Exemplar Problems

Problem 9 : Determine the standard enthalpy change (∆H°) for the reaction:

N₂(g) + 3H₂(g) → 2NH₃(g) Given the standard enthalpies of formation (∆H°f) for N₂, H₂, and NH₃ as 0 kJ/mol, 0 kJ/mol, and -45 kJ/mol, respectively.

Solution :

To determine ∆H° for the reaction, we can use the standard enthalpies of formation:

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

∆H° = [2∆H°f(NH₃)] - [∆H°f(N₂) + 3∆H°f(H₂)]

∆H° = [2(-45 kJ/mol)] - [0 kJ/mol + 3(0 kJ/mol)]

∆H° = -90 kJ/mol

So, the standard enthalpy change for the reaction is -90 kJ/mol.