5.1.4 The Energy Change of Reactions

When a chemical reaction occurs, the amount of energy in the system changes. This change in energy can either involve absorbing energy from the surroundings or releasing energy to the surroundings. This is explained by the principle of conservation of energy, which states:

Energy cannot be created or destroyed; it can only be transferred from one form to another.

System and Surroundings

In the context of energy changes during reactions:

• The system refers to the chemical reaction being studied.
• The surroundings include everything outside the reaction, such as the reaction container and the surrounding environment.

We can also state the principle of the conservation of energy in terms of the universe:

The total energy of the universe remains constant.

Types of Energy Change in Reactions

Chemical reactions can be categorised based on whether energy is released or absorbed:

1. Exothermic Reactions:

• In exothermic reactions, the energy stored in the reactants is greater than the energy stored in the products.
• As a result, energy is released to the surroundings, often in the form of heat, causing the temperature of the surroundings to increase.
• Example: Combustion (burning) and neutralisation reactions. If the temperature goes up: The reaction is exothermic.

2. Endothermic Reactions:

• In endothermic reactions, the energy stored in the products is greater than the energy stored in the reactants.
• Therefore, energy must be absorbed from the surroundings for the reaction to occur, which often results in a decrease in the temperature of the surroundings.
• Example: Photosynthesis and thermal decomposition reactions. If the temperature goes down: The reaction is endothermic. You can also look at reaction profiles (energy diagrams) to see how energy changes during a reaction and whether energy is being released or absorbed.

Bond Energies

Every chemical bond will have a specific chemical energy associated with it and these bond energies can be used to calculate the overall energy change of a reaction. To do this, we subtract the energy of the bonds formed by that of the bonds broken.

$$Energy Change=Sum of Bonds Broken−Sum of Bonds Made$$

This means that the reaction releases energy overall.

• If the energy change is negative, the reaction is exothermic.
• When the energy change is positive, the energy released in making bonds is less than what is used to break bonds and so the reaction absorbs energy. This means it that the reaction is endothermic.