Hess Law Formula

Formula to calculate the enthalpy change using Hess’s Law is expressed mathematically as the sum of the enthalpy changes for a series of reactions being equal to the enthalpy change for the overall reaction. The formula is:

ΔH°_reaction = ∑n × ΔH_f°_(products) − ∑n × ΔH_f°_(reactants)

Where,

• ΔH°_reaction is the enthalpy change for the reaction of interest,
• ΔH°_products is the standard enthalpy of formation of the products,
• ΔH°_reactants ​ is the standard enthalpy of formation of the reactants, and
• ∑n is the coefficient of each substance in the balanced chemical equation.

Enthalpy Change

Enthalpy change of a reaction is a measure of the heat evolved or absorbed during the reaction at constant pressure.

Enthalpy, denoted as H, is the heat content of a system at constant pressure. It is determined by the energies needed to break chemical bonds and the energies needed to form new bonds. The heat that is absorbed or released by a reaction at constant pressure is the same as the enthalpy change, given the symbol ΔH.

Example of Hess Law

For example, to find the enthalpy change for the reaction of carbon with hydrogen gas to form acetylene (C₂H₂), you could use the following equations:

C(s) + O₂(g) → CO₂(g) ΔH = -393.5 kJ/mol

2H₂(g) + O₂(g) → 2H₂O(l) ΔH = -572 kJ/mol

C₂H₂(g) + 2O₂(g) → 2CO(g) + H₂O(l) ΔH = +1299.6 kJ/mol

To find the enthalpy change for the formation of one mole of acetylene, you would add the enthalpy changes for the first two equations and then subtract the enthalpy change for the third equation, multiplied by a factor of 1/2 (since there is half a mole of CO₂ in the third equation):

ΔH(acetylene) = ΔH(CO₂) + ΔH(H₂O) – (1/2)ΔH(CO)

⇒ ΔH(acetylene) = (-393.5 kJ/mol) + (-572 kJ/mol) – (1/2)(1299.6 kJ/mol)

⇒ ΔH(acetylene) = -965.5 kJ/mol + 649.8 kJ/mol – 649.8 kJ/mol

⇒ ΔH(acetylene) = -315.7 kJ/mol

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.