  Context. A molecular survey recently performed ahead of HH~2 supports the
idea that the observed molecular enhancement is due to UV radiation from the HH
object. Aims. The aim of the present work is to determine whether all HH
objects with enhanced HCO$^+$ emission ahead of them also exhibit the same
enhanced chemistry as HH~2. We thus observed several molecular lines at several
positions ahead of five Herbig-Haro objects where enhanced HCO$^+$ emission was
previously observed. Methods. We mapped the five Herbig-Haro objects using the
IRAM-30 m. For each position we searched for more than one molecular species,
and where possible for more than one transition per species. We then estimated
the averaged beam column densities for all species observed and also performed
LVG analyses to constrain the physical properties of the gas. Results. The
chemically richest quiescent gas is found ahead of the HH~7-11 complex, in
particular at the HH~7-11 A position. In some regions we also detected a high
velocity gas component. We find that the gas densities are always higher than
those typical of a molecular cloud while the derived temperatures are always
quite low, ranging from 10 to 25 K. The emission of most species seems to be
enhanced with respect to that of a typical dense clump, probably due to the
exposure to a high UV radiation from the HH objects. Chemical differentiation
among the positions is also observed. We attempt a very simple chemical
analysis to explain such differentiation.
