Heat Energy Changes

• In an exothermic reaction, heat energy is given out, so the reaction mixture and surroundings increase in temperature
• In an endothermic reaction, heat energy is taken in, so the reaction mixture and surroundings decrease in temperature
• In a chemical reaction, bonds are broken and then rearranged to form the products
• • Breaking bonds is endothermic
  • Forming bonds is exothermic
• Overall in an exothermic reaction, energy is released to the surroundings because more heat energy is released making the product's bonds than needed to break the bonds in the reactants
• Overall in an endothermic reaction, energy is taken in from the surroundings because less heat energy is released making the product's bonds than needed to break the bonds in the reactants
• Reaction Profiles model:

  the energy changes that happen during reactions
• Exothermic

  • energy level of the reactants > energy level of the products
  • the energy change of the reaction is negatuve (transferred to the surroundings)
  • the activation energy is the minimum energy needed to start a reaction.

  1. may be supplied by heating or applying a flame
• Exothermic

  • the energy level of the reactants < energy level of the products
  • the energy change of the reaction is positive (transferred from the surroundings)
  • the activation energy may be supplied by

  1. continually heating
  2. electrolysis
• energy change = total energy in - total energy out
• 1370-1856= -486kJ mol-1


  The negative sign shows it is exothemic