Exothermic and endothermic reactions

During chemical reaction reactants (the substances we start with) are changed into the products (the substances we are left with at the end of the reaction). To convert reactants into products in a chemical reaction there is always a transfer of heat energy between the system and the surroundings. The system is the chemical reactants and products.

The surroundings are the environment around the system. This may be the solution in which the reactants are contained, the container in which the solution is placed or the air surrounding the container. This heat energy may be released from the system to the surroundings or absorbed from the surroundings into the system.

Exothermic reactions

An exothermic reaction is a reaction which releases heat energy. The heat energy released by the system is transferred from the system into the surroundings causing the temperature of the surroundings to increase. The temperature rise of the surroundings can be measured using a thermometer and used to calculate the amount of heat energy transferred. Examples of exothermic reactions include:

• Combustion reactions. When ethanol reacts with oxygen to produce carbon dioxide and water, heat energy is released
• Neutralisation reactions. When an acid such as hydrochloric acid reacts with an alkali such as sodium hydroxide, heat energy is released and the temperature of the solution increases
• Reaction between water and calcium oxide. When water is added to calcium oxide, sufficient heat energy is produced to boil the water and create steam. This leads to the formation of calcium hydroxide

Endothermic reactions

An endothermic reaction is one in which heat energy is taken in (absorbed) from its surroundings, causing the temperature of the surroundings to decrease.

This decrease in temperature can be measured with a thermometer and used to calculate the amount of heat energy transferred to the system. One example of an endothermic reaction is the melting of ice in a glass of water. The ice absorbs heat energy from the water and the water becomes colder. The surroundings (the water in the glass) have transferred heat energy to the system (the ice). Other examples of endothermic reactions include:

• Electrolysis
• The reaction between ethanoic acid and sodium carbonate
• Thermal decomposition of calcium carbonate in a blast furnace

Remember

Heat energy is given out in an exothermic reaction and

taken in by an endothermic reaction.