For the combustion of gaseous methane: CH4(g) + 2 O2(g) -> CO2(g) + 2 H2O(l), given the standard enthalpies of formation:
ΔfH°(CH4) = -74.8 kJ/mol
ΔfH°(CO2) = -393.5 kJ/mol
ΔfH°(H2O(l)) = -285.8 kJ/mol
what is the standard enthalpy of combustion (ΔcH°) of methane?
Answer and explanation
Correct answer: A. -890.3 kJ/mol
- Step 1: According to Hess's Law: .
- Step 2: Sum of enthalpies of products: .
- Step 3: Sum of enthalpies of reactants: .
- Step 4: Subtract reactants from products: .
Answer
-890.3 kJ/mol
Why not the others: '-604.5 kJ/mol' fails to multiply the enthalpy of formation of water by its stoichiometric coefficient of 2; '-965.1 kJ/mol' forgets to subtract the enthalpy of formation of methane; '+890.3 kJ/mol' has the wrong positive sign (combustion is exothermic).
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