard enthalpy of formation of octane is -250.40 kJ.
The energy released or spent when one mole of a substance is formed is measured by the standard enthalpy of formation.
2 C₈H₁₈ (l) + 25 O₂ (g) → 16 CO₂ (g) + 18 H₂O (l)
The ΔH°rxn will be given by:
ΔH°rxn = 16 ΔH°f (CO₂ (g)) + 18 ΔH°f (H₂O (l)) - (2 ΔH°f (C₈H₁₈) (l) + 25 ΔH°f O₂ (g))
ΔH°rxn and all the ΔH°fs except for O₂ which is zero and the ΔH°(C₈H₁₈) (l) is then calculated:
On substituting the values:
-1.0940 × 10⁴ kJ = 16 mol x (-393.5 kJ/mol) + 18mol x (-285.8 kJ/mol) - 2 (ΔH°f (C₈H₁₈) (l) -1.094 x10⁴ kJ
= -6296 kJ + ( -5144.40 kJ ) - 2(ΔH°f(C₈H₁₈) (l)
-500.40 kJ/2 = ΔH°f(C₈H₁₈) (l)
-250.40 kJ = ΔH°f(C₈H₁₈) (l)
Thus, the standard enthalpy of formation of octane is -250.40 kJ.
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