Bond Energy (Edexcel IGCSE Chemistry)

• During a chemical reaction energy must be taken in to break bonds
  • Because energy is being taken in, bond breaking is an endothermic process
• During a chemical reaction, energy is released when new bonds are formed
  • Because energy is released, bond making is an exothermic process
• Whether a reaction is endothermic or exothermic overall depends on the difference between the energy needed to break existing bonds and the energy released when the new bonds are formed
  

Exothermic reactions

• In an exothermic reaction:
  • The energy released when new bonds are formed is greater than the energy taken in to break bonds
  • The change in energy is negative since the reactants have more energy than the products
  • Therefore an exothermic reaction has a negative ΔH value

Making new bonds gives off heat from the reaction to the surroundings

Endothermic reactions

• In an endothermic reaction:
  • The energy needed to break existing bonds is greater than the energy released when new bonds are formed
  • The change in energy is positive since the products have more energy than the reactants
  • Therefore an endothermic reaction has a positive ΔH value

• Each chemical bond has a specific bond energy associated with it
• This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed
• This energy can be used to calculate how much heat would be released or absorbed in a reaction
• To do this it is necessary to know the bonds present in both the reactants and products
• We can calculate the total change in enthalpy for a reaction if we know the bond energies of all the species involved
• Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’
• Add together the bond energies for all the bonds in the products – this is the ‘energy out’
• Calculate the enthalpy change using the equation:

Enthalpy change (ΔH) = Energy taken in - Energy given out