Bond Breaking & Bond Forming (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award)): Revision Note

Bond breaking & bond forming

Extended tier only

• Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break existing bonds and the energy released when the new bonds are formed

• Bond breaking is always an endothermic process as energy needs to be taken in from the surroundings to break the chemical bonds

• Bond making is always an exothermic process as energy is transferred to the surroundings as the new bond is formed

Exothermic reactions

• If more energy is released than is absorbed, then the reaction is exothermic

• More energy is released when new bonds are formed than energy required to break the bonds in the reactants

• The change in energy is negative since the products have less energy than the reactants

• Therefore, an exothermic reaction has a negative ΔH value

  • This can be shown in reaction pathway diagrams and calculations

Making new chemical bonds releases energy which radiates outwards from the reaction to the surroundings in the form of heat

Endothermic reactions

• If more energy is absorbed to break bonds than is released to form new bonds, this reaction is endothermic overall

• The change in energy is positive since the products have more energy than the reactants

• The symbol ΔH is used to show the change in heat energy

  • H is the symbol for enthaply, which is a measure of the total heat of reaction of a chemical reaction

• Therefore, an endothermic reaction has a positive ΔH value

  • This can be shown in reaction pathway diagrams and calculations

Breaking chemical bonds requires energy which is taken in from the surroundings in the form of heat