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We can use Hess's law to calculate enthalpy changes that cannot be measured. One such reaction...

We can use Hess's law to calculate enthalpy changes that cannot be measured. One such reaction is the conversion of methane to ethylene: 2CH4(g)⟶C2H4(g)+2H2(g) 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 ΔH∘=−3120.8kJ

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Answer #1

Multiply first equation with 2,

2CH4(g) + 4O2(g) ----> 2CO2(g) + 4H2O(l) ------- dHo = -1780.6 kJ

Invert the second equation,

C2H6(g) -----> C2H4(g) + H2(g) ----------------------dHo = 136.3 kJ

Divide the third equation with 2 and invert,

H2O(l) ----> H2(g) + 0.5O2(g) -------------------------dHo = 285.8 kJ

Divide the fourth equation with 2 and invert,

2CO2(g) + 3H2O(l) -----> C2H6(g) + 3.5O2(g) ----dHo = 1560.4 kJ

Add all four equations and cancel common terms,

2CH4(g) ----> C2H4(g) + 2H2(g) -----dHo = -1780.6 + 136.3 + 285.8 + 1560 = 201.5 kJ

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