  Chemical equilibrium is fully characterized at thermodynamic level by the
free energy: the equilibrium is written in terms of equality of chemical
potentials. Rate of chemical reactions can be calculated for ideal solutions by
using mass-action equations, with reaction rate constants related to energy
barriers through the transition state theory. The two approaches merge in the
case of equilibrium and ideal solutions. Here, we discuss how to extend the
transition state theory in the case of non-ideal solutions, i.e. of interacting
particles, at equilibrium. We find bounds of the exchange reaction rate.
However, we show that the value of the exchange reaction rate cannot be
calculated from equilibrium thermodynamic parameters (i.e. from the free
energy), i.e. it is possible to find different Hamiltonians giving the same
free energy but different exchange reaction rate.
