3.2.2 - reaction rates

• The equation to calculate rate is

  rate = change in concentration / time
• The unit for rate of reaction is: mol dm-3 s-1
• In order for particles to react, they must collide with sufficient energy ( activation energy ) and the correct orientation
• The minimum energy particles need to react is the activation energy
• the majority of collisions do not result in a reaction
• The factors that affect rate of reaction are:

  temperature

  pressure

  concentration

  surface area

  catalyst
• Effect of increasing temp on reaction rate:

  increasing temperature = increased rate of reaction

  higher proportion of particles have energy greater than the activation energy

  more successful collisions per second
• Activation energy is 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 (does not change)

  curve starts at the origin = no molecules have zero energy

  curve does not cross x axis = no maximum energy

  Only molecules with energy greater than Ea can react
• In a Boltzmann distribution

  x axis = energy

  y axis = number of molecules with a given energy
• Boltzmann curve showing reaction with and without increase in temperature - graph shifts to the right and is squashed

  
• Effect of increasing concentration on rate:

  increased concentration/pressure = increased rate of reaction

  more particles in a given volume

  more frequent successful collisions
• A catalyst is a substance which increases the rate of reaction but is not used up in the reaction
• Catalysts work by providing an alternate reaction pathway with a lower activation energy.

  due to the lower Ea, more particles have energy greater than the activation energy

  so more frequent and successful collisions
• Boltzmann curve showing effect of catalyst on rate of reaction.

  Ea with a catalyst is lower so is to the left on the graph.

  
• The variables that must be monitored to calculate rate of reaction are:

  • Concentration of reactant or product
  • gas volume of products
  • mass of substances formed
• To calculate rate from a concentration time graph:

  draw a tangent

  work out the gradient of the tangent

  change in y / change in x
• A homogenous catalyst is a catalyst that is in the same phase as the reactants

  eg, a liquid catalyst mixed with liquid reactants
• What is a homogenous catalyst?

  a catalyst that is in the same phase as the reactants
• What is a heterogenous catalyst?

  a catalyst that is in a different phase to the reactants
• a heterogenous catalyst is when the catalyst used in the reaction is in a different phase to the reactants

  eg, gaseous reactants passed over a solid catalyst
• Catalytic convertors are present in vehicles to reduce toxic emission and prevent photochemical smog