Exothermic & Endothermic (Edexcel IGCSE Chemistry)

Spec point — know that chemical reactions in which heat energy is given out are described as exothermic, and those in which heat energy is taken in are described as endothermic

Exothermic and endothermic reactions

• The changes in heat content can be determined and measured with a thermometer

• Note that the overall amount of energy does not change as energy is conserved in reactions

  • This is known as the law of conservation of energy

• This means that it cannot be created or destroyed but it can be transferred

• So, if energy is transferred to the surroundings during a chemical reaction, then the products formed must have less energy than the reactants by the same amount as that transferred

Exothermic reactions

• An exothermic reaction releases heat energy into the surroundings

  • This means that the temperature of the reaction mixture and its surroundings increases
  • This change can be measured with a thermometer

• Examples of exothermic reactions include neutralisation and combustion

Exothermic reaction diagram

In exothermic reactions, the temperature of the surroundings increases and the heat content of the system falls

Endothermic reactions

• An endothermic reaction takes heat energy in from the surroundings

  • This means that the temperature of the reaction mixture and its surroundings decreases
  • This change can be measured with a thermometer

Endothermic reaction diagram

In endothermic reactions, the temperature of the surroundings falls and the heat content of the system increases

• The following are some examples of heat changes in reactions

  • Neutralisation reactions:

    • These always give energy out

  • Displacement reactions:

    • These can either take energy in or give it out

  • Combustion reactions:

    • These always give energy out

Worked Example A student was investigating the temperature change for four different chemical reactions. The table shows the chemicals that the student combined for each reaction along with the initial and final temperatures of the reaction.

Experiment	Chemicals	Initial temperature (oC)	Final temperature (oC)
1	10 cm3 NaOH	10 cm3 HCl	19	21
2	10 cm3 NaHCO3	2 g citric acid	20	16
3	10 cm3 CuSO4	0.5 g Mg powder	20	26
4	10 cm3 H2SO4	3 cm Mg ribbon	19	31

a) Identify each reaction as endothermic or exothermic.

b) Explain your answer.

Answers:

a)

Exothermic reactions = 1, 3 and 4

Endothermic reaction = 2

b)

The exothermic reactions all show an increase in temperature, while the endothermic reaction shows a decrease in temperature

Exam Hint You need to able to work with experimental data to identify exothermic and endothermic reactions and justify your decisions.