Edexcel A Level Chemistry:复习笔记1.9.3 Maxwell-Boltzmann Distributions

Temperature Effects

 

Maxwell-Boltzmann distribution curve

  • A Maxwell-Boltzmann distribution curve is a graph that shows the distribution of energies at a certain temperature
  • In a sample of a gas, a few particles will have very low energy, a few particles will have very high energy, but most particles will have energy in between

1.8-Reaction-Kinetics-Boltzmann-Distribution-Curve

The Maxwell-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 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 Maxwell-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
    • A greater proportion of the molecules having kinetic energy greater than the activation energy

     

Exam Tip

The increase in proportion of molecules having kinetic energy greater than the activation has a greater effect on the rate of reaction than the increase in effective collisions

Effects of Adding a Catalyst

  • Catalysis is the process in which the rate of a chemical reaction is increased, by adding a catalyst
  • A catalyst increases the rate of a reaction by providing the reactants with an alternative reaction pathway which is lower in activation energy than the uncatalysed reaction
  • Catalysts can be divided into two types:
    • Homogeneous catalysts
    • Heterogeneous catalysts

     

  • Homogeneous means that the catalyst is in the same phase as the reactants
    • For example, the reactants and the catalysts are all in solution

     

  • Heterogeneous means that the catalyst is in a different phase to the reactants
    • For example, the reactants are gases but the catalyst used is a solid

     

1.8-Reaction-Kinetics-Catalyst-Reaction-Pathway

The diagram shows that the catalyst allows the reaction to take place through a different mechanism, which has a lower activation energy than the original reaction

 

Maxwell-Boltzmann distribution curve

  • Catalysts provide the reactants another pathway which has a lower activation energy
  • On the graph below, the original number of successfully reacting particles is shown by the dark shaded area
  • By lowering Ea, a greater proportion of molecules in the reaction mixture have the activation energy, and therefore have sufficient energy for an effective collision
    • This is shown by the combined number of particles in the light and dark shaded areas
  • As a result of this, the rate of the catalysed reaction is increased compared to the uncatalysed 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

 

 

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