Exemplar Problems

Problem 19 : Calculate the standard enthalpy change (∆H°) for the following reaction: C(graphite) + 2H₂(g) → CH₄(g)

Given the standard enthalpies of formation: ∆H°f(C(graphite)) = 0 kJ/mol ∆H°f(H₂(g)) = 0 kJ/mol ∆H°f(CH₄(g)) = -74.81 kJ/mol

Solution :

∆H° = Σ(∆H°f(products)) - Σ(∆H°f(reactants))

∆H° = ∆H°f(CH₄(g)) - [∆H°f(C(graphite)) + 2∆H°f(H₂(g))]

∆H° = -74.81 kJ/mol - [0 kJ/mol + 2(0 kJ/mol)] ∆H° = -74.81 kJ/mol

So, the standard enthalpy change (∆H°) for the reaction is -74.81 kJ/mol.