Answer to Question #59673 in General Chemistry for tom

Question

What is Hess law and how is it applied to chemical processes?

Answer 1

Germain Henri Hess, in 1840, discovered a very useful principle which is named for him:
The enthalpy of a given chemical reaction is constant, regardless of the reaction happening in one step or many steps.
It is usually presented like this:
ΔH°r = Σ ΔH°f (products) - Σ ΔH°f (reactants)
Hess's law is now understood as an expression of the principle of conservation of energy, also expressed in the first law of thermodynamics, and the fact that the enthalpy of a chemical process is independent of the path taken from the initial to the final state (i.e. enthalpy is a state function). Reaction enthalpy changes can be determined by calorimetry for many reactions.
Hess's law of constant heat summation is useful in the determination of enthalpies of the following:
1. Heats of very slow reactions
2. Heats of formation of unstable intermediates like CO(g) and NO(g).
3. Heat changes in phase transitions and allotropic transitions.
4. Lattice energies of ionic substances by constructing Born-Haber cycles if the electron affinity to form the anion is known, or
5. Electron affinities using a Born-Haber cycle with a theoretical lattice energy