Exothermic and Endothermic Reactions

Reactions can be split into exothermic and endothermic reactions.

Exothermic Reaction

In an exothermic reaction, heat energy is released to the surroundings, causing an increase in the surrounding temperature. Exothermic reactions can be very useful. For example, let’s look at the combustion of wood:

In this case, we are burning a fuel (CxHy) which is reacting with oxygen (O2) from the air. This forms carbon dioxide (CO2) and water (H2O). Energy is also released in the process.

The heat energy will be transferred to the surrounding particles, which will make them move faster. We can also use the release of heat energy to keep ourselves warm, to cook food and many other uses.

  • Two more examples of exothermic reactions are oxidation and neutralisation

Some uses of exothermic reactions are:

  • Self-heating cans
  • Hand warmers
  • A fireworks display

Endothermic Reaction

An Endothermic reaction involves taking in heat energy, which causes a reduction in the temperature of the surroundings. Endothermic reactions can also be useful. As you can see below, an example of an endothermic reaction is water electrolysis.

The electrolysis of water (H2O) forms hydrogen (H2) and oxygen (O2) molecules. For this reaction to take place, electrical energy must be absorbed, which is why this is an endothermic reaction.

  • Another endothermic reaction is thermal decomposition.

An example of a thermal decomposition reaction is heating calcium carbonate to produce carbon dioxide and quicklime (calcium oxide).

Calcium carbonateCarbon dioxide + Calcium oxide

CaCO3 CO2 + CaO

Some uses of endothermic reactions are:

  • Ice sports injury packs
  • Water evaporation

Testing for Exothermic or Endothermic Reactions

You can complete an experiment to test whether a reaction is exothermic or endothermic. For this experiment, you will need a polystyrene cup, a thermometer and your reactants.

1. Add your first reactant to the cup

2. Put the thermometer in the cup and measure the starting temperature of the reactant

3. Add the second reactant to the cup and measure the temperature change

We use a polystyrene cup in this experiment is because it is a very good insulator. This means that it won’t affect the temperature change. The thermometer is measuring the temperature of the surroundings.

Exothermic Reaction

If there is a increase in surrounding temperature, this would mean that an exothermic reaction is taking place, as the reaction is giving out heat energy.

Endothermic Reaction

If there is a decrease in the surrounding temperature, this would mean an endothermic reaction is taking place, as the reaction is taking in heat energy.