Exothermic & Endothermic Reactions

• Whenever chemical reactions occur, there are changes in energy. This means that when chemicals get together, things either heat up or cool right off
• Energy is moved around in chemical reactions
• Chemicals store a certain amount of energy - and different chemicals store different amounts
• If the products of a reaction store more energy than the original reactants, then they must have taken in the difference in energy between the products and reactants from the surroundings during the reaction. But if they store less, then the excess energy was transferred to the surroundings during the reaction
• The overall amount of energy in a doesn't change. This is because energy is conserved in reactions - it can't be created or destroyed, only moved around. This means the amount of energy in the universe always stays the same
• In an exothermic reaction, heat is given out
• An exothermic reaction is one which transfers energy to the surroundings, usually by heating. This is shown by a rise in temperature
• The best example of an exothermic reaction is burning fuels - also called combustion. This gives out a lot of energy
• Neutralisation reactions (acid + alkali) are exothermic
• Many oxidisation reactions are exothermic. E.g. adding sodium to water releases energy, so it must be exothermic. The reaction releases energy and the sodium moves about on the surface of the water as it is oxidised
• Exothermic reactions have lots of everyday uses:

  • Some hand warmers use the exothermic oxidation of iron in air (with a salt solution catalyst) to release energy
  • Self heating cans of hot chocolate and coffee also rely in exothermic reactions between chemicals in their bases
• Physical processes can also take in or release energy. E.g. freezing is an exothermic processs, melting is endothermic
• In an endothermic reaction heat is taken in
• An endothermic reaction is one which takes in energy from the surroundings. This is shown by a fall in temperature
• Endothermic reactions are much less common than exothermic reactions, but they include:

  • The reaction between citric acid and sodium hydrogencarbonate
  • Thermal decomposition - e.g. heating calcium carbonate causes it to decompose into calcium oxide (also called quicklime) and carbon dioxide:

  > CaCO3 (+ HEAT) -> CO2 + CaO
• Endothermic reactions are used in some sports injury packs - the chemical reaction allows the pack to become instantly cooler without having to put it in the freezer
• Sometimes it's not enough to just know if a reaction is endothermic or exothermic. You may also need to know how much energy is absorbed or released
• Reaction profiles show energy changes
• Reaction profiles are diagrams that show the relative energies of the reactants and products in a reaction and how the energy changes over the course of the reaction
• This shows an exothermic reaction - the products are at a lower energy than the reactants. The difference in height represents the overall energy change in the reaction per mole

  
• The initial rise in energy represents the energy needed to start the reaction. This is the activation energy (Ea)

  
• The activation energy is the minimum amount of energy the reactants need to collide with each other and react. The greater the activation energy, the more energy needed to start the reaction - this has to be supplied, e.g. by heating the reaction mixture

  
• This shows an endothermic reaction because the products are at a higher energy than the reactants

  
• The difference in height represents the overall energy change during the reaction (the energy taken in) per mole

  
• Reaction profiles are sometimes called energy level diagrams