.3 the energy change of reactions

• During a chemical reaction:

  energy must be supplied to break bonds in the reactants

  energy is released when bonds in the products are formed.
• The energy needed to break bonds and the energy released when bonds are formed can be calculated from bond energies.
• The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed is the overall energy change of the reaction.
• In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds
• In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.