1.5 - Kinetics

• What is the activation energy (EA)?

  Minimum energy needed for a reaction
• What does the Maxwell-Boltzmann energy distribution illustrate?

  Spread of energies in gas or liquid
• Why does the energy distribution curve go through the origin?

  There are no molecules with no energy
• Why should the energy distribution curve never meet the x-axis?

  There is no maximum energy for molecules
• Where is the mean energy of particles located on the curve?

  Not at the peak of the curve
• What does the area under the Maxwell-Boltzmann curve represent?

  Total number of particles present
• Why do a few particles have low energies?

  Collisions cause some particles to slow down
• How many particles typically have energy greater than the activation energy (EA)?

  Only a few particles
• How can a reaction go to completion if few particles exceed EA?

  Particles can gain energy through collisions
• What happens to the energy distribution as temperature increases?

  Shifts towards more higher energy molecules
• What remains constant as temperature increases in the energy distribution?

  Total area under the curve
• How does the range of energies change with increasing temperature?

  Molecules have a wider range of energies
• What happens to the Emp and mean energy at higher temperatures?

  Both shift to higher energy values
• What is the rate of reaction defined as?

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

  mol dm^-3s^-1
• How is the rate of reaction determined from a concentration vs. time graph?

  Gradient of the curve indicates the rate
• What is the initial rate of reaction?

  Rate at the start of the reaction
• How can reaction rates be calculated from graphs?

  By drawing a tangent and calculating the gradient
• What effect does increasing concentration have on reaction rates?

  Increases frequency of effective collisions
• How does the shape of the energy distribution curve change with increased concentration?

  Curves become higher, area increases
• What happens to the number of molecules with energy > EA when concentration increases?

  More molecules exceed the activation energy
• How do different volumes of the same initial concentrations affect initial rates?

  Same initial rate, different final amounts
• What factors increase the rate of reaction?

  Higher concentration, temperature, surface area
• How is the reaction rate measured in the sodium thiosulfate and hydrochloric acid experiment?

  As 1/time for cross disappearance
• Why is measuring 1/time an approximation for reaction rate?

  It does not include concentration
• What happens to successful collisions with increased surface area?

  They occur more frequently
• What is the definition of a catalyst?

  Increases reaction rates without being used
• How do catalysts increase reaction rates?

  By providing an alternative route with lower EA
• What effect does a lower activation energy have on reaction rates?

  More particles exceed EA, faster reaction
• How does increasing temperature affect particle collisions?

  Increases frequency and energy of collisions
• What does the graph show at higher temperatures regarding particle energy?

  Bigger proportion have energy > EA
• What happens to the frequency of successful collisions at higher temperatures?

  Increases with more particles exceeding EA
• What are the effects of increasing temperature on reaction rates?

  • Increases particle energy
  • Increases collision frequency
  • More particles exceed activation energy
  • Higher frequency of successful collisions
• What are the effects of increasing concentration and pressure on reaction rates?

  • More particles per unit volume
  • Higher frequency of effective collisions
  • Shape of energy distribution curves remains unchanged
  • Area under the curves increases
• What are the effects of increasing surface area on reaction rates?

  • More frequent successful collisions
  • Increases the rate of reaction
• What are the effects of catalysts on reaction rates?

  • Increase reaction rates
  • Provide alternative route with lower activation energy
  • Not consumed in the reaction
• How do reaction rates vary with different initial concentrations?

  • Same initial rate for different volumes
  • Different final amounts of product
  • Proportional relationship between moles of reactant and product