Exothermic and Endothermic Reactions

Reactions can be exothermic or endothermic.

Exothermic Reaction

Exothermic reactions release energy to the surroundings, usually in the form of heat. This causes an increase in the surrounding temperature. These types of reactions can be very useful, such as the combustion of wood, which provides a source of heat.

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, cook food and for many other uses.

Two more examples of exothermic reactions are oxidation and neutralisation.

Some uses of exothermic reactions are:

  • Self-heating cans
  • Hand warmers
  • Fireworks displays

Endothermic Reaction

Endothermic reactions absorb energy, resulting in a decrease in the surrounding temperature. These reactions can also be very useful, such as water electrolysis, which is an endothermic process that produces hydrogen gas.

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 it is an endothermic reaction.

  • Another example of an endothermic reaction is thermal decomposition

Thermal decomposition is a type of chemical reaction in which a compound breaks down into simpler compounds or elements when heated. An example of this is the breakdown of calcium carbonate into carbon dioxide and quicklime (calcium oxide) upon heating.

Calcium carbonateCarbon dioxide + Calcium oxide

CaCO3 CO2 + CaO

Some uses of endothermic reactions are:

  • Instant ice packs
  • Refrigerators

Testing for Exothermic or Endothermic Reactions

You can carry out 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. Insert the thermometer into the cup and measure the starting temperature of the reactant.

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

The change in temperature during the reaction will indicate whether the reaction is endothermic (heat is being absorbed) or exothermic (heat is being released).

We use a polystyrene cup in this experiment because it is a very good insulator and does not affect the temperature change. The thermometer measures the temperature of the reactants.

Exothermic Reactions

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

Endothermic Reactions

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.