basically because concentration is an approximate concept.
is the number of molecules of specie B. In phase space (ideal gas), there is no quantum correlations between molecules and each molecule is a well-defined specie. In real phase (gas, and specially solutions and solids) there is not real molecules. There is a global system formed by N particles. The situation is even worse in electrochemical systems because there is charges and electrostatic Coulomb forces are large correlation ones.
Therefore it is difficult to define cB
in a system where B (in the usual chemical sense) really does not exist
However in some sense "B" is inside the total system of N particles and therefore it is natural to believe in some substitute for cB
: e.g. activity aB
Some books attempt to claim that one may
always substitute cB
in non ideal phases. This is not true
In fact, that ad hoc
rule was source (and continue to be) of confusion in the past. E.g. if one follows that "standard rule" one obtains from chemical kinetics
B + D ---> products
/ part t = - kaB
but this equation does not work. The correct is
/ part t = - kaB
and, therefore, some textbooks do
/ part t = - k'cB
and introduces nonideal effects (e.g. strengh of an external electric field directly in the rate constant).
From macroscopic canonical science one obtains the correct answers to these questions without adtitional asumptions or wrong reasoning. Moreover, at equilibrium the rate part "collapses" and survives only the activity part (right part) and by this reason in equilibrium formulas it appears the activity instead of concentration. For instance the equlibrium constant is computed from both products and products-1
of activities instead of concentrations.
Moreover, from microscopic canonical science, one obtain the details of why. There is no real molecules in the ideal gas sense, therefore there is not typical concentration.
Perhaps some chemists are perplexed of that molecules (in the usual "chemical" sense of individual entities) do not exist, but let me remember that usual quantum chemistry computations are only valid for idealized phases, where each individual molecule is a well-defined entity.
In fact, my profesor of quantum mechanics said us an anecdote (i believe is real one)of an inorganic chemist that using the Gaussian computed an entalpy, apply it, and find a sound discrepancy. He just forgot that computed "Gaussian entalpy" was valid only for ideal gas of that molecule. In condensed matter situations, one may introduce solvent effects and in more sophisticated approaches simply ignore the language of wavefunctions, Schrödinger equation, and all that.