Also called the law of constant heat summation: the total enthalpy change for a chemical process between defined initial and final states is independent of the pathway taken and equals the sum of enthalpy changes of constituent steps.

Principle

Enthalpy is a state function; therefore ΔH for a transformation depends only on initial and final states, allowing additive combination of known enthalpy changes to obtain unknown ΔH values.

Demonstration

To determine the enthalpy of formation of a compound, one combines measured enthalpies of combustion for reactants and products in a thermochemical cycle so that intermediate steps cancel, yielding the desired ΔHf.

Misapplication

Treating non-state properties (heat q, work w) as path-independent or neglecting phase and temperature dependencies of enthalpy (heat capacities) when summing steps can produce incorrect results.

Consequence

Hess's law enables indirect determination of reaction enthalpies, construction of thermochemical tables, and simplification of energetic analyses by breaking complex transformations into measurable steps.

Reversal

For path-dependent quantities (e.g., heat transferred in a specific process, irreversible work), summing stepwise values without accounting for path yields different totals; thus not all energetic measures obey Hess's law.

Boundary

Applies only to state functions (enthalpy) and requires that initial and final states be well-defined; temperature, pressure, phase, and composition must be specified because ΔH generally depends on those variables.

Semantic Tension

Often conflated with conservation of energy generally; Hess's law is a specific consequence of enthalpy being a state function rather than a separate conservation statement about heat or work paths.

Synthesis

Hess's law formalizes that enthalpy changes are path-independent state-function values, permitting construction of enthalpies for complex reactions by algebraic summation of simpler, measured steps when conditions (state definitions) are consistent.