A clear visual guide to Hess’s Law calculations where the usual universal formula cannot be applied directly, showing how the algebraic method and Hess cycle method provide reliable alternatives.

The resource uses two worked examples of increasing complexity.

Example 1 – Standard Enthalpy of Formation of Ethane

Students calculate ΔH°f for:

2C(s) + 3H₂(g) → C₂H₆(g)

from:

2C(s) + 2H₂(g) → C₂H₄(g) ΔH = +52 kJ mol⁻¹

C₂H₄(g) + H₂(g) → C₂H₆(g) ΔH = −137 kJ mol⁻¹

This example illustrates an important situation in which one mole of hydrogen is effectively carried forward unchanged.

Using both the algebraic and Hess cycle approaches gives:

ΔH°f(C₂H₆) = −85 kJ mol⁻¹

Example 2 – Standard Enthalpy of Formation of Sucrose

A more demanding example calculates ΔH°f for sucrose from combustion data:

12C(s) + 11H₂(g) + 5½O₂(g) → C₁₂H₂₂O₁₁(s)

Students see how the supplied equations must be multiplied, reversed and combined to construct the required formation equation.

The resource reinforces the essential rule:

Reverse an equation → reverse the sign of ΔH.

Both approaches give:

ΔH°f(C₁₂H₂₂O₁₁) = −2234 kJ mol⁻¹

The resource is designed to follow naturally from simpler Hess’s Law examples where students can use the convenient products − reactants or reactants − products formulae.

The key progression is:

Use the shortcut formula when the data allow it — but understand Hess’s Law well enough to use algebra or construct a cycle when they do not.

Suitable for A-level Chemistry and equivalent courses covering Hess’s Law, thermochemical equations, enthalpies of formation and combustion, manipulation of equations and multi-step enthalpy calculations.

Ideal for classroom teaching, worked examples, revision and independent study.