heat energy changes

• when a chemical reaction happens energy is transferred to or from the surrounding for example:

  • salts dissolving in water
  • neutralisation reactions
  • displacement reactions
  • precipitation reactions
• exothermic reactions - heat energy is given out and the temperature of the reaction mixture increases
• endothermic reactions - heat energy is take in and the temperature of the reaction mixture decreases
• breaking bonds is an endothermic reaction as energy is being taken in

  forming bonds is an exothermic reaction as energy is being given out
• overall heat energy change for a reaction is:

  • exothermic, more energy is released in forming bonds in the products than required in breaking bonds in the reactants
  • endothermic, less heat is released in forming bonds than is required in breaking bonds in the reactants
• energy change in a reaction given the bond energies

  • add together the bond energies for all the bonds broken in the reactants 'energy in'
  • add together the bond energies for all the bonds formed in the products 'energy given out'
  • energy change = energy in - energy given out OR bonds broken - bonds formed
• activation energy is the minimum energy needed for 'successful collisions' (a reaction to occur)
• a catalyst is a substance that increases the rate of reaction without being used up it does this by lowering the activation energy
• exothermic

  energy change is negative as products have less energy than reactants overall energy released is down

  A) activation energy

  B) products

  
• endothermic

  energy change is positive as products have more energy the overall change arrow points up

  A) activation energy

  B) reactants