Question

Calculate the ΔH∘ for this reaction using the following thermochemical data:

CH4(g)+2O2(g)⟶CO2(g)+2H2O(l)	ΔH∘=−890.3kJ
C2H4(g)+H2(g)⟶C2H6(g)	ΔH∘=−136.3kJ
2H2(g)+O2(g)⟶2H2O(l)	ΔH∘=−571.6kJ
2C2H6(g)+7O2(g)⟶4CO2(g)+6H2O(l)

Answer 1

Multiply first equation with 4,

4CH4(g) + 8O2(g) ---> 4CO2(g) + 8H2O(l)   dHo = -3561.2 kJ

Multiply the second equation and reverse it,

2C2H6(g) ----> 2C2H4(g) + 2H2(g)      dHo = 272.6 kJ

Add the two and subtract third equation from it,

    2C2H6(g) + 4CH4(g) + 8O2(g) ---> 4CO2(g) + 8H2O(l) + 2C2H4(g) + 2H2(g)

-   2H2(g) + O2(g) ----> 2H2O(l)

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2C2H6(g) + 4CH4(g) + 7O2(g) ----> 4CO2(g) + 6H2O(g) + 2C2H4(g)

pl. note we need another equation to cancel out CH4 and C2H4 on each side.

dHo = -3561.2 + 272.6 + 571.6 = -2717.0 kJ