The energy change associated with a chemical reaction is determined by subtracting the energy of the reactants from the energy of the products.

In a chemical reaction, energy can be absorbed or released, and this energy change is commonly referred to as the enthalpy change, denoted by the symbol $\Delta H$. The enthalpy change can be calculated using the formula:

where $H$ represents the total energy of the molecules involved in the reaction.

To compute the energy change, you first need to obtain the energies of both the reactants and the products. These energy values are typically expressed in kilojoules per mole ($\text{kJ/mol}$) and can be found in tables of standard enthalpies of formation. The energy of the reactants corresponds to the energy needed to break the bonds in the reactants, while the energy of the products refers to the energy released when new bonds are formed in the products.

To find the energy change, subtract the total energy of the reactants from the total energy of the products. If the result is negative, the reaction is classified as exothermic, indicating that it releases energy. Conversely, if the result is positive, the reaction is classified as endothermic, meaning it absorbs energy.

For example, if the energy of the reactants is $500 \, \text{kJ/mol}$ and the energy of the products is $300 \, \text{kJ/mol}$, the energy change can be calculated as follows:

This result indicates that the reaction is exothermic, releasing $200 \, \text{kJ}$ of energy per mole of reactant.

It is important to understand energy changes in reactions, as they provide insight into whether a reaction will occur spontaneously. Reactions that release energy (exothermic reactions) are generally more likely to proceed than those that absorb energy (endothermic reactions).