Step 1: The standard enthalpy change of reaction is given by Hess's law using enthalpies of formation.
ΔHr∘xn=∑nΔHf∘(products)−∑nΔHf∘(reactants)
where n is the stoichiometric coefficient.
Step 2: Identify enthalpies of formation with units (all in kJ mol−1 at 298 K):
ΔHf∘(\ceCO2(g))=−393.5
ΔHf∘(\ceH2O(l))=−285.8
ΔHf∘(\ceCH4(g))=−74.8
ΔHf∘(\ceO2(g))=0
Step 3: Calculate sum for products.
Products: \ce1⋅CO2(g)+2⋅H2O(l)
∑nΔHf∘(products)=[1×(−393.5)]+[2×(−285.8)]kJmol−1
=−393.5+2×(−285.8)kJmol−1
=−393.5−571.6kJmol−1
=−965.1kJmol−1
Step 4: Calculate sum for reactants.
Reactants: \ce1⋅CH4(g)+2⋅O2(g)
∑nΔHf∘(reactants)=[1×(−74.8)]+[2×0]kJmol−1
=−74.8+0kJmol−1
=−74.8kJmol−1
Step 5: Subtract sums.
ΔHr∘xn=[−965.1]−[−74.8]kJmol−1
=−965.1+74.8kJmol−1
=−890.3kJmol−1
\textbf{-890 kJ mol^{-1}}