Topic 6

• Rate of reaction


  • Amount of reactant used / Time
  • Amount of product formed / Time
• 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)
• Reversible reaction symbol


  ⇌ (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 (just lost one way, gained the other)
• 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 also use quantity of reactants in terms of moles and units for rate of reaction in mol/s
• 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
• If a change is made to the conditions of a system at equilibrium

  The system responds to counteract the change (Le Chatelier's principle)
• 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 fewer moles of gas
• In gaseous reactions, if pressure is decreased


  The equilibrium shifts to the side of the equation with the more moles of gas
• Effect of pressure on yield for reactions with larger or smaller gas volumes

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