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Using the bond energies given below, calculate the standard enthalpy of combustion of methane gas CH4(g). Bond energies (kJ/mol): O=O: 495, C-H: 410, C=O: 800, O-H: 460 Resonance energy of CO2 = -140 kJ/mol Enthalpy of vaporisation of H2O(l) = 40 kJ/mol
- -990 kJ
- -1030 kJ
- -950 kJ
- -240 kJ
Correct answer: -1030 kJ
Solution
Breaking 4 C-H and 2 O=O requires 2630 kJ. Forming CO2 releases 1600+140=1740 kJ (including resonance) and forming 2H2O(l) releases 4*460+2*40=1920 kJ. Net: 2630-3660 = -1030 kJ.
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