Question

The standard reduction potential for `Zn^(2+)//Zn, Ni^(2+)//Ni` and `Fe^(2+)//Fe` are `-0.76, -0.23` and `-0.44V` respectively. The reaction `X + Y^(2) rarr X^(2+) + Y` will be spontaneous when:
A. `x=Zn,y=Ni`
B. `x=Ni,y=Fe`
C. `x=Mn,y=Zn`
D. `x=Fe,y=Zn`.

Answer 1

Correct Answer - A
The reaction `x+y^(2+)rarrx^(2+)+y` will be spontaneous if `E_("cell")^(@)` is +ve
(A) `Zn+Ni^(2+)rarrZn^(2+)+Ni`
`E_("cell")^(@)=E_(cu^(2+)//Ni)^(@)=E_(Zn^(+2)//Zn)^(@)`
`=-0.23-(-0.76)V=0.53V`
(B) `Ni+Fe^(2+)rarrNi^(2+)+Fe`
`E_("cell")^(@)=E_(Fe^(2+)..Fe)^(@)-E_(Ni^(2+)//Ni)^(@)=-0.44-(-.23)V`
=-0.21V
(C) `Ni+Zn^(2+)rarrNi^(2+)+Zn`
`E_("cell")^(@)+Zn ^(2+)rarrNi^(2+)+Zn`
`E_("cell")^(@)=0.53V` from (A)
(D) `Fe+Zn^(2+)rarrFe^(2+)+Zn`
`E_("cell")^(@)=E_(Zn^(2+)//Zn)^(@)-E_(Fe^(2+)//Fe)^(@)`
`=-0.76-(-0.44)=-0.32V`