Collision Theory
Collision theory
For us to be able to understand what a reaction is, we must ask ourselves the question:
For a reaction to occur, what needs to happen at the molecular level?
If we think about this question carefully, there are 3 conditions have to be met:
- The molecules need to meet.
- The molecules need to have enough energy.
- The molecules need to be in the correct orientation.
The molecules need to meet.
This point is hopefully pretty obvious. If the molecules of the two substances do not interact in some way, then how is any reaction going to take place? Clearly it won't. So for any reaction to occur, the individual molecules have to come together.
The molecules need to have enough energy.
The molecules meeting is not enough on its own. Why are there many reactions which only happen when heat is applied? The two reactants are mixed together and therefore the molecules are meeting, which fulfils the first criteria but no reaction is occurring. Why? That is because the energy of the molecules must be high enough to cause the existing bonds to break. If the energy is not sufficient when the molecules collide, then the molecules will not react.
The molecules need to have the correct orientation
This is the least obvious of the 3 criteria. Even if the molecules meet and they have sufficient energy, a reaction still might not take place. Why? This is because the molecules often need to meet in a certain orientation. In large molecules it is especially likely that only certain parts of the molecule is reactive, therefore a reaction will only take place when the molecules collide in those regions.
In addition to these points, it is important to keep in mind that the energy of all the molecules will not be exactly the same, and there will be variations in their energies. This is partly why, whenever a reaction occurs with an evenly mixed substance, it does not all happen at the same time. Just like with things like a group of people's heights, the energies of the molecules will follow a distribution pattern. This means that often some of the molecules have sufficient energy to react, whilst others do not.