The arrangements in Daniel cell
Anode reaction
#Zn(s)->Zn^(2+)(aq)+2e^-#
Cathode reaction
#Cu^(2+)(aq)+2e^(-) ->Cu(s)#
Overall reaction
#Zn(s)+Cu^(2+)(aq)->Cu(s)+Zn^(2+)(aq)#
The cell notation
#Zn(s)|Zn^(2+) ||Cu^(2+)|Cu(s)#
The Cell Emf by Nernst equation
#E_(cell)=E_(cell)^@-(RT)/(nF)xx([Zn^(2+)])/([Cu^(2+)])......[1]#
If we add sodium sulphide #(Na_2S)# to cupper sulphate in daniel cell, it interacts with #Cu^(2+)# in solution and lowers the concentration of #Cu^(2+)# by precipitating it as insoluble #CuS(s)# as per the following equation.
#Cu^(2+)(aq)+Na_2S->CuS(s)darr+2Na^+#
As a result the ratio #([Zn^(2+)])/([Cu^(2+)])# in equation [1] increases and this decreases the #E_(cell)# to a great extent.
