← Thermochemical principles and the properties of particles and substances

4 Hess’s Law

Hess’s Law: the total enthalpy change for a reaction is independent of the route taken. It depends only on the initial and final states. This lets you calculate a ΔH you can’t measure directly.

Two ways to use it

From enthalpies of formation ΔH_r° = Σ ΔH_f°(products) − Σ ΔH_f°(reactants) Remember to multiply each by its coefficient in the balanced equation. From combustion data ΔH_r° = Σ ΔH_c°(reactants) − Σ ΔH_c°(products) Note this is reactants − products (the reverse of the formation version). ✓

Rules for adding equations in a Hess cycle

• Reverse an equation → change the sign of ΔH.
• Multiply an equation → multiply ΔH by the same factor.
• Cancel any species appearing on both sides once the equations are added.

Worked example Hess’s law from formation data Find ΔH° for C₂H₄(g) + H₂(g) → C₂H₆(g), given ΔH_f°: C₂H₄ = +52, C₂H₆ = −85 kJ mol⁻¹ (H₂ is an element, so 0). ΔH_r° = Σ ΔH_f°(products) − Σ ΔH_f°(reactants) = [ΔH_f°(C₂H₆)] − [ΔH_f°(C₂H₄) + ΔH_f°(H₂)] = (−85) − (+52 + 0) = −137 kJ mol⁻¹ ✓ ΔH° = −137 kJ mol⁻¹ (exothermic: an addition/hydrogenation reaction).

Practise just this