On the basis of standard electrode potential values, suggest which of the following reactions would take place? (Consult the book for `E^(ϴ)` value)
(a) `Cu+Zn^(2+)toCu^(2+)+Zn`
(b) `Mg+Fe^(2+)toMg^(2+)+Fe`
(c ) `Br_(2)+2Cl^(-)toCl_(2)+2Br^(-)`
(d) `Fe+Cd^(2)toCd+Fe^(2+)`

Explanation

As we know that,
`E_(Cu^(2+)//Cu)^(@)=0.34V, E_(Zn^(2+)//Zn)^(@)=-0.76V`
`E_(Mg^(2+)//Mg)^(@)=-2.37V, E_(Fe^(2+)//Fe)^(@)=-0.74V`
`E_(Br_(2)//Br)^(@)+1.08V, E_(Xr_(2)//Cl^(-))^(@)=+1.36V`
`E_(Cd^(2+)//Cd)^(@)=-0.44V`
(a) `E_(cu^(2+)//Cu=+0.34V` and `E_(Zn^(2+)//Zn)^(@)=-0.76V`
`Cu+Zn^(2+)toCu^(2+)+Zn`
In the given cell reaction, Cu is oxidised to `Cu^(2+)`, therefore `Cu^(2+)//Cu` couple acts as anode and `Zn^(2+)` is reduced to Zn, therefore, `Zn^(2+)//Zn` couple acts as cathode.
`E_("cell")^(@)=E_("cathode")^(@)-E_("anode")^(@)`
`E_("cell")^(@)=-0.76-(+0.34)=-1.10V`
Negative value of `E_("cell")^(@)` indicates that the reaction will not occur.
(b) `Mg+Fe^(2+)toMg^(2+)+Fe`
`E_(Mg^(2+)//Mg)=-2.37V` and `E_(Fe^(2+)//Fe)=-0.74V`
In the given cell reaction, Mg is oxidised to `Mg^(2+)` hence, `Mg^(2+)//Mg` couple acts as anode and `Fe^(2+)` is reduced to Fe hence, `Fe^(2+)//Fe` couple acts as cathode.
`E_("cell")^(@)=E_("cathode")^(@)-E_("anode")^(@)`
`E_("cell")^(@)=-0.74-(-2.7)=+1.63V`
Positive value of `E_("cell")^(@)5` indicates that the reaction will occur.
( C) `Br_(2)+2Cl^(-)toCl_(2)+2Br^(-)`
`E_(Br^(-)//Br_(2))^(@)=+1.08V` and `E_(Cl^(-)//Cl_(2))^(@)=+1.36V`
In the given cell reaction, `Cl^(-)` is oxidised to `Cl_(2)` hence, `Cl^(-)//Cl_(2)` couple acts as anode and `Br_(2)` is reduced to `Br^(-)` hence, `Br^(-)//Br_(2)` couple acts as cathode.
`E_("cell")^(@)=E_("cathode")^(@)-E_("anode")^(@)`
`E_("cell")^(@)=+1.08-(+1.36)-0.28V`
Negative value of `E_("cell")^(@)` indicates that the reaction will occur
(d) `Fe+Cd^(2+)toCd+Fe^(2+)`
`E_(Fe^(2+)//Fe)=-0.74V`, and `E_(Cd^(2+)//Cd)^(@)=-0.44V`
In the given cell reaction, Fe is oxidised to `Fe^(2+)` hence, `Fe^(2+)//Fe` couple acts as anode and `Cd^(2+)` is reduced to Cd hence, `Cd^(2+)//Cd` couple acts as cathode.
`E_("cell")^(@)=E_("cathode")^(@)-E_("anode")^(@)`
`E_("cell")^(@)=-0.44-(-0.74)=+0.30V`
Positive value `E_("cell")^(@)` indicates that the reaction will occur.