Chapter 21.2, Problem 8SP

Interpretation Introduction

Interpretation: Whether the given redox reaction will occur spontaneously or not needs to be identified.

Concept Introduction: Whether a metal can reduce other metals from its solution or not can be determined from its reduction electrode potential. The metal with a high reduction electrode potential cannot reduce the metal with a low reduction electrode potential.

Explanation of Solution

The given redox reaction is as follows:

  $3{\text{Zn}}^{2+}\left(aq\right)+2\text{Cr}\left(s\right)\to 3\text{Zn}\left(s\right)+2{\text{Cr}}^{3+}\left(aq\right)$

The two half-reactions are as follows:

  $\begin{array}{l}{\text{Zn}}^{2+}\left(aq\right)+2e\to \text{Zn}\left(s\right)\\ \text{Cr}\left(s\right)\to {\text{Cr}}^{3+}\left(aq\right)+3e\end{array}$

Here, the reduction of Zn and oxidation of Cr take place. Whether the given reaction is spontaneous or not can be determined by checking the reduction electrode potentials of the two reactions.

The metal with more reduction electrode potential value can easily be reduced and the metal with less reduction electrode potential value can easily be oxidized.

The reduction electrode potential of the two half-reactions are as follows:

  $\begin{array}{l}{\text{Zn}}^{2+}\left(aq\right)+2e\to \text{Zn}\left(s\right)-\text{0}\text{.76 V}\\ \text{Cr}\left(s\right)\to {\text{Cr}}^{3+}\left(aq\right)+3e-\text{0}\text{.73 V}\end{array}$

Here, Cr has more reduction electrode potential thus, it will undergo reduction, and oxidation of Zn takes place.

Thus, the given redox reaction will not occur spontaneously.