##  [Hess's Law](/hesss-law-0) 

 Definition

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

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

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

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

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

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

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

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

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.