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Energy and reactions

What you need to know

Reflections and Exam tips


Types of reactions

During a chemical reaction energy is transferred as bonds form and break.

Reactions that transfer energy to the surroundings are called exothermic reactions and this energy usually heats the surroundings. Combustion, respiration and neutralisation are examples of exothermic reactions.

Reactions which take energy from the surroundings are called endothermic reactions. Some reactions will cause a decrease in temperature while others require a supply of energy. Thermal decomposition reactions require high temperatures input to keep them going, and photosynthesis uses light energy.

Reversible reactions and energy

Forward and backward reactions in reversible reactions require equal but opposite energy transfers.

In general if a reaction is exothermic:

  1. an increase in temperature decreases the yield, so less product is formed
  2. a decrease in temperature will increase the yield

In general if a reaction is endothermic:

  1. an increase in temperature increases the chemical yield
  2. a decrease in temperature decreases the yield




Endothermic reactions involve energy being taken in from their surroundings.

Exothermic reactions give out energy to their surroundings such as heat, light or sound

Know some examples of exothermic reaction types: combustion, oxidation and neutralisation.  

Understand that in an equilibrium the rates of the forward and reverse reactions are equal

Understand that in a reversible reaction the energy change in the forwards direction is opposite to that in the reverse direction