ENERGY CHANGES

• Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place. If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred

  
• Exothermic reaction


  A reaction that transfers energy to the surroundings so the temperature of the surroundings increases
• Endothermic reaction


  A reaction that takes in energy from the surroundings so the temperature of the surroundings decreases
• Exothermic reactions include


  • Combustion, many oxidation reactions and neutralisation
• Endothermic reactions include


  • Thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate

  
• Students should be able to

  1. Distinguish between exothermic and endothermic reactions on the basis of the temperature change of the surroundings

  2. Evaluate uses and applications of exothermic and endothermic reactions given appropriate information
• Required practical 4


  • Investigate the variables that affect temperature changes in reacting solutions such as, eg acid plus metals, acid plus carbonates, neutralisations, displacement of metals. AT skills covered by this practical activity: 1, 3, 5 and 6. This practical activity also provides opportunities to develop WS and MS.
• Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy
• Students should be able to

  1. Draw simple reaction profiles (energy level diagrams) for exothermic and endothermic reactions showing the relative energies of reactants and products, the activation energy and the overall energy change, with a curved line to show the energy as the reaction proceeds

  2. Use reaction profiles to identify reactions as exothermic or endothermic

  3. Explain that the activation energy is the energy needed for a reaction to occur
• During a chemical reaction: energy must be supplied to break bonds in the reactants, and energy is released when bonds in the products are formed
• In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds. In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds
• Students should be able to


  Calculate the energy transferred in chemical reactions using bond energies supplied
• Cells contain chemicals which react to produce electricity. The voltage produced by a cell is dependent upon a number of factors including the type of electrode and electrolyte
• Students should be able to


  Interpret data for relative reactivity of different metals and evaluate the use of cells
• AT6


  • Safe and careful use of liquids