Hess’s Law

Basics of Hess’s Law

Hess’s Law states that the total enthalpy change for a chemical reaction is independent of the pathway by which the reaction occurs. It solely depends on the initial and final states.
Named after German-Swiss Chemist Germain Henri Hess.
This principle is a direct result of the law of conservation of energy, which stipulates that energy cannot be created or destroyed.
Hess’s Law is extremely useful because it allows scientists to calculate the enthalpy change using simpler reactions rather than carrying out complex or hazardous reactions directly.

Hess’s Law can be applied to calculate the enthalpy of formation or enthalpy of combustion using other known enthalpy changes.
For example, the enthalpy change for a combustion reaction of a particular substance can be found using the known enthalpies of formation for the reactants and products.
Hess’s Law is also applied in the calculation of bond enthalpies, i.e., the amount of energy needed to break a particular type of bond in a molecule.

To determine the overall enthalpy change (ΔH) for a reaction from Hess’s Law, addition or subtraction of individual enthalpy changes is performed, depending on whether reactions are reversed or multiplied.
If a reaction is reversed, the sign of its enthalpy change is also reversed.
If a reaction is multiplied by a factor, the enthalpy change of that reaction is also multiplied by the same factor.

Hess’s Law assumes that all reactions are carried out under constant conditions of temperature and pressure.
It cannot be applied if the reactant or product molecules are unstable. Only stable molecules with low energy levels can be used.
Hess’s Law does not provide any information about the rate of a reaction. It can tell us the overall energy change, but not how quickly this change will occur.
The use of Hess’s law assumes that all substances involved act ideally and do not interact with each other in unusual ways. This is often not the case, especially with gases and solutions.