Enthalpy Change

• Enthalpy Change

  • the enthalpy, H, of a system refers to the energy within the system
  • enthalpy change refers to the energy exchanged with the surroundings, it is the amount of energy involved in a reaction
  • enthalpy change = total energy of products - total energy of reactants
  • all reactions involve the breaking of bonds and formation of new bonds
  • bond breaking is endothermic, bond forming is exothermic
• Endothermic Reactions


  • the system takes in heat from the surroundings
  • temperature of the surroundings decreases
  • enthalpy change is positive as heat is gained from the surroundings
  • examples :

  1. evaporation
  2. melting
  3. sublimation
  4. photosynthesis
  5. thermal decomposition
• Bond energy


  • the energy required to break 1 mol of a covalent bond is known as bond energy
  • the stronger the bond, the more energy needed to break the bond and the higher the bond energy
• Exothermic Reactions


  • the system gives out heat to the surroundings
  • temperature of the surroundings increases
  • enthalpy change is negative is lost to the surroundings
  • Examples :

  1. condensation
  2. freezing
  3. vapour deposition
  4. dissolving of anhydrous salts
  5. dissolving of acids in water
  6. combustion of fuels
  7. respiration
  8. neutralisation

  
• Energy Level Diagram

  
• Energy Profile Diagrams


  • a chemical reaction occurs when particles of the reactants collide with enough energy to break their bonds to form new bonds
  • amount of energy that the particles have should be equal to or greater than activation energy
• Activation energy

  • the minimum amount of energy that colliding reactants particles must possess to react with each other
  • represents an energy barrier that must be overcome by the reactants before they can react to form products