Chapter ER, Problem 13PQ

Interpretation Introduction

Interpretation:

The reaction, which occurs when iron and zinc metals are to the solutions containing ${\text{Fe}}^{\text{2+}}{\text{,Zn}}^{\text{2+}}$ ions has to be predicted.

Concept Introduction:

Standard reduction potential:

The standard reduction potential is in a category known as the standard cell potentials or standard electrode potentials.  The standard cell potential is the potential difference between the cathode and anode.  The standard potentials are all measured at $\text{298 K}$ , 1$\text{atm}$  and with 1 $\text{M}$ solution.

The relation between the reduction and oxidation potential is,

  ${\text{E}}^{\text{o}}\text{reduction}\text{=}{\text{-E}}^{\text{o}}\text{oxidation}$

Answer to Problem 13PQ

Zinc metal will reduce ${\text{Fe}}^{\text{2+}}$ ions.  Hence, option (D) is correct.

Explanation of Solution

Reason for the correct answer:

Considering the choice (D) the overall reaction is,

  $\begin{array}{l}{\text{Zn}}_{\left(\text{s}\right)}\stackrel{}{\to }{\text{Zn}}^{\text{2+}}+2e^{-}{}_{\text{(aq)}}\text{+0}\text{.76V}\\ {\text{Fe}}^{\text{2+}}{}_{\left(\text{aq}\right)}\text{+2e}\stackrel{}{\to }{\text{Fe}}_{\left(\text{s}\right)}\text{-0}\text{.44V}\\ \text{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}\\ {\text{Fe}}^{\text{2+}}{}_{\left(\text{aq}\right)}{\text{+Zn}}_{\left(\text{s}\right)}\stackrel{}{\to }{\text{Fe}}_{\left(\text{s}\right)}{\text{+Zn}}^{\text{2+}}{}_{\text{(aq)}}\text{+0}\text{.32V}\end{array}$

The overall standard electrode potential is $\text{+0}\text{.32V}$ this positive cell potential value is associated with the spontaneous reaction.  Thus, option D is correct.

Reason for incorrect answer:

The overall reaction for option (A) is ,

  $\begin{array}{l}{\text{Fe}}_{\left(\text{s}\right)}\stackrel{}{\to }{\text{Fe}}^{\text{2+}}{}_{\left(\text{aq}\right)}{\text{+2e}}^{-}+\text{0}\text{.44V}\\ {\text{Zn}}^{\text{2+}}{}_{\left(aq\right)}+2e^{-}\stackrel{}{\to }{\text{Zn}}_{\left(s\right)}-\text{0}\text{.76V}\\ \text{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}\\ {\text{Fe}}_{\left(S\right)}{\text{+Zn}}^{\text{2+}}{}_{\left(aq\right)}\stackrel{}{\to }{\text{Fe}}^{\text{2+}}{}_{\left(\text{aq}\right)}+{\text{Zn}}_{\left(s\right)}-\text{0}\text{.32V}\end{array}$

The overall standard electrode potential is $\text{-0}\text{.32V}$ this negative cell potential a value confirms thus, option (A) is incorrect.

The overall potential is zero in case of option (C) thus it is not a correct choice.

The overall potential is also negative in the case of option (B).  Thus it is also not a correct choice.