The rate and extent of Chemical change

• Finding rate of reaction graphically


  1. Draw tangents to curves

  2. Use the slope of the tangent as a measure of rate of reaction at a specific time
• Factors which affect the rates of chemical reactions


  • Concentration
  • Pressure
  • Surface area
  • Temperature
  • Catalysts
• Collision theory


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


  The minimum amount of energy that particles must have to react
• Increasing concentration, pressure, surface area


  Increases the frequency of collisions and so increases the rate of reaction
• Increasing temperature


  Increases the frequency of collisions and makes the collisions more energetic, and so increases the rate of reaction
• Catalysts


  Substances that speed up chemical reactions without being changed or used up during the reaction
• Enzymes act as catalysts in biological systems
• Catalysts are not included in the equation for a reaction
• How catalysts work


  • Decrease the activation energy, increasing the proportion of particles with energy to react
  • Provide a different pathway for a chemical reaction that has a lower activation energy

  
• Reversible reactions


  Reaction can be changed by changing conditions (e.g. hot for forwards, cool for reverse)
• Reversible reaction example


  • Hydrogen + nitrogen ⇌ ammonia
• The same amount of energy is transferred each way in a reversible reaction
• Equilibrium


  When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction
• Le Chatelier's principle


  • If a system is at equilibrium and a change is made to any of the conditions, the system responds to counteract the change
• Effect of changing concentration

  1. If concentration of reactants is increased, position of equilibrium shifts towards products

  2. If concentration of products is increased, position of equilibrium shifts towards reactants
• If temperature is increased

  Equilibrium moves in the direction of the endothermic reaction
• If temperature is decreased


  Equilibrium moves in the direction of the exothermic reaction
• Effect of temperature on yield for exothermic/endothermic reactions

  • Exothermic: Increase in temp decreases yield, Decrease in temp increases yield
  • Endothermic: Increase in temp increases yield, Decrease in temp decreases yield
• Effect of pressure changes on equilibrium


  1. Increase in pressure favours the reaction that produces the least number of molecules

  2. Decrease in pressure favours the reaction that produces the greater number of molecules
• Effect of pressure on yield for reactions with larger/smaller volume of gas

  • Larger volume of gas: Increase in pressure decreases yield, Decrease in pressure increases yield
  • Smaller volume of gas: Increase in pressure increases yield, Decrease in pressure decreases yield