Bond Breaking & Bond Forming (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

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 

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 

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