standard enthalpies of formation of CO2 and H2O and CH4 are -393.5, -286.2 and 74.8 kJmol-1 respectively. calculate the enthalpy of combustion of methane.

We have to calculate the enthalpy of combustion of methane that is we have to calculate ΔH for the following reaction  

CH₄ + 2O₂ --------> CO₂ + 2H₂O  .........................................(a)

We will calculate the value of ΔH for the above reaction by adding and subtracting the provided equations. The corresponding ΔH values for the reactions will also be multiplied, added and subtracted in the same manner. We are given that

C(gr) + O₂(g) ------------> CO₂(g) ΔH = -393.5 KJ/mol  .....................(i)

H₂(g) + 1/2O₂(g) -----------> H₂O(l) ΔH⁰ = -286.2KJ/mol  ...........................(ii)

C(gr) + 4H -----------------> CH₄  ΔH = 74.8 KJ/mol  .............................(iii)

To obtain the required equation (a), we will adopt the following steps:

(1) Multiply the (ii) equation by 2 and then add equation (i) and (ii), The corresponding ΔH values would also be multiplied by the respective numbers and added. Thus the net equation and the corresponding ΔH that we will obtain are as follows :

C + 2H₂ + 2O₂ ---------> CO₂ + 2H₂O  ΔH = -965.9KJ/mol  ............................(iv)

(2) Now invert the equation (iii), and then add it with equation (iv), The corresponding ΔH values would also be changed accordingly, we will get

CH₄ + 2O₂ --------> CO₂ + 2H₂O  ΔH = -1040.7 KJ/mol

Hence the heat of combustion for above reaction = -1040.7 KJ/mol