Energy changes

Breaking and making bonds

During a chemical reaction:

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.

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  • If more heat energy is released when making the bonds than was taken in, the reaction is exothermic
  • If more heat energy was taken in when making the bonds than was released, the reaction is endothermic

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
Graph of energy against progress of reaction. Horizontal line represents reactants energy. Drops vertically as energy released. Horizontal line at a lower level represents products' energy.An energy level diagram for an exothermic reaction

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.

Graph of energy against progress of reaction. A horizontal line represents the reactants' energy. It rises vertically as energy is absorbed. A horizontal line at a higher level then represents the products' energy.An energy level diagram for an endothermic reaction

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.