The standard enthalpy of formation of the given isomer of C₈H₁₈(g) is -230.2 kJ.
Heat of formation of any compound is the change in enthalpy takes place for the formation of one mole of that compound.
Combustion reaction for the C₈H₁₈ will be written as:
C₈H₁₈(g) + 12.5O₂(g) ⟶ 8CO₂(g) + 9H₂O(g)
Total heat of reaction = -5093.7 kJ = -5094 kJ (as heat is released)
Heat of formation of O₂(g) = 0 kJ/mol
Heat of formation of CO₂(g) = -393.5 kJ/mol
Heat of formation of H₂O(g) = -241.8 kJ/mol
Heat of the reaction will be calculated by substracting the sum of heat of formations of reactants from the sum of the heat of formation of products as:
ΔH(reaction) = ΔHf(products) - ΔHf(reactants)
ΔHrxn = [8(CO₂) + 9(H₂O)] - [x + 12.5(O₂)]
On putting all these values on the above equation we get,
5094kJ = [8(-393.5) + 9(-241.8)] - [x + 12.5(0)]
-5094 kJ = [-3148 + (-2176.2)] - [x + 0]
-5094 kJ = -5324.2 - x
x = -230.2 kJ
Hence, standard enthalpy of formation of C₈H₁₈ is -230.2 kJ/mole.
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