AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Maxwell–Boltzmann distribution of molecular energies in gases.
Full specification wording
Maxwell–Boltzmann distribution: Maxwell–Boltzmann distribution of molecular energies in gases.
The Maxwell–Boltzmann distribution is a graph of the number of molecules against molecular kinetic energy for a gas at a fixed temperature.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The Maxwell–Boltzmann distribution is a graph of the number of molecules against molecular kinetic energy for a gas at a fixed temperature. Its shape is fixed by physics: it starts at the origin, because no molecule has zero energy, rises steeply to a peak at the most probable energy, then falls away with a long tail to the right that never touches the axis, because there is no upper limit to the energy a molecule can have. The mean energy sits slightly to the right of the peak because that tail drags the average up. The area under the whole curve equals the total number of molecules, so it stays constant at a fixed amount of gas, and the area beyond Eₐ represents the molecules able to react.
What earns the marks
- A curve starting at the origin, since no molecules have zero energy.
- A single peak at the most probable energy with the curve falling away to the right.
- An asymptotic tail that never meets the energy axis.
- Stating that the area under the curve equals the total number of molecules.
Common mistakes and how to put them right
Drawing a symmetrical, normal distribution rather than a skewed curve with a long tail.
Letting the curve start above the origin or cut the energy axis at high energy.
Labelling the peak as the mean energy rather than the most probable energy.
Labelling the y axis as energy and the x axis as number of molecules.
Assuming a molecule at the peak of the curve has enough energy to react.
Exam tips
- Label the axes number of molecules and energy; an unlabelled sketch usually scores nothing.
- Practise drawing the curve in one stroke from the origin, so the start and the tail both come out right.
- Mark Eₐ well to the right of the peak and shade the area beyond it when explaining rate.
Try the lesson
The start of the Maxwell–Boltzmann distribution: Maxwell–Boltzmann distribution of molecular energies in gases. lesson, made from checked teaching notes for this specification point.
Have a go first
On a Maxwell–Boltzmann distribution curve for a gas, what does the area under the curve represent?
Listen
The Maxwell–Boltzmann distribution is a graph of the number of molecules against molecular kinetic energy for a gas at a fixed temperature. Its shape is fixed by physics: it starts at the origin, because no molecule has zero energy, rises steeply to a peak at the most probable energy, then falls away with a long tail to the right that never touches the axis, because there is no upper limit to the energy a molecule can have. The mean energy sits slightly to the right of the peak because that tail drags the average up. The area under the whole curve equals the total number of molecules, so it stays constant at a fixed amount of gas, and the area beyond Eₐ represents the molecules able to react. A strong answer earns marks for these points. a curve starting at the origin, since no molecules have zero energy. a single peak at the most probable energy with the curve falling away to the right. an asymptotic tail that never meets the energy axis. stating that the area under the curve equals the total number of molecules. Exam tip: Label the axes number of molecules and energy; an unlabelled sketch usually scores nothing.
Key words
Say what each word means before you turn the card.
most probable energy
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mean energy
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asymptotic tail
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area under the curve
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See every definition
- most probable energy
- The energy at the single peak of the curve, where the number of molecules is highest.
- mean energy
- The average energy, which sits slightly to the right of the peak because the tail drags the average up.
- asymptotic tail
- The long tail to the right that never touches the axis, because there is no upper limit to the energy a molecule can have.
- area under the curve
- The whole area equals the total number of molecules, so it stays constant at a fixed amount of gas.
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