5.2.7Additional lessons
Maxwell-Boltzmann Distribution Curves — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 102 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Shape of the Curve
The distribution shows that in any gas or liquid the particles have a wide spread of kinetic energies.
Axes
- Axes The x-axis is the kinetic energy of the molecules and the y-axis is the number of molecules having that energy.
Starts at origin
- Starts at origin The curve begins at the origin because no molecules in a sample have zero energy.
Peak
- Peak The peak marks the most probable energy, and the curve is skewed because a few molecules have very high energies.
No end point
- No end point The curve approaches the x-axis but never touches it, since there is no upper limit to molecular energy.
Section 2 of 4Area and Activation Energy
Area and Activation Energy
Total area
- Total area The area under the whole curve equals the total number of molecules, which stays constant at a fixed amount of substance.
Eₐ marker
- Eₐ marker A vertical line at the activation energy divides the curve;
- the area to its right is the molecules able to react.
Small fraction
- Small fraction For most reactions at room temperature only a very small share of molecules lies to the right of Eₐ.
Rate link
- Rate link The larger the shaded area beyond Eₐ, the more successful collisions happen and the faster the reaction.
Section 3 of 4Temperature and Catalysts
Two ways of increasing the area beyond Eₐ give the same result for the rate.
1
At a higher temperature the peak moves to the right and becomes lower and broader, but the total area is unchanged.
2
The area beyond Eₐ is therefore larger, so a greater proportion of molecules can react.
3
At the higher temperature the average energy is greater, but the Eₐ line is still in the same position.
4
A catalyst does not change the curve at all; it moves the Eₐ line to the left, which also increases the area beyond it.
5
Both changes raise the rate for the same reason: more collisions have energy equal to or above the activation energy.
Section 4 of 4Exam Checklist & Tips
5.2.7 – Key Points for the Exam
- Maxwell-Boltzmann distribution A curve showing how the kinetic energies of molecules are spread in a sample at a given temperature.
- Most probable energy The energy at the peak of the curve, where the most molecules are found.
- Fraction of molecules The area beyond Eₐ divided by the total area under the curve.
- Activation energy The minimum energy needed for a successful collision, marked on the x-axis.
Common Mistakes
- Drawing the high-temperature curve with a higher peak than the low-temperature curve, when its peak should be lower.
- Letting the curve touch or cross the x-axis at high energy.
- Claiming that a catalyst shifts the peak of the curve to the right.
- Forgetting that both curves must enclose the same total area.
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