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Molecular Speed of Gases

Skills to develop

  • Explain the distribution of molecular speed of a gas.
  • Molecular Speed of Gases

    The quantitative relationship of temperature effect on chemical reaction rates are discussed in form of activation energy, depicted as Ea in the diagram. For reactions having large Ea values, high temperatures are required to have a measurable reaction rate. The reason behind this is due to the small number of molecules having sufficient energy to overcome Ea, the energy barrier of forming an activated reaction intermediate.

    At a certain temperature, not all gas molecules are moving with the same speed, some fast and some slow. The kinetic energies of molecules are not all equal. The fast moving ones have high kinetic energy, and they may have enough energy to overcome the reaction energy barrier Ea.

    A plot of number of gas molecules at certain speed versus speed gives a distribution curve. Careful studies of gases show that the distribution is not a bell or NORMAL distribution, but a Maxwell- Boltzmann distribution.


    A sketch of a Maxwell distribution is given here. The peaks are not symmetrical. There are more molecules at higher speed than at lower speeds. When the temperature increases, the peak shifts to the right. A more carefully plotted diagram is shown below, and a computer simulated plots will be more illustrative, however the simulation program was only available in the DOS version of CAcT earlier.

    You will learn the theory and the statistics at a higher year. For the moment, you may notice how the distribution curve shift as temperature increases.

    Confidence Building Questions

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