  The validity of two fundamental concepts of classical chemical kinetics - the
notion of "Chemical Equilibrium" and the "Law of Mass Action" - are re-examined
for reversible \textit{diffusion-limited} reactions (DLR), as exemplified here
by association/dissociation $A+A \rightleftharpoons B$ reactions. We consider a
general model of long-ranged reactions, such that any pair of $A$ particles,
separated by distance $\mu$, may react with probability $\omega_+(\mu)$, and
any $B$ may dissociate with probability $\omega_-(\lambda)$ into a geminate
pair of $A$s separated by distance $\lambda$. Within an exact analytical
approach, we show that the asymptotic state attained by reversible DLR at $t =
\infty$ 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 picture holds \text{only} in physically unrealistic case when
$\omega_+(\mu) \equiv \omega_-(\mu)$ for any value of $\mu$.
