5.2.1—Hess’s law to construct simple energy cycles

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.