Statistical explanation

Consider our hypothetical reaction  n_A A + ν_B B --> ν_C C + n_D D. 

The chemical potential for each chemical,  j, can be derived from the partition function

Using this as a starting point, we can determine the equilibrium constant in terms of the molecular partition function

or, more generally,

So how does this contribute to the equilibrium? 

Lets consider a simplified gas phase reaction R P.

A and B will have different partition functions, depending on all the factors we saw earlier, q_j = q_elecq_vibq_rotq_trans.  and in addition, the lowest level of q_R will differ from that of q_P by E₀.  We can look at the distribution of states resulting from a particular reactant and product to visualize what these contributions mean.

Fig. 17.20 (from Atkins): Here, we see an endothermic reaction where the density of states in the products is similar to the density of states in the reactants, i.e., not a large entropy difference between them.  Obviously, the lower manifold of states (reactants) is populated more, according to Boltzmann.  Hence, this is an enthalpy driven reaction.

Figure 17.21 (from Atkins): Again, we have an endothermic reaction but this time, the density of states in the products is higher than in the reactants, thus, Boltzmann puts most of the population in the reactants.  Hence, this is an entropy driven reaction.

Clearly, we can see that both the enthalpy and the entropy have an important contribution to the position of the equilibrium and it all boils down ultimately to randomnes.