AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to: draw and interpret distribution curves for…
Full specification wording
Maxwell–Boltzmann distribution: Students should be able to: draw and interpret distribution curves for different temperatures.
When a gas is heated the total number of molecules is unchanged, so the area under the curve must stay the same, but the molecules are redistributed towards higher energies.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When a gas is heated the total number of molecules is unchanged, so the area under the curve must stay the same, but the molecules are redistributed towards higher energies. The curve for the higher temperature has its peak lower and shifted to the right, and it crosses the original curve exactly once before running above it at all energies past that crossing point. That combination is what examiners look for: a lower, flatter, right shifted maximum, an unchanged total area, and a single crossing. The consequence is read off the tail. Beyond a marked Eₐ the shaded area under the hotter curve is many times larger, so many more molecules have energy greater than or equal to the activation energy and the rate rises sharply.
What earns the marks
- Eₐ marked on the energy axis to the right of the peaks.
- The original curve drawn with the correct shape from the origin.
- The higher temperature curve with a lower peak shifted to the right, crossing the first curve only once.
- Stating that at the higher temperature many more molecules have energy greater than or equal to Eₐ.
Common mistakes and how to put them right
Drawing the hotter curve taller as well as shifted, so the total area is no longer constant.
Drawing curves that cross twice or run parallel without crossing.
Shifting the peak to the right but keeping it at the same height.
Marking Eₐ at the peak instead of far out on the tail.
Leaving the axes unlabelled so no shape marks can be awarded.
Exam tips
- Draw the lower temperature curve first, then fit the hotter one under and to the right of it, checking there is exactly one crossing.
- Write T₁ and T₂ on the curves themselves, and state which is hotter.
- Finish with the sentence about the area beyond Eₐ; the shape marks and the explanation mark are separate.
Try the lesson
The start of the Maxwell–Boltzmann distribution: Students should be able to: draw and interpret distribution curves for different temperatures. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement correctly describes how a Maxwell–Boltzmann distribution curve changes when the temperature of a gas is increased?
Listen
When a gas is heated the total number of molecules is unchanged, so the area under the curve must stay the same, but the molecules are redistributed towards higher energies. The curve for the higher temperature has its peak lower and shifted to the right, and it crosses the original curve exactly once before running above it at all energies past that crossing point. That combination is what examiners look for: a lower, flatter, right shifted maximum, an unchanged total area, and a single crossing. The consequence is read off the tail. Beyond a marked Eₐ the shaded area under the hotter curve is many times larger, so many more molecules have energy greater than or equal to the activation energy and the rate rises sharply. A strong answer earns marks for these points. Eₐ marked on the energy axis to the right of the peaks. the original curve drawn with the correct shape from the origin. the higher temperature curve with a lower peak shifted to the right, crossing the first curve only once. stating that at the higher temperature many more molecules have energy greater than or equal to Eₐ. Exam tip: Draw the lower temperature curve first, then fit the hotter one under and to the right of it, checking there is exactly one crossing.
Key words
Say what each word means before you turn the card.
constant area under the curve
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peak lower and shifted right
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single crossing point
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area beyond the activation energy
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See every definition
- constant area under the curve
- The total number of molecules is unchanged when a gas is heated, so the area under the curve must stay the same.
- peak lower and shifted right
- The curve for the higher temperature has its peak lower and shifted to the right.
- single crossing point
- The hotter curve crosses the original curve exactly once before running above it at all energies past that crossing point.
- area beyond the activation energy
- Beyond a marked Eₐ the shaded area under the hotter curve is many times larger, so many more molecules have energy greater than or equal to the activation energy.
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