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1.9.3Physical Chemistry

Maxwell-Boltzmann Distributions — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 40 of 42

📚 Edexcel AS Chemistry Hub / Topical Revision
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Section 1 of 5Reading the Curve

1.9.3 Maxwell-Boltzmann Distributions
Axes
  • The x-axis shows molecular energy and the y-axis shows the number of molecules with that energy.
Shape
  • The curve starts at the origin, rises to a peak and has a long tail that does not touch the axis.
Area under curve
  • This represents the total number of molecules and stays constant for a fixed sample.
Peak
  • The most probable energy lies at the maximum of the curve.
Energy range
  • Molecules in a gas have a wide spread of energies because they constantly collide and exchange energy with each other.

Section 2 of 5Activation Energy on the Graph

Activation Energy on the Graph
Marking Ea
  • A vertical line on the x-axis at the activation energy splits the curve.
Molecules to the right
  • Only the area to the right of Ea represents molecules able to react on collision.
Fraction reacting
  • This area is a small part of the total, which explains why most collisions are unsuccessful.
Link to rate
  • A larger area to the right of Ea means a faster reaction.

Section 3 of 5Effect of Temperature

1
At higher temperature the average kinetic energy of molecules increases.
2
The peak of the curve moves to the right and becomes lower and broader.
3
The total area under the curve remains the same because the number of molecules is unchanged.
4
A much larger proportion of molecules has energy equal to or greater than Ea.
5
More collisions are successful in a given time, so the rate increases.
ChangePeakArea right of Ea
Raise temperatureLower and moves rightLarger
Lower temperatureHigher and moves leftSmaller

Section 4 of 5Effect of a Catalyst

Lower Ea
  • A catalyst provides a different pathway with a lower activation energy.
Curve unchanged
  • The distribution of molecular energies stays the same shape, because temperature has not changed.
Second Ea line
  • Draw a new vertical line further to the left to mark the catalysed activation energy.
Result
  • More of the area now lies beyond Ea, so a greater proportion of molecules can react.
Same outcome
  • Both a catalyst and a higher temperature raise the proportion of molecules able to react, but they do so in different ways.

Section 5 of 5Exam Checklist & Tips

1.9.3 – Key Points for the Exam

  • Maxwell–Boltzmann distribution Graph showing the spread of molecular energies in a gas.
  • Activation energy Minimum energy needed for a reaction.
  • Most probable energy Energy at the peak of the distribution.
  • Proportion of molecules The fraction of the total area under the curve.

Common Mistakes

  • Drawing a curve that touches the x-axis at high energy.
  • Drawing the peak of the higher temperature curve higher than the lower temperature curve.
  • Moving the Ea line when temperature changes; Ea stays fixed unless a catalyst is added.
  • Saying that more molecules exist when temperature is raised.

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