Exothermic and Endothermic Reactions

Energy cannot be created or destroyed. However, it can be changed from one form to another. Energy changes occur in chemical reactions and even in some physical processes, such as when a solid dissolves in water.

4.10 recall that chemical reactions in which heat energy is given out are described as exothermic and those in which heat energy is taken in are endothermic

Exothermic Reactions

Reactions that give out heat energy to the surroundings are called exothermic reactions.

What are the characteristics of exothermic reactions?

When an exothermic reactions occurs,

• heat is liberated and is transferred from the chemicals to the surroundings, and 
• the temperature of the reaction mixture rises. The container feels hot.  

In an exothermic reaction, the temperature of the reaction mixture initially rises until the highest temperature is reached. When the reaction is completed, the temperature of the reaction mixture falls until it reaches room temperature.

Examples of exothermic reactions include:

• the combustion of fuels
• the rusting of iron
• the corrosion of metals
• the reaction between acid and alkali (neutralisation)
• respiration

Endothermic Reactions

Not all reactions give out heat. Reactions that absorb heat from the surroundings are called endothermic reactions. 

What are the characteristics of endothermic reactions?

When an endothermic reactions occurs, 

• heat energy is absorbed and is transferred from the surroundings to the reactants, and, 
• the temperature of the reaction mixture falls. The container feels cold. 

Initially, the temperature of the reaction mixture falls until the lowest temperature is reached. When the reaction is completed, the temperature of the reaction mixture rises until it reaches room temperature. 

Examples of endothermic reactions include:

• photosynthesis
• the action of light on silver bromide in photographic film
• thermal decomposition