Reaction rates

• Particles must collide with sufficient energy (activation energy) and the correct orientation in order to react
• Effect of increasing temperature on rate of reaction: Increasing temperature → increased rate of reaction. Much higher proportion of particles have energy greater than the activation energy → many more successful collisions per second → increased rate
• Calculate rate from a concentration time graph

  Draw a tangent. Work out the gradient of the tangent using the equation Gradient = change in y / change in x
• Unit for rate of reaction: mol dm-3s-1
• How catalysts work and how they increase the rate of reaction: Not provided
• 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
• Factors that affect rate of reaction

  • Temperature
  • Pressure
  • Concentration
  • Surface area
  • Catalyst
• Definition of a catalyst: A substance which increases the rate of reaction but is not used up in the reaction
• Effect of increasing concentration/pressure on rate of reaction: Increased concentration/pressure → increased rate of reaction. There are more particles in a given volume → more frequent successful collisions → increased rate
• Equation used to calculate rate: Rate = change in concentration / time
• Do most collisions result in a reaction? No
• What are catalytic converters?
• How do catalysts work and how do they increase the rate of reaction?
• Name some important features of Boltzmann distribution
• Draw a labelled Boltzmann Curve with labels of average energy, activation energy, and most probable energy. Draw in a different color the effect of increasing temperature
• Homogeneous catalyst


  A catalyst that is in the same phase as the reactants. Eg. liquid catalyst mixed with liquid reactants
• What does heterogeneous catalyst mean?
• How catalysts work

  1. They provide an alternate reaction pathway (with a lower activation energy)

  2. Due to lower activation energy, more particles have energy > activation energy, so more frequent successful collisions, so increased reaction rate
• Define activation energy
• 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
• Draw a labelled Boltzmann Curve showing the effect of a catalyst on the rate of reaction
• Heterogeneous catalyst


  Catalyst used in the reaction is in a different phase to the reactants. Eg. gaseous reactants passed over solid catalyst
• Catalysts increase the rate of reaction but are not used up in the reaction
• What does homogeneous catalyst mean?
• Activation energy

  The minimum energy that particles must collide with for a reaction to occur
• What are the axes in a Boltzmann distribution?
• If the order is zero with respect to a reactant, it means
• Calculate the overall order of a reaction

  Sum of individual orders
• Effect on rate in a second order reaction
• Write a generic rate equation and state what each term means

  Rate = k [X]x [Y]y; k = rate constant for the reaction, [X] and [Y] are concentrations of species X and Y respectively, x and y are the orders of reaction with respect to X and Y
• Changing the concentration of the reactant has no effect on the rate
• Do zero order reactants appear on rate equation? Why?
• Calculate the rate of reaction at a given instant

  Rate of reaction = change in concentration of reactants or products / time
• Effect on rate in a first order reaction
• What is the symbol for half life?
• How could you measure the rate of reaction experimentally (different methods)?

  Use a colorimeter at suitable intervals if there is a colour change. If gas is evolved, use a gas syringe to collect volume of gas evolved, or measure the change in mass of the reaction mixture
• When the overall order is 3, what would be the units for rate constant?
• What is the relationship between first order reactions and half life?
• How is overall order of a reaction calculated?

  Sum of individual orders
• What is the equation that is used to determine rate constant using half life in a first order reaction?