Equilibrium for very small amounts of reactants
Thermodynamics is only applicable to the systems of size in between astronomic and microscopic scales. Systems with a small number of particles can spontaneously abandon
equilibrium state due to fluctuations. Statistical physics tells us [1] that the relative magnitude of the fluctuations of thermodynamic quantities $x$ (e.g. pressure, temperature, volume, internal energy, entropy etc.) is inversely proportional to the root of the number of particles in the system:

$$
\frac{
\sqrt{\left\langle\left(\Delta x \right)^2\right\rangle}
}{
\left\langle x \right\rangle
}
\sim
\frac{1}{\sqrt{N}}
$$

Lets arbitrarily set the detection limit for experimental determination of the relative change of $x$ to optimistic value of $10^{-8}$. In this case we would need at least $N = (10^{-8})^{-2} = 10^{16}$ particles belonging to the thermodynamic system before we can speak of equilibrium at all. 

References


Landau, L. D.; Lifshitz, E. M. Statistical Physics, 3rd Edition, Part 1: Vol. 5; Butterworth-Heinemann: Amsterdam u.a, 1980.