2.6.3Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Maxwell-Boltzmann Distributions — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 39 of 53 Unit quiz
Part 1 of 4Features of the Curve
The distribution graphs the number of molecules against their energy at a fixed temperature.
Shape
- The curve starts at the origin, rises to a peak and then tails off without touching the axis.
Origin
- It begins at the origin because no molecules have zero energy.
Peak
- The peak is the most probable energy, with only a few molecules at very low or very high energy.
Area
- The area under the curve equals the total number of molecules, so it is constant for a given sample.
Activation energy
- A line marked at Eₐ divides the molecules that can react from those that cannot.
Part 2 of 4Effect of Raising the Temperature
1
At a higher temperature the peak moves to the right and becomes lower, because the energies are more spread out.
2
The total area under the curve stays the same because the number of molecules has not changed.
3
The curve is no longer symmetrical, with a longer tail at high energy.
4
The area to the right of Eₐ increases, so a greater proportion of molecules has enough energy to react.
5
More collisions are successful per second, so the rate of reaction increases.
6
Label the axes with energy on the x-axis and number of molecules with that energy on the y-axis.
Part 3 of 4Interpreting Questions
Interpreting Questions
Sketching
- Draw both curves on the same axes, label them T₁ and T₂, and make the higher temperature curve lower and wider.
Explaining
- State that the proportion of molecules above Eₐ rises, not just that molecules have more energy.
Common error
- Do not let the curves touch the x-axis at the high-energy end or cross the y-axis.
Link to rate
- A small temperature rise can give a large increase in rate because the tail is steep.
Typical error
- A common slip is labelling the y-axis as energy; it actually shows the number of molecules with each energy.
Part 4 of 4Exam Checklist & Tips
2.6.3 – Key Points for the Exam
- Maxwell-Boltzmann distribution A curve showing how molecular energies are shared in a gas or liquid.
- Activation energy The minimum energy needed for a successful collision.
- Most probable energy The energy at the peak of the distribution curve.
- Proportion The fraction of molecules with energy at or above a particular value.
Common Mistakes
- Drawing the curve so that it touches the x-axis at high energy.
- Saying the area under the curve increases at higher temperature.
- Moving the peak up instead of lowering it for a higher temperature.
- Claiming the activation energy decreases when temperature increases.
Last updated:
\n