Exothermic Reactions

Definition of Exothermic Reactions

Exothermic reactions are chemical reactions that release energy to the surroundings, usually in the form of heat. This causes the temperature of the surroundings to increase.

Common examples of exothermic reactions include:

• Combustion, such as burning fuels like petrol or wood
• Respiration, the process cells use to release energy from glucose

When an exothermic reaction occurs, you can often feel the container or surroundings getting warmer.

Energy Changes in Reactions

Chemical reactions involve breaking and forming bonds between atoms. Energy changes happen because:

• Energy is needed to break bonds in the reactants (energy input)
• Energy is released when new bonds form in the products

In exothermic reactions, the energy released when bonds form is greater than the energy used to break bonds. This means the overall energy change is negative, indicating a release of energy.

For example, in combustion, the bonds in the fuel and oxygen break, and new bonds form in carbon dioxide and water. The energy released by forming these new bonds is more than the energy needed to break the original bonds, so heat is given out.

For instance, if the energy needed to break bonds is $400 \text{ kJ}$ and the energy released forming bonds is $600 \text{ kJ}$, the overall energy change is:

$\text{Energy change} = 400 \text{ kJ (in)} - 600 \text{ kJ (out)} = -200 \text{ kJ}$

The negative sign shows energy is released to the surroundings.

Reaction Profiles for Exothermic Reactions

A reaction profile is a graph showing the energy changes during a reaction. For exothermic reactions:

• The energy level of the reactants is higher than that of the products
• The difference in energy between reactants and products is the energy released
• This energy release is shown as a downward arrow on the graph
• There is an initial rise in energy at the start, called the activation energy, which is the energy needed to start the reaction

The activation energy is always present, even in exothermic reactions, but the overall energy change is negative because more energy is released than taken in.

For example, in the combustion of methane, the reaction profile shows reactants at a higher energy level than products, with a peak representing the activation energy.

Everyday Examples and Uses

Exothermic reactions are very important in everyday life and technology:

• Hand warmers: These often use exothermic oxidation reactions of iron powder to release heat and keep hands warm in cold weather.
• Respiration: The exothermic reaction inside cells releases energy needed for all biological processes.
• Combustion engines: Burning fuel in car engines releases energy that powers the vehicle.

These examples show how exothermic reactions provide useful energy in practical ways.

Example: When a hand warmer is activated, iron reacts with oxygen in the air:

$\text{4Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3 + \text{heat}$

This reaction releases heat energy, warming the hand warmer.