  On example of diffusion-limited reversible $A+A \rightleftharpoons B$
reactions we re-examine two fundamental concepts of classical chemical kinetics
- the notion of "Chemical Equilibrium" and the "Law of Mass Action". We
consider a general model with distance-dependent reaction rates, such that any
pair of $A$ particles, performing standard random walks on sites of a
$d$-dimensional lattice and being at a distance $\mu$ apart of each other at
time moment $t$, may associate forming a $B$ particle at the rate $k_+(\mu)$.
In turn, any randomly moving $B$ particle may spontaneously dissociate at the
rate $k_-(\lambda)$ into a geminate pair of $A$s "born" at a distance $\lambda$
apart of each other. Within a formally exact approach based on Gardiner's
Poisson representation method we show that the asymptotic $t = \infty$ state
attained by such diffusion-limited reactions is generally \textit{not a true
thermodynamic equilibrium}, but rather a non-equilibrium steady-state, and that
the Law of Mass Action is invalid. The classical concepts hold \text{only} in
case when the ratio $k_+(\mu)/k_-(\mu)$ does not depend on $\mu$ for any $\mu$.
