Question

Question: (a) One method to reduce the concentration of unwanted ${\mathbf{\text{Fe}}}^{\mathbf{3}\mathbf{+}}\mathbf{\left(}\mathit{a}\mathit{q}\mathbf{\right)}$in a solution of ${\mathbf{\text{Fe}}}^{\mathbf{2}\mathbf{+}}\mathbf{\left(}\mathit{a}\mathit{q}\mathbf{\right)}$is to drop a piece of metallic iron into the storage container. Write the reaction that removes the ${\mathbf{\text{Fe}}}^{\mathbf{3}\mathbf{+}}$, and compute its standard cell potential.

(b) Referring to problem 23, suggest a way to remove unwanted Mn³⁺(aq) from solutions of Mn²⁺(aq).

Short answer

Answer

a) By applying 1.179V to the cell we can remove the unwanted Fe³⁺ ion from solution

b) By applying 2.54 to the cell we can remove the unwanted Mn³⁺ ion from solution.

Step-by-step solution

Redox reaction

A redox reaction can be stated as a chemical reaction in which electrons are transferred between two reactants participating in it. This transfer of electrons can be found by observing the changes in the oxidation states of the reacting species.

The redox reaction  to

The reduction reaction

$$\begin{gathered} {\text{Fe}}^{3+}\left(aq\right)+e^{-}\to {\text{Fe}}^{2+}\left(aq\right) \\ {E}^0==0.770\text{V} \end{gathered}$$

The oxidation reaction

$$\begin{gathered} \text{Fe}\left(s\right)\to {\text{Fe}}^{2+}\left(aq\right)+2e^{-} \\ {E}^0=-0.409\text{V} \end{gathered}$$

The overall reaction

$$\begin{gathered} 2{\text{Fe}}^{3+}\left(aq\right)+\text{Fe}\left(s\right)\to 3{\text{Fe}}^{2+}\left(aq\right) \\ {E}^0=0.770\text{V}-(-0.409\text{V}) \\ {E}^0=1.179\text{V} \end{gathered}$$

The overall reaction for Mn3+(aq) to Mn2+(aq) 

The reduction reaction

$$\begin{gathered} {\text{Mn}}^{3+}\left(aq\right)+e^{-}\to {\text{Mn}}^{2+}\left(aq\right) \\ E=1.51\text{V} \end{gathered}$$

The oxidation reaction

$$\begin{gathered} \text{Mn}\left(s\right)\to {\text{Mn}}^{2+}\left(aq\right)+2e^{-} \\ {E}^0=-1.029\text{V} \end{gathered}$$

The overall reaction

$$\begin{gathered} {\text{2Mn}}^{3+}\left(aq\right)+\text{Mn}\left(s\right)\to 3{\text{Mn}}^{2+}\left(aq\right) \\ {E}^0=1.51\text{V}-(-1.029\text{V}) \\ =2.54\text{V} \end{gathered}$$