【正确答案】Only in the presence of Cl^-ion is Ag a sufficiently good reducing agent to be very useful for prereductions.
   In the presence of Cl^-ion,the half-reaction occurring in the Walden reduc tot is
   Ag(s) +Cl^-→AgCl(s)+e^-
   The excess HCl increases the tendency of this reaction to occur by the common ion effect.