Energy Changes in Chemical Reactions

Breaking and Making Bonds

When chemical reactants take place:

  • The bonds in the reactants breakThis process is endothermic as energy is absorbed to break bonds
  • New bonds form in the productsThis process is exothermic as energy is released when new bonds form

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

Remember:

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

Reaction Profiles

Reaction profiles show the energy changes that occur during a chemical reaction.

Endothermic Reactions

Below is a reaction profile for an endothermic reaction:

As you can see from the diagram, the reactants have less energy than the products. So we know that energy was taken in from the surroundings.

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

Exothermic Reactions

Below is a reaction profile for an exothermic reaction

As you can see from the diagram, the reactants have greater energy than the products. So we know that energy is being released to the surroundings.

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

Activation Energy

The energy needed to begin a chemical reaction is called activation energy. It’s the “push” needed to get the reaction started, and it’s usually represented by the initial rise in energy on a reaction profile.

Activation energy is the smallest amount of energy needed for a reaction to happen.