Bond energies

• Bond Energies


  Energy required to break or form chemical bonds
• Energy must always be supplied to break bonds
• Chemical reaction


  1. Bond breaking

  2. Bond forming
• Bond breaking


  Endothermic process
• Bond forming


  Exothermic process
• In exothermic reactions, the energy released by forming bonds is greater than the energy used to break them
• In endothermic reactions, the energy required to break bonds is greater than the energy released by forming them
• Bond energies need to be practised to calculate the overall energy change for a reaction
• Calculating energy change for a reaction

  1. Find energy required to break original bonds

  2. Find energy released by forming new bonds

  3. Overall energy change = Energy to break bonds - Energy released by forming bonds
• You can't compare the energy changes of different reactions unless you know the specific bond energies involved
• Bromine and hydrogen reaction


  Br-Br bonds are weaker than H-Cl bonds, so less energy is required to break the bonds
• Bond energy calculations may look hard, but with practice they are easy and can earn you marks