Particles in any sample of matter have energy spread out in a random fashion. This distribution follows the statistical patterns first described by Maxwell and Boltzmann.
This is important in reaction kinetics as only particles with high enough energy can cause reaction to occur when they collide.
By understanding this distribution and how it varies under different situations we can better understand how the rate of chemical reactions changes.
Distribution of energy
The distribution of energy in a sample of particles is shown by the Maxwell-Boltzmann curve.
The curve is arrived at by statistical analysis.
Statistics and probability tells us that if the energy is distributed randomly over a large number of particles there will be a much greater probability that some arrangements are found rather than others.
The Maxwell Boltzmann curve shows us that are few particles with small amounts of energy and few particles with large amounts of energy. The majority of the particles possess an intermediate amount of energy.
Increasing the temperature flattens the curve as more particles attain higher energies and the median energy (the hump) moves to the right (higher energy) side.
For chemical reaction to occur bonds must be broken and formed. The energy required to break the bonds comes from the collision. The activation energy is the minimum amount of energy required for reaction to occur between colliding particles.
Not all collisions between suitable particles are going to result in successful reaction. There are some collisions that simply do not have enough energy to break bonds. The collisions in which the particles do have enough energy for bond breaking can react. The activation energy of a reaction is given the symbol Ea.
In a reaction profile the activation energy is the highest energy step taken during the course of the reaction. (Note: there may be several activation energy steps in a multi-step reaction)
The activation energy of a reaction is a constant for that specific reaction - it can only be changed if the mechanism of the reaction itself changes. This occurs during catalysis (see 6.25)
Effect of temperature
The shape of the Maxwell Boltzmann curve is temperature dependent. As the temperature increases there are more particles with higher energy than at a lower temperature. The shape of the curve broadens and flattens (it must flatten if it gets broader, as the total number of particles cannot change).
We can see from the curves that there are more particles with higher energy at a higher temperature. This means that there will be more successful colisions, i.e. collision with the required activation energy.
For this reason, increasing the temperature of a reaction increases the rate. As a rough guide, the rate doubles for every ten degree increase in temperature.
|It is tempting to say that the reason temperature affects rate is that there are more collisions, due to the fact that the particles are travelling faster. While it is true that there are more collisions, this factor is much less important than the activation energy factor.|
Collision geometry - the orientation factor
When two cars collide the degree of damage depends on the angle with which they collide. Likewise, a car that reverses into a tree is not going to break its headlights!
Taking these things to a logical conclusion, when two particles collide the orientation of the collision is important for two reasons:
- The collision must occur with enough energy to result in reaction
- The particles must collide in such a way as to break the required bonds.
The second of these collision requirements is called the orientation factor. Not all collisions result in reaction, even if the combined energy of the particles is theoretically large enough, because the orientation is incorrect.
A catalyst is a substance that increases the rate of a chemical reaction without itself getting consumed in the reaction. A catalyst can be recovered chemically unchanged at the end of the reaction.
Catalysts work by offering a different route to the products (mechanism) with a lower activation energy.
The consequence is that there are now more particles at a given temperature that have the required activation energy to react.