IB DP Chemistry: SL复习笔记6.1.7 Maxwell-Boltzmann Curves

Maxwell-Boltzmann Curves

Boltzmann distribution curve

  • The Boltzmann distribution curve is a graph that shows the distribution of energies at a certain temperature
  • In a sample of a substance, a few particles will have very low energy, a few particles will have very high energy, and many particles will have energy in between

1.8-Reaction-Kinetics-Boltzmann-Distribution-Curve

The Boltzmann distribution curve shows the distribution of the energies and the activation energy

  • The graph shows that only a small proportion of molecules in the sample have enough energy for an effective or successful collision and for a chemical reaction to take place

Changes in temperature

  • When the temperature of a reaction mixture is increased, the particles gain more kinetic energy
  • This causes the particles to move around faster resulting in more frequent collisions
  • Furthermore, the proportion of successful collisions increases, meaning a higher proportion of the particles possess the minimum amount of energy (activation energy) to cause a chemical reaction
  • With higher temperatures, the Boltzmann distribution curve flattens and the peak shifts to the right

1.8-Reaction-Kinetics-Boltzmann-Distribution-Curve-at-higher-Temperature

The Boltzmann distribution curve at T oC and when the temperature is increased by 10 oC

  • Therefore, an increase in temperature causes an increased rate of reaction due to:
    • There being more effective collisions as the particles have more kinetic energy, making them move around faster
    • greater proportion of the molecules having kinetic energy greater than the activation energy

Effect of a catalyst on the Maxwell-Boltzmann distribution curve

  • A catalyst provides the reactants another pathway which has a lower activation energy
  • By lowering Ea, a greater proportion of molecules in the reaction mixture have sufficient energy for a successful collision
  • As a result of this, the rate of the catalysed reaction is increased compared to the uncatalyzed reaction

1.8-Reaction-Kinetics-Catalyst-Boltzmann-Distribution

The diagram shows that the total shaded area (both dark and light shading) under the curve shows the number of particles with energy greater than the Ea when a catalyst is present. This area is much larger than the dark shaded area which shows the number of particles with energy greater than the Ea without a catalyst

Exam Tip

Make sure you know how to sketch and label the axes in Maxwell-Boltzmann distribution curves. The curve must start at the origin and it approaches, but never touches the x- axis

 

 

 

 

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