5.2 Hess’s law

Hess’s law states that the enthalpy change for a reaction is independent of the route taken because enthalpy is a state function.

Manipulate known equations until they sum to the target equation. Reverse an equation and change the sign of ΔH; multiply an equation and multiply ΔH by the same factor.

If A→B has ΔH₁ and B→C has ΔH₂, then A→C has ΔH₁+ΔH₂. If a route uses C→B instead, subtract that step’s enthalpy.

Never add enthalpies without first checking that the chemical equations cancel correctly. The target equation, not the diagram’s layout, decides the final sign.

In an energy cycle, arrows represent enthalpy changes between the same states. Choose a route to the target, then combine the signed values along that route.

Write the target reaction explicitly, align intermediate species, and apply reversal/multiplication rules before calculating. A final sign check should match whether the target is exothermic or endothermic.

If direct A→C is unknown, but A→B=−80 kJ mol⁻¹ and B→C=+25 kJ mol⁻¹, then ΔH(A→C)=−55 kJ mol⁻¹.

Do not treat every number around a cycle as positive. A value belongs to its arrow direction; reversing that arrow reverses its sign.