Application of Hess Law

Hess’s Law of Constant Heat Summation, also known as Hess’s Law, has several applications in chemistry. Some specific applications and uses of Hess’s Law include:

Determination of Enthalpy of Formation

Hess Law can be used to determination of enthalpy of formation of any reaction. For example, lets calculate the enthalpy of formation of methane.

The enthalpy change for the reaction of carbon with hydrogen to form methane is -74.8 kJ/mol, and the enthalpy change for the combustion of methane to form carbon dioxide and water is -890.3 kJ/mol.

Therefore, the enthalpy of formation of methane is -74.8 kJ/mol + (-890.3 kJ/mol) = -965.1 kJ/mol.

Calculation of Standard Enthalpies of Reaction

Hess’s Law can be applied to calculate the standard enthalpies of reaction. For example, consider the reaction:

2H2(g) + O2(g) → 2H2O(l)

The standard enthalpies of formation for hydrogen gas and water are 0 kJ/mol and -285.8 kJ/mol, respectively. Using Hess’s Law, we can calculate the standard enthalpy change for the reaction as:

ΔH° = 2ΔH°_f(H₂O_(l)) – 2ΔH°_f(H_2(g)) – ΔH°_f(O_2(g))

ΔH° = 2(-285.8 kJ/mol) – 0 – 0 = -571.6 kJ/mol

Therefore, the standard enthalpy change for the reaction is -571.6 kJ/mol. This example illustrates how Hess’s Law can be used to calculate the standard enthalpy change for a reaction of interest by summing up the enthalpy changes of a series of reactions that lead to the formation of the products and reactants of the reaction of interest

Hess’s Law is a fundamental principle in thermodynamics that states that the total enthalpy change for a chemical reaction is independent of the pathway taken to reach the final state.

Hess’s Law is based on the first law of thermodynamics, which states that energy cannot be created or destroyed but can be converted from one form to another. Hess Law is also called Hess Law of Constant Heat. In this article, we will see Hess Law, its forms, applications, etc.