Maxwell-Boltzmann Distribution Curves (DP IB Chemistry): Revision Note
Maxwell-Boltzmann distribution curves
What is a Maxwell-Boltzmann distribution curve?
A Maxwell-Boltzmann distribution curve is a graph that shows the distribution of energies at a certain temperature
In a sample of a substance:
A few particles will have very low energy
A few particles will have very high energy
Many particles will have energy in between
Maxwell-Boltzmann distribution curve
The graph shows that only a small proportion of particles in the sample have enough energy for an effective or successful collision and for a chemical reaction to take place
The most probable energy of a particle is represented by the highest point on the curve's peak
This is sometimes written as EMP
Effect of changes in temperature on the Maxwell-Boltzmann distribution curve
When the temperature of a reaction mixture is increased, the particles gain more kinetic energy
This causes the particles to move around faster, resulting in more frequent collisions
The proportion of successful collisions increases, meaning a higher proportion of the particles possess the Ea to cause a chemical reaction
With higher temperatures, the Maxwell-Boltzmann distribution curve flattens and the peak shifts to the right
Graph of the effect of temperature on the Maxwell-Boltzmann distribution curve
Therefore, an increase in temperature causes an increased rate of reaction due to:
More effective collisions as the particles have more kinetic energy, making them move around faster
A greater proportion of the molecules having kinetic energy greater than the Ea
Examiner Tips and Tricks
When drawing Maxwell-Boltzmann distribution curves at different temperatures, make sure that:
The peak of the curve of the higher temperature is lower and to the right of the peak of the curve of the lower temperature
The two curves should only cross each other once
The tail of the curve of the higher temperature should be higher than that of the lower temperature
Careful: Examiners currently prefer to ask about the effect of reducing the temperature of a reaction, rather than increasing the temperature
The underlying theory is still the same, but you need to apply it in the opposite direction
Effect of a catalyst on the Maxwell-Boltzmann distribution curve
A catalyst provides the reactants with another reaction pathway which has a lower activation energy
By lowering Ea, a greater proportion of molecules in the reaction mixture will have sufficient energy for a successful collision
As a result of this, the rate of the catalysed reaction is increased compared to the uncatalysed reaction
Maxwell-Boltzmann distribution curve with a catalyst
Examiner Tips and Tricks
Make sure you know how to sketch and label the axes in Maxwell-Boltzmann distribution curves
The curve must start at the origin, and it approaches, but never touches the x-axis
If you are asked to show the area that represents the particles with an energy greater than the activation energy with a catalyst, make sure you highlight the total shaded area, not just the light-shaded area
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