Question

Calculate the heat of combustion of ethane, in the reaction C2H6(g) + 3 1/2O2(g) → 2CO2(g) + 3H2O(l) where the heats of formation of ethane gas, carbon dioxide gas and water liquid are –84.7 kJ mol^-1, -393.5 kJ mol^-1 and –285.8 kJ mol^-1 respectively.

(a) -1559.7 kJ mol^-1

(b) -155.7 kJ mol^-1

(c) -159.7 kJ mol^-1

(d) -559.7 kJ mol^-1

This question was addressed to me by my college director while I was bunking the class.

Asked question is from Thermodynamics topic in chapter Thermodynamics of Chemistry – Class 11

Answer 1

Correct answer is (a) -1559.7 kJ mol^-1

To explain: Given that C2H6(g) + 3 1/2 O2(g) → 2CO2(g) + 3H2O(l) As we know that, ΔHr = Σ ΔHf[products] – Σ ΔHf[reactants]; ΔHc(C2H6) = 2 X (-393.5 kJ mol^-1) + 3 X (-285.8 kJ mol^-1) – (- 84.7 kJ mol^-1) – 2 X (0 kJ mol^-1) = ΔHc(C2H6) = – 1559.7 kJ mol^-1.