AQA Chemistry AS/A-level Year 1
3.1
Physical chemistry
3.1.5 Kinetics
Essential Notes In addition, before a reaction can occur, it is often necessary for the orientation of molecules to be correct on collision.
3.1.5.1 Collision theory When two substances react, particles (molecules, atoms or ions) of one substance must collide with particles of the other. However, not all collisions result in a reaction; i.e. not all collisions are productive. This situation arises because particles will react only when they collide with sufficient energy. The minimum energy necessary for reaction is known as the activation energy. In practice, most collisions do not lead to a reaction because the particles do not collide with sufficient energy. If the activation energy is not reached, particles simply bounce apart unchanged.
3.1.5.2 Maxwell–Boltzmann distribution In a sample of gas or liquid, the molecules are in constant motion and collide both with each other and with the walls of their container. Such collisions are said to be elastic; i.e. no energy is lost during the collision, but energy can be transferred from one molecule to another.
Fig 1 Distribution of energies at a particular temperature
Number of molecules
Consequently, at a given temperature, molecules in a particular sample will have a spread of energies about the most probable energy. James Clark Maxwell and Ludwig Boltzmann derived a theory from which it is possible to draw curves showing how these energies are distributed. A plot of the number of molecules with a particular energy against that energy (see Fig 1) is known as the Maxwell–Boltzmann distribution curve.
The area under the curve represents the total number of molecules in the sample
Energy
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