3.2.2 Reaction Rates

• What is the definition of Activation Energy?

  The minimum energy which particles need to collide to start a reaction
• Why do reactions only occur when collisions take place between particles?

  Because sufficient energy is needed to break the relevant bonds in the reactant molecules
• What happens to the frequency of effective collisions when concentration or pressure increases?

  The frequency of effective collisions increases
• How is the rate of reaction defined?

  As the change in concentration of a substance in unit time
• What is the usual unit for measuring reaction rates?

  mol dm^-3s^-1
• In the experiment between sodium thiosulfate and hydrochloric acid, how is the reaction rate typically measured?

  As 1/time, where time is the time taken for a cross to disappear due to cloudiness
• Why is measuring reaction rate as 1/time an approximation?

  Because it does not include concentration, but assumes the amount of sulfur produced is fixed
• What should you mention if a question refers to a doubling of concentration or rate?

  Double the number of particles per unit volume and double the frequency of effective collisions
• What factors affect the rate of reaction?

  • Concentration
  • Temperature
  • Surface area
  • Presence of catalysts
• How can you distinguish between different finishing volumes when comparing rate curves?

  By calculating or comparing initial moles of reactants
• What happens to the initial rate of reaction if different volumes of the same initial concentrations are used?

  They will have the same initial rate if other conditions are the same
• What does a steeper gradient in a reaction rate graph indicate?

  A faster rate of reaction
• How is the rate of reaction calculated from a graph of concentration vs time?

  By drawing a tangent to the curve and calculating the gradient of the tangent
• What is the definition of a catalyst?

  A substance that increases reaction rates without getting used up
• How do catalysts increase reaction rates?

  By providing an alternative route with a lower activation energy
• What are the benefits of using catalysts in reactions?

  They can lower temperatures and pressures, saving energy costs and reducing CO2 emissions
• What are the types of catalysts?

  • Heterogeneous catalysts: usually solids, reactants are gaseous or in solution
  • Homogeneous catalysts: in the same phase as the reactants
• What is heterogeneous catalysis?

  A process where the catalyst and reactants are in different phases
• What is homogeneous catalysis?


  A process where the catalyst and reactants are in the same phase
• What are some techniques to investigate rates of reaction?

  • Measurement of gas volume change
  • Titration of samples
  • Colorimetry
  • Measurement of change in mass
  • Measuring change in electrical conductivity
• How can the change in volume of a gas be measured in a reaction?

  Using a gas syringe
• What is colorimetry used for in reaction rate measurement?

  To measure the change in color of the reacting mixtures
• What is the Maxwell-Boltzmann energy distribution?

  It shows the spread of energies that molecules of a gas or liquid have at a particular temperature
• What does the area under the Maxwell-Boltzmann curve represent?

  The total number of particles present
• Why does the Maxwell-Boltzmann distribution curve not meet the x-axis?

  Because there is no maximum energy for molecules
• How does increasing temperature affect the Maxwell-Boltzmann distribution?

  It shifts the distribution towards having more molecules with higher energies
• What happens to the mean energy of particles as temperature increases?

  The mean energy shifts to higher energy values
• How does increasing surface area affect the rate of reaction?

  It causes collisions to occur more frequently, increasing the rate of reaction
• What is the effect of increasing temperature on the frequency of successful collisions?

  It increases the frequency of successful collisions
• How does a lower activation energy affect the number of particles with energy greater than the activation energy?

  More particles will have energy greater than the activation energy