Chapter 10 - Reaction Rates

• Why does rate of reaction change with time?

  • Rate of reaction is fastest at the start as lots of successful collisions per second
  • Rate of reaction slows down as the reaction proceeds because reactants are used up and less successful collisions per second
  • Reaction stops when reactants have been completely used up
• Why are some collisions effective and others ineffective?

  • Particles must collide with the correct orientation
  • Particles must have sufficient energy to overcome the activation energy barrier of the reaction
• How does increasing the concentration affect the rate of reaction?

  • More particles per unit volume
  • More successful collisions per second
• How does increasing the pressure affect the rate of reaction?

  • Particles are closer together
  • More successful collisions per second
• How does increasing the temperature affect the rate of reaction?

  • Particles have more kinetic energy
  • More successful collisions per second
• State two ways in which we can monitor the rate of reaction for a reaction that produces a gas?

  • Monitoring the volume of gas produced at regular time intervals using a gas syringe
  • Monitoring the loss of mass of reactants using a balance at regular time intervals
• How can we determine the rate of reaction from experimental results?

  • Plot a graph of volume of gas/mass (y axis) against time (x axis)
  • Draw a tangent at t = 0
  • Calculate the rate from the gradient of the tangent (rate = change in y / change in x)
• A catalst increases the rate of reaction by providing an alternative pathway with a lower activation energy
• Homogeneous catalyst:

  Catalyst is the same physical state as the reactants
• Heterogeneous catalyst:

  Catalyst is a different physical state as the reactants
• What is the mechanism for a heterogenous catalyst?

  • Reactants absorb onto catalyst surface
  • Bonds weaken and break
  • New bonds form
  • Products desorb from catalyst surface
• What is the sustain and economic importance of a catalyst?

  • Reduces temperature needed for the process and the energy requirements
  • Less electricity used or fossil fuels burnt
  • Less carbon dioxide given off