The rate and extent of chemical changes

• 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 reactions


  Reactions where the products can react to produce the original reactants
• Reversible reactions

  Reaction can be changed by changing conditions (e.g. hot for forwards, cool for reverse)
• Symbol for reversible reactions

  ⇌ instead of →
• Example of reversible reaction

  • Hydrogen + nitrogen ⇌ ammonia (Haber Process)
• 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
• If the concentration of one of the reactants or products is changed

  The system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again
• If the concentration of reactants is increased

  The position of equilibrium shifts towards the products so more product is produced until equilibrium is reached again
• If the concentration of products is increased

  The position of equilibrium shifts towards the reactants so more reactant is produced until equilibrium is reached again
• 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
• In gaseous reactions, if pressure is increased

  The equilibrium shifts to the side of the equation with the least number of moles of gas
• In gaseous reactions, if pressure is decreased

  The equilibrium shifts to the side of the equation with the greater number of 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