Energy Changes in Chemical Reactions

Breaking and Making Bonds

When chemical reactants take place:

The bonds in the reactants break.

  • This process is endothermic as energy is absorbed to break bonds

New bonds form in the products.

  • This process is exothermic as energy is released when new bonds form

We determine whether a reaction is exothermic or endothermic depends based on the difference between the energy required to break the bonds and the energy required to form the new bonds.

If the energy required to break the bonds is greater than the energy released when new bonds form then the reaction is endothermic.

If the energy required to break the bonds is less than the energy released when new bonds form then the reaction is exothermic.

  • Energy to break > Energy to make = Endothermic
  • Energy to make < Energy to break = Exothermic

Reaction Profiles

Reaction profiles can be used to show energy changes in endothermic and exothermic reactions.

Endothermic Reactions

Below is the reaction profile for an endothermic reaction:

As you can see from the diagram, the reactants have more energy than the products. This is because energy was taken in from the surroundings.

The difference in energy between the reactants and the products tells us the amount of energy that has been taken in (energy absorbed), in the reaction.

Exothermic Reactions

Below is the reaction profile for an exothermic reaction

As you can see from the diagram, the reactants have greater energy than the products. This is because energy is being released to the surroundings.

The difference between the energy of the reactants and products (energy released) tells us how much energy is being released to the surroundings.

Activation Energy

The initial rise in potential energy is the energy required to start the reaction. This is known as the activation energy. This is better defined as the minimum amount of energy required to start a chemical reaction.