Energy Changes: Exothermic Reactions

Understanding Exothermic Reactions

Exothermic reactions are reactions that release energy to the surroundings, usually in the form of heat.
During exothermic reactions, the energy required to break the bonds in the reactants is less than the energy released when new bonds are formed in the products.
Consequently, the surrounding temperature increases during exothermic reactions.
Combustion reactions (i.e., burning) and neutralisation reactions (acid + base producing a salt and water) are common examples of exothermic reactions.

Energy Changes in Exothermic Reactions

In a chemical reaction, the energy difference between the reactants and products is termed ΔH, where ΔH = energy of bonds broken - energy of bonds formed.
For exothermic reactions, ΔH will be negative, indicating that energy is released.
The energy level diagram of an exothermic reaction shows the energy of products to be lower than the energy of reactants.

Measuring Energy Change

The energy change in a reaction can be measured using a calorimeter, where the heat given out or taken in by a solution is used to determine the amount of energy involved in the reaction.
The experimentally determined quantity of heat can be used to calculate the enthalpy change, ΔH.

Applications of Exothermic Reactions

Hand warmers and self-heating cans utilise the principles of exothermic reaction to provide heat.
The process of respiration, which is essential to life, is an example of an exothermic reaction.
Most combustion processes, like burning wood or petrol, are exothermic.

Factors Affecting Exothermic Reactions

Factors such as temperature, surface area, concentration, and presence of a catalyst can impact the rate at which an exothermic reaction proceeds.
Increasing the temperature, concentration, or surface area, or adding a catalyst, typically accelerates the rate of reaction.