Hess's law

Hess's law states that a reaction's enthalpy change is independent of the route between its initial and final states. It lets unknown reaction enthalpies be calculated by adding known ones.

Enthalpy: A thermodynamic state function measuring a system's heat content at constant pressure. Hess's law applies to changes in this state function.
Born–Haber cycle: A thermochemical cycle that accounts for the formation of an ionic solid through gaseous ions and atoms. It uses Hess's law to infer lattice enthalpy from other measured quantities.
Chemical equation: A symbolic representation of a chemical reaction showing reactants, products, and their stoichiometric proportions. Hess-law calculations combine equations while preserving atom balance.
Gibbs free energy: A thermodynamic state function that combines enthalpy and entropy to determine spontaneity at constant temperature and pressure. Like enthalpy, it is path-independent, but it answers a different thermodynamic question.
State function: A property whose value depends only on a system's current state, not the path taken to reach it. Its path independence is the thermodynamic basis of Hess's law.
Lattice energy: The energy change associated with forming an ionic solid from separated gaseous ions, under a specified convention. Born–Haber calculations use Hess's law to estimate this difficult-to-measure quantity.
Stoichiometry: The quantitative relationships among reactants and products in a chemical reaction. Stoichiometric coefficients determine how each reaction enthalpy must be scaled.
Entropy: A thermodynamic state function that quantifies energy dispersal and the number of microscopic configurations available to a system. Entropy changes also depend only on endpoints, but do not add as reaction enthalpies.
Thermodynamic cycle: A sequence of thermodynamic changes that returns a system to its initial state. The enthalpy changes around a closed cycle sum to zero.
Combustion calorimetry: The measurement of heat released when a substance burns in a calorimeter. Measured combustion enthalpies can be combined to obtain enthalpies for other reactions.

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