  We study a mesoscopic model of a chemically active colloidal particle which
on certain parts of its surface promotes chemical reactions in the surrounding
solution. For reasons of simplicity and conceptual clarity, we focus on the
case in which only electrically neutral species are present in the solution and
on chemical reactions which are described by first order kinetics. Within a
self-consistent approach we explicitly determine the steady state product and
reactant number density fields around the colloid as functionals of the
interaction potentials of the various molecular species in solution with the
colloid. By using Teubner's reciprocal theorem, this allows us to compute and
to interpret -- in a transparent way in terms of the classical Smoluchowski
theory of chemical kinetics -- the external force needed to keep such a
catalytically active colloid at rest (\textit{stall} force) or, equivalently,
the corresponding velocity of the colloid \textit{if} it is free to move. We
use the particular case of triangular-well interaction potentials as a
benchmark example for applying the general theoretical framework developed
here. For this latter case, we derive explicit expressions for the dependences
of the quantities of interest on the diffusion coefficients of the chemical
species, the reaction rate constant, the coverage by catalyst, the size of the
colloid, as well as on the parameters of the interaction potentials. These
expressions provide a detailed picture of the phenomenology associated with
catalytically-active colloids and self-diffusiophoresis.
