  A magnetohydrodynamic model of a steady, transverse C-type shock in a dense
molecular cloud is presented. A complete gas-grain chemical network is taken
into account: the gas-phase chemistry, the adsorption of gas species on dust
grains, various desorption mechanisms, the grain surface chemistry, the ion
neutralization on dust grains, the sputtering of grain mantles. The population
densities of energy levels of ions CI, CII and OI and molecules H$_2$, CO,
H$_2$O are computed in parallel with the dynamical and chemical rate equations.
The large velocity gradient approximation is used in the line radiative
transfer calculations. The simulations consist of two steps: (i) modelling of
the chemical and thermal evolution of a static molecular cloud and (ii) shock
simulations. A comparison is made with the results of publicly available models
of similar physical systems.
  The focus of the paper is on the chemical processing of gas material and ice
mantles of dust grains by the shock. Sputtering of ice mantles takes place in
the shock region close to the temperature peak of the neutral gas. At high
shock speeds, molecules ejected from ice mantles are effectively destroyed in
hot gas, and their survival time is low - of the order of dozens of years.
After a passage of high-speed C-type shock, a zone of high abundance of atomic
hydrogen appears in the cooling postshock gas that triggers formation of
complex organic species such as methanol. It is shown that abundances of some
complex organic molecules (COMs) in the postshock region can be much higher
than in the preshock gas. These results are important for interpretation of
observations of COMs in protostellar outflows.
