The​ ​rate​ ​and​ ​extent​ ​of chemical​ ​change

• Rate of reaction


  • Amount of reactant used / Time
  • Amount of product formed / Time
• Calculating rates of reactions

  • Quantity of reactant or product can be measured by mass in grams or volume in cm3
  • Units of rate of reaction may be g/s or cm3/s
  • Can use quantity of reactants in moles and units for rate of reaction in mol/s
• 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


  • Catalysts decrease the activation energy, which increases the proportion of particles with energy to react
  • Catalysts provide a different pathway for a chemical reaction that has a lower activation energy
• Reversible reaction


  A chemical reaction where the products can react to produce the original reactants
• Reversible reaction


  1. Reactants

  2. Products

  3. Direction of reaction can be changed by changing conditions
• If a reversible reaction is endothermic one way


  It is exothermic in the opposite direction
• 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, then 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 and endothermic reactions

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

  1. If pressure is increased, equilibrium shifts to side of equation with fewer moles of gas

  2. If pressure is decreased, equilibrium shifts to side of equation with more moles of gas
• Effect of pressure on yield for reactions with larger or 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
• Equilibrium exam question structure

  Equilibrium shifts to the left/right to counteract the increase/decrease in___and yield increases/decrease of product/reactant