Bond Making & Breaking (Oxford AQA IGCSE Chemistry)

Bond Making & Breaking

• 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

• In an exothermic reaction:

  • The energy released when new bonds are formed is greater than the energy taken in to break bonds

• In an endothermic reaction:

  • The energy needed to break existing bonds is greater than the energy released when new bonds are formed

• The energy level diagram below is for the reaction to produce ammonia from hydrogen and nitrogen

  • The energy needed to break the bonds is 2253 kJ

  • The energy released when new bonds are formed is 2346 kJ

  • The overall reaction is therefore exothermic because more energy was released when new bonds are formed

  • The overall energy change for the reaction is -93 kJ

Energy Level Diagram showing bond breaking and making

Bond Dissociation Energy Calculations

• 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 energy for a reaction if we know the bond energies of all the species involved

  1. Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’

  2. Add together the bond energies for all the bonds in the products – this is the ‘energy out’

  3. Calculate the energy change using the equation:

Energy change = Energy taken in - Energy given out