Hess's Law Calculator

Formula

The formula to calculate the total enthalpy change using Hess's Law is:

\[ \Delta H = \sum \Delta H_f \text{(products)} - \sum \Delta H_f \text{(reactants)} \]

Where:

\(\Delta H\) is the total enthalpy change (kJ/mol)
\(\sum \Delta H_f \text{(products)}\) is the sum of the enthalpy of formation of the products (kJ/mol)
\(\sum \Delta H_f \text{(reactants)}\) is the sum of the enthalpy of formation of the reactants (kJ/mol)

What is Hess's Law?

Hess's Law, named after Russian chemist Germain Hess, states that the total enthalpy change during the complete course of a chemical reaction is the same whether the reaction is made in one step or in several steps. This is because enthalpy is a state function, meaning its value depends only on the current state of the system and not on the path taken to reach that state. Therefore, the total energy change in a chemical reaction is independent of the route by which the chemical reaction takes place.

Example Calculation

Let's assume the following values:

Sum of Enthalpy of Formation of Products (Σ ΔHf products) = -500 kJ/mol
Sum of Enthalpy of Formation of Reactants (Σ ΔHf reactants) = -300 kJ/mol

Using the formula:

\[ \Delta H = -500 - (-300) = -200 \, \text{kJ/mol} \]

The total enthalpy change (ΔH) is -200 kJ/mol.