Exo/Endo Reactions

• Energy is not created or destroyed. It is transferred and conserved between objects.
• Exothermic reactions - Energy from the reacting chemicals is transferred TO the surroundings, which often increase in temperature as a result.
• Endothermic reactions - Energy FROM the surrounding is transferred to the reacting chemicals, causing the temperature of the surroundings to decrease.
• Oxidation is an exothermic reaction.
• Combustion is an exothermic reaction
• Disposable hand warmers use the energy released by iron oxidation.

  Reusable hand warmers source their energy from the crystallisation of salt solutions. Boiling the pack re-dissolves the crystals.
• Neutralisation is an endothermic reaction
• Thermal decomposition is an endothermic reaction.
• Sports injury packs are an endothermic reaction.

  When squeezed forcefully, ammonium nitrate and water mix in the pack resulting in instant cooling.
• Citric acid and baking soda is an endothermic reaction.
• In order for a reaction to happen, particles of the reactants have to collide, and the collisions must take place with enough energy. The threshold amount of energy is the activation energy.
• Collisions between reacting particles are needed for chemical reactions to take place. Additionally, these collisions must have sufficient energy. 


  This is collision theory.
• Reaction profiles show the progress of a reaction on the x-axis and energy level on the y-axis.
• Reaction profiles tell us:

  • The amount of energy contained within the reactants and the products
  • The activation energy of a reaction
  • The overall energy change that happens as a result of a reaction
• Exothermic reactions go down from reactants to products.
• Endothermic reactions go up between reactants and products.
• Catalysts can increase reaction rats by lowering the activation energy, which increases the likelihood of successful collisions.
• All chemical bonds have a bond energy that measures the strength of chemical bonds.
• By subtracting the total bond energies of the products from the total bond energies of the reactants, we can see if a reaction is endothermic or exothermic.
• If the overall energy change is greater than 0, the reaction is endothermic
• If the overall energy change is less than 0, the reaction is exothermic.
• Bond breaking requires energy and is thus endothermic
• Bond energy is the measure of the strength of chemical bonds and is measures in kilojoules per mole (kJ/mol)