5.1.3 The Energy Change of Reactions (HT only)

During a chemical reaction, energy changes occur as bonds between atoms are broken and new bonds are formed. Understanding these energy changes is crucial for determining whether a reaction is exothermicA reaction that releases heat, where the products have less energy than the reactants. or endothermicA reaction that ‘absorbs’ heat, where the products have greater energy than the reactants., as well as for quantifying the amount of energy transferred during a reaction.

Energy Changes in Chemical Reactions

To break the bonds in the reactants, energy must be supplied. This energy is called the bond dissociation energy or bond energy. It represents the amount of energy required to break a specific bond. Each type of bond has a characteristic bond energy associated with it.

Conversely, when new bonds are formed in the products, energy is released. This energy is referred to as the bond formation energy. Like bond dissociation energy, it represents the energy released when a particular bond is formed.

The overall energy change of a reaction can be calculated by considering the difference between the energy needed to break the bonds in the reactants and the energy released when the bonds in the products are formed. If more energy is released during bond formation than is required to break the bonds, the reaction is exothermic, and the overall energy change is negative. In an exothermic reaction, the products have a lower energy content than the reactants.

On the other hand, in an endothermic reaction, the energy needed to break the existing bonds is greater than the energy released from forming new bonds. As a result, the overall energy change is positive, and energy is absorbed from the surroundings. The products of an endothermic reaction have a higher energy content than the reactants.

Bond energies can be used to estimate the overall energy change of a reaction. By summing the bond dissociation energies of the bonds broken in the reactants and subtracting the sum of the bond formation energies of the bonds formed in the products, an approximation of the overall energy change can be obtained.

Bond Energy Calculation

Bond energy, also known as bond dissociation energy, is the amount of energy required to break a specific type of bond in a molecule. It is typically reported in kilojoules per mole (kJ/mol). The bond energy values can be found in reference tables or obtained from experimental data.

To calculate the energy transferred in a reaction using bond energies, the following steps can be followed:

1. Write the balanced chemical equation for the reaction of interest.
2. Identify the bonds that are broken and formed during the reaction.
3. Determine the bond energies for the bonds being broken and formed. These values can be obtained from reference tables or experimental data.
4. Multiply the number of bonds broken by their corresponding bond energy values to calculate the energy required to break the bonds.
5. Multiply the number of bonds formed by their corresponding bond energy values to calculate the energy released from bond formation.
6. Subtract the total energy required to break the bonds from the total energy released from bond formation. This will give the overall energy change of the reaction.

Bond energy values represent average values and may vary depending on the specific molecules and environments involved in the reaction. Additionally, bond energies do not take into account other factors that can influence the energy changes in a reaction, such as changes in molecular geometry or intermolecular forces.

Conclusion

During a chemical reaction, energy is required to break bonds in the reactants, while energy is released when new bonds are formed in the products. The overall energy change of a reaction is determined by comparing the energy needed to break bonds with the energy released from bond formation. Exothermic reactions release more energy than is required to break bonds, while endothermic reactions require more energy to break bonds than is released during bond formation.