Energy changes
Energy is absorbed to break bonds and released when bonds are made. Energy changes in a reaction are calculated by bond energies and shown by energy diagrams. Heat energy is released when fuels burn.
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.
- 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
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.