Energy changes

• Energy is conserved in chemical reactions.

  If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred.
• Exothermic reaction 


  -energy is transferred to the surroundings so the temperature of the surroundings increases
• Examples of exothermic reactions


  -cumbustion(burning) e.g methane(reacts with oxygen) gas

  -many oxidation reactions, e.g respiration

  -neutralisation: used with hand warmers & self-heating cans
• Endothermic reaction


  -takes in energy from their surroundings so the temperature of the surroundings decreases
• Examples of endothermic reactions


  -thermal decompositions e.g calcium carbonate

  -reaction of citric acid + sodium hydrogencarbonate -in sherbet sweets & sports injury packs
• Reaction profiles


  Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy
• Activation energy


  The minimum amount of energy that particles must have to react
• Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction
• reaction profiles diagram


  -In an exothermic reaction the energy of the products is lower than the energy of the reactants so there is a loss of energy because the energy is transferred to the surroundings

  -In an endothermic reaction the reactants are at a lower energy level than the products so energy transferred from the surroundings
• Labels of a reaction profile diagram


  Activation energy - difference between the peak and the energy of the reactants

  Overall energy change - difference between the reactants and the products
• 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.