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• Factors which affect the rates of chemical reactions

  • Concentration of reactants in solution
  • Pressure of reacting gases
  • Surface area of solid reactants
  • Temperature
  • Presence of catalysts
• Required Practical: Investigate the rate of reaction by colour change

  1. Add 50cm³ of dilute sodium thiosulfate solution to a conical flask

  2. Place conical flask on a piece of paper with a black cross

  3. Add 10cm³ of dilute hydrochloric acid to conical flask, swirl and start stop clock

  4. Record time when cross can no longer be seen

  5. Measure and record temperature of reaction mixture
• Sodium thiosulfate solution can be hazardous, sulphur dioxide can cause irritation, do not breathe over glass and ensure room is ventilated
• Collision theory


  Chemical reactions occur when reacting particles collide with each other and with sufficient energy
• Activation energy


  Minimum amount of energy that particles need to react
• Increasing concentration of reactants in solution, pressure of reacting gases, and surface area of solid reactants

  Increases the rate of reaction
• Increasing temperature


  Increases the frequency and energy of collisions, increasing the rate of reaction
• Catalysts


  Change the rate of chemical reactions but are not used up during the reaction
• Catalysts


  • Increase the rate of reaction by providing an alternative pathway for the process that has a lower activation energy
• Reversible reactions


  Reactions where the products can react to produce the original reactants
• If a reversible reaction is exothermic in one direction

  It is endothermic in the opposite direction, with the same amount of energy transferred
• Equilibrium


  Reached when the rate of the forward reaction equals the rate of the reverse reaction
• Le Chatelier's Principle: If a system at equilibrium is subjected to a change in one of the conditions, the system will shift to counteract the change and establish a new equilibrium
• Increasing the concentration of a reactant


  Shifts the equilibrium position to favour the formation of more products
• Increasing the temperature of an endothermic reaction


  Shifts the equilibrium position to favour the formation of more products
• Increasing the pressure of a gaseous reaction

  Shifts the equilibrium position to favour the side with the smaller number of molecules