Kinetics 1

• Rate law - The relationship between the rate of a chemical reaction and the concentration of its reacting species
• Order of Reaction - The sum of the exponents on the molar concentrations of all the substances involved in the reaction, raised to their respective powers.
• Collision Theory - States that reactions occur when two particles collide with sufficient energy (activation energy) to overcome the repulsive forces holding them together.
• Activation Energy - The minimum amount of energy required for a reaction to proceed from reactants to products.
• Collision Theory


  Chemical reactions occur when reactant particles collide
• Successful collision

  • Energy greater than or equal to the activation energy of the reaction
  • Particle orientation must be correct
• Activation energy


  Minimum amount of energy required for two particles to react
• Maxwell-Boltzmann Distribution


  Pattern of energy distribution among molecules in a substance
• Changing reaction conditions

  Alters the shape of the Maxwell-Boltzmann distribution curve
• The total area under the Maxwell-Boltzmann distribution curve represents the total number of molecules in the sample, and so it must remain constant
• Reaction conditions


  Impact the collisions of the particles and can be altered to give the particles more energy
• Increasing the likelihood of collisions with sufficient energy

  Leads to a greater rate of reaction
• Rate of reaction

  Can be calculated from empirical data plotted on graphs
• Finding rate of reaction on a concentration-time graph

  1. Draw a tangent to the curve at a given time

  2. The gradient of the tangent is the rate of reaction at that time

  3. Drawing a tangent at time = 0 finds the initial rate of reaction
• Overall rate of reaction

  Can be calculated using the equation: Rate = Change in concentration / Change in time
• Effect of Temperature


  1. Thermal energy is transferred to the substance

  2. Kinetic energy of molecules increases

  3. Collisions occur more often and with greater energy

  4. More collisions have energy greater than activation energy
• Increasing the reaction temperature

  Increases the rate of reaction
• Increased temperature


  Maxwell-Boltzmann distribution shifts to the right
• A greater proportion of molecules have energy greater than or equal to the activation energy at increased temperature
• Effect of Concentration and Pressure

  1. Molecules are packed closer together

  2. Collisions between molecules become more likely

  3. Probability of collision with energy greater than or equal to activation energy increases
• Increasing concentration or pressure

  Increases the rate of reaction
• Increasing concentration or pressure does not change the energy of individual particles
• The shape of the Maxwell-Boltzmann distribution does not shift with changes in concentration or pressure
• Effect of Surface Area

  1. Increasing surface area increases the number of exposed reactant particles

  2. More frequent, successful collisions
• Increasing surface area


  Increases the rate of reaction
• Increasing surface area does not change the energy of individual particles
• The shape of the Maxwell-Boltzmann distribution does not change with changes in surface area
• Catalyst


  A substance that increases the rate of reaction without being used up in the reaction
• How a catalyst works


  Provides an alternative reaction path with lower activation energy
• The Maxwell-Boltzmann distribution curve is unchanged in shape but the position of the activation energy is shifted to the left when in the presence of a catalyst
• A greater proportion of molecules have sufficient energy to react with the use of a catalyst
• Benefits of using catalysts

  • Lower the energy costs of the reaction process
  • Allow lower temperatures and pressures to be used
  • Achieve the same rate of reaction
  • Give a higher atom economy
• Surface area


  Increasing the surface area of a reactant, for example by crushing it into a powder, increases the number of exposed reactant particles
• Increasing the number of exposed reactant particles

  Increases the rate of reaction
• Increasing concentration and pressure changes does not change the energy of the individual particles, so the shape of the Maxwell-Boltzmann distribution does not change
• Catalyst


  A substance that increases the rate of reaction without being used up in the reaction
• How a catalyst works


  • It provides an alternative reaction path that requires a lower activation energy for the reaction to occur
• The Maxwell-Boltzmann distribution curve is unchanged in shape but the position of the activation energy is shifted to the left so that a greater proportion of molecules have sufficient energy to react
• Catalysts used in industry

  • They lower the energy costs of the reaction process
  • They allow lower temperatures and pressures to be used, whilst still achieving the same rate of reaction
  • They can give a higher atom economy
• Reaction profile of a catalysed reaction

  There is a dip in the energy profile for the catalysed reaction, representing the intermediate formed during the reaction. The intermediate is less stable (and therefore higher in energy) than the reactants and products.