When a reaction takes place energy is involved, as energy is transferred as chemical bonds are broken and/or formed.
Transferring energy from the reacting chemicals to the surroundings is called exothermic reactions. This means they heat up the surroundings, cause an increase in temperature.
Neutralisation between acids and alkalis is exothermic.
Transferring energy from the surroundings to the reacting chemicals is called endothermic reactions. These mean they take in heat from surroundings, causing a decrease in temperature.
Thermal decomposition and photosynthesis are endothermic.
Bond energy calculations
- During a chemical reaction, energy is supplied to break forms and energy is released when bonds form.
- In an exothermic reaction, the energy released when new bonds form is greater than the energy supplied to break existing bonds.
- In an endothermic reaction, the energy supplied to break existing bonds is greater than the energy released when new bonds form.
Calculating overall energy change:
- Calculate total energy needed to break all bonds in reactants (energy in)
- Calculate total energy released in making bonds in products (energy out)
- Overall energy change = energy in – energy out
- If energy change is positive, the reaction is endothermic.
- If energy change is negative, reaction is exothermic.