Now we have discussed, and looked at, various factors affecting the reactivity of a species, and which types of reagents attack where, we can look at the variation of these factors from one structure to another, and hope these factors also affect the course, and rate, of a reaction.

Firstly, a basic tenet of chemistry is that systems tend towards their lowest energy state. 

Therefore, we might reasonably expect that the greater the difference in energy betweenstarting material and products, the more  the equilibrium will favour the lower in energy.  We can use this idea to work out how much of a given starting material will react, i.e. the extent of the reaction. The larger the arrow in the diagram below, the bigger the energy gap, and the more the reaction favours products over starting materials:

A brief look at the thermodynamics course in the physical chemistry section is recommended if this material is unfamiliar.  For the purposes of a brief summary (this material is presented in more depth in the physical chemistry section) we will now look at some basic thermodynamics.

The energy change of a reaction ΔH can be measured as the heat of the reaction.  It is, however, not adequate for measuring the difference  in stability between the reactants and products.  This is essentially due to the Second Law of Thermodynamics, which paraphrased states that: systems tend towards maximum entropy (disorder).  This leads us to define the Gibb's free energy, so that we might consider the entropy change of both the system and the surroundings in one. Hence

ΔG = ΔH - TΔS

This expression, it is found, relates the Gibb's energy change to the extent of a reaction via:

ΔG = - RTlnK

Where K is the equilibrium constant, and is equal to the concentration of products over the concentration of reactants.  The larger the value of K, the further the reaction has proceeded to products.  This equation makes intuitive sense, for if we set ΔG = 0, we find that K = 1, which corresponds to 50% products, and 50% reactants.  In order for a reaction to occur, ΔG < 0

Now we have a measure of the extent of a reaction, we can work on finding an expression for the rate of a reaction.