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Breaking and making bonds

During a chemical reaction:

• bonds in the reactants are broken
• new bonds are made in the products

Energy is absorbed to break bonds. Bond-breaking is an endothermic process.

Energy is released when new bonds form. Bond-making is an exothermic process.

Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when new bonds form.

Energy diagrams

Energy diagrams show the level of energy of the reactants and of the products. The bigger the difference between the energy of the reactants and the energy of the products, the more energy is given out or taken in.

It is easy to see from an energy level diagram whether the reaction is exothermic or endothermic:

• in exothermic reactions the reactants are higher than the products
• in endothermic reactions the reactants are lower than the products

In an exothermic reaction, the products are at a lower energy than the reactants. The difference between the energy of the reactants and the energy of the products is called the enthalpy change (∆H) of the reaction. For an exothermic reaction, the enthalpy change is always negative.

In an endothermic reaction, the products are at a higher energy than the reactants. This means that the enthalpy change of the reaction (∆H) is positive.

Activation energy

The activation energy is the minimum amount of energy required for a reaction to take place.

This can be represented on the energy profile as a ‘hump’. This shows how much energy is needed for the reactants to react and become products.

The activation energy for an exothermic and an endothermic reaction is shown below: