C5 Energy Changes

• When chemical reactions occur, energy is conserved
• The amount of energy in the universe at the beginning is the same as at the end
• 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
• Product molecules in an exothermic reaction must have less energy than the reactants, by the amount transferred
• Exothermic reactions


  • Combustion
  • Many oxidisation reactions
  • Neutralisation
• Everyday examples of exothermic reactions

  • Self-heating cans (e.g. for coffee)
  • Hand warmers
• Endothermic reaction


  A reaction that takes in energy from the surroundings so the temperature of the surroundings decreases
• Product molecules in an endothermic reaction must have more energy than reactants
• Examples of endothermic reactions

  • Thermal decomposition
  • Reaction of citric acid and sodium hydrogencarbonate
• Some sports injury packs are based on endothermic reactions
• Activation energy


  Minimum amount of energy that reactants need to collide with each other and react
• Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy
• Reaction profile


  Can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction
• You can tell which reaction is exothermic or endothermic by looking at whether the reactants or products have more energy (higher up energy scale=higher energy)
• Exothermic: heat released to the surroundings, so products have less energy than reactants
• Endothermic: heat taken in from surroundings, so products have more energy than reactants
• During a chemical reaction

  1. Energy must be SUPPLIED to BREAK bonds in the reactants

  2. Energy is RELEASED when bonds in the products are FORMED
• The energy needed to break bonds and energy released when bonds are formed can both be calculated from bond energies
• Sum of energy taken in to break bonds - sum of energy released to form bonds = overall energy change
• Energy taken in to break > energy released when formed = ENDOTHERMIC (because overall energy has been taken in)
• Energy taken in to break < energy released when formed = EXOTHERMIC (because overall energy has been released)
• What type of Reaction does this Reaction Profile show?

  Exothermic

  
• What type of Reaction does this Reaction profile show?

  Endothermic

  
• Experiment


  1. Measure out an exact volume of the acid solution into the polystyrene cup

  2. Record the initial temperature of the acid solution

  3. Add one metal powder and stir the mixture

  4. Place a lid on the polystyrene cup to reduce the amount of energy transferred to the surroundings

  5. Take the temperature of the mixture every 30 seconds and record the highest temperature

  6. Repeat the experiment for each different metal

  7. Use the same volume and concentration of acid each time you repeat the experiment

  8. Make sure the acid starts at the same temperature each time you repeat the experiment

  9. Use the same number of moles and the same surface area of metal each time you repeat the experiment
• Describe how you can tell from a reaction profile that a reaction is exothermic.

  In exothermic reactions, the overall change is negative. The products have less energy than the reactants
• Describe how you can tell from a reaction profile that a reaction is endothermic.

  In the profile for an endothermic reaction, the overall change is positive. You can tell this because the products have more energy than the reactants
• If a reaction releases more energy when new bonds are made than the amount of energy absorbed to break bonds in the reactants, what is the reaction? Exothermic Reaction