Reaction Rates

• Rate


  Change in concentration / time
• The unit for rate of reaction is mol dm-3s-1
• Particles must do to react

  Collide with sufficient energy (activation energy) and the correct orientation
• Most collisions do not result in a reaction
• Factors that affect rate of reaction

  • Temperature
  • Pressure
  • Concentration
  • Surface area
  • Catalyst
• Increasing temperature


  Increased rate of reaction
• Much higher proportion of particles have energy greater than the activation energy, leading to many more successful collisions per second
• Increasing concentration/pressure


  Increased rate of reaction
• There are more particles in a given volume, leading to more frequent successful collisions
• Variables in an experiment that can be monitored to calculate the rate of reaction

  • Concentration of reactant or product
  • Gas volume of products
  • Mass of substances formed
• How to calculate rate from a concentration time graph

  1. Draw a tangent

  2. Work out the gradient of the tangent using the equation

  3. Gradient = change in y / change in x
• Catalyst


  A substance which increases the rate of reaction but is not used up in the reaction

  
• How catalysts work


  Provide an alternate reaction pathway (with a lower activation energy)
• Due to lower activation energy, more particles have energy greater than activation energy, leading to more frequent successful collisions
• Homogeneous catalyst


  A catalyst that is in the same phase as the reactants (e.g., liquid catalyst mixed with liquid reactants)
• Heterogeneous catalyst


  Catalyst used in the reaction is in a different phase to the reactants (e.g., gaseous reactants passed over solid catalyst)
• Catalytic converters are present in vehicles to reduce toxic emission and prevent photochemical smog
• Activation energy


  The minimum energy that particles must collide with for a reaction to occur
• Important features of Boltzmann distribution

  • Area under the curve = total number of molecules
  • Area under the curve does not change when conditions alter
  • The curve starts at the origin
  • Curve does not touch or cross the energy axis
  • Only the molecules with energy greater than activation energy can react

  
• X axis in a Boltzmann distribution is energy
• Y axis in a Boltzmann distribution is number of molecules with a given energy

  
• Draw a labelled Boltzmann Curve with labels of average energy, activation energy and most probable energy
• Draw a labelled Boltzmann Curve showing the effect of catalyst on rate of reaction