Exemplar Problems

Problem 1 : Calculate the standard enthalpy change (∆H°) for the combustion of methane (CH₄) to form carbon dioxide (CO₂) and water (H₂O) according to the reaction: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

Given the standard enthalpies of formation (∆H°f) for CH₄, CO₂, and H₂O as -74.8 kJ/mol, -393.5 kJ/mol, and -285.8 kJ/mol, respectively.

Solution :

To calculate ∆H° for the combustion of methane, we can use the standard enthalpies of formation:

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

∆H° = [∆H°f(CO₂) + 2∆H°f(H₂O)] - [∆H°f(CH₄) + 2∆H°f(O₂)]

∆H° = [(-393.5 kJ/mol) + 2(-285.8 kJ/mol)] - [(-74.8 kJ/mol) + 2(0 kJ/mol)]

∆H° = (-393.5 kJ/mol - 571.6 kJ/mol) - (-74.8 kJ/mol) = -965.1 kJ/mol

So, the standard enthalpy change for the combustion of methane is -965.1 kJ/mol.