Question

Consider the reaction

${\mathbf{Fe}}^{\mathbf{3}\mathbf{+}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{SCN}}^{\mathbf{-}}\left(\mathrm{aq}\right)\stackrel{}{\mathbf{\rightleftharpoons }}{\mathbf{FeSCN}}^{\mathbf{2}\mathbf{+}}\left(\mathrm{aq}\right)$

How will the equilibrium position shift if

a. water is added, doubling the volume?

b. ${\mathbf{AgNO}}_3\left(\mathrm{aq}\right)$ is added? (AgSCN is insoluble.)

c. NaOH(aq) is added? [$\mathbf{Fe}{\left(\mathrm{OH}\right)}_3$ is insoluble.]

d. $\mathbf{Fe}{\left({\mathbf{NO}}_3\right)}_3\left(\mathrm{aq}\right)$ is added?

Short answer

a. It will shift the reaction to the left when the volume is doubled

b. It will shift the reaction to the left.

c. It will shift the reaction to the left.

d. It will shift the reaction to the right.

Step-by-step solution

Subpart a)

When the volume is increased by a factor of 2 by adding the water, the concentration of each species will decrease by a factor of $\frac{1}{2}$. Hence, the reaction will move toward the reactants (left).

Subpart b)

When soluble silver nitrate is added to the reaction, the reaction will shift to the left side to reach the equilibrium again. Because ${\mathrm{Ag}}^{+}$ in the silver nitrate solution combines with ${\mathrm{SCN}}^{-}$ to form the precipitate as $\mathrm{AgSCN}$, therefore the concentration of ${\mathrm{SCN}}^{-}$ will decrease. So, the reaction will start to produce more ${\mathrm{SCN}}^{-}$ to reestablish the equilibrium. Hence, it will shift the reaction to the left.

Reactions:

$\begin{array}{l}\mathrm{AgN}{\mathrm{O}}_3\stackrel{}{\to }{\mathrm{Ag}}^{+}+{\mathrm{NO}}_3\\ {\mathrm{Ag}}^{+}+\mathrm{SC}{\mathrm{N}}^{-}\stackrel{}{\rightleftharpoons }\mathrm{AgSCN}\downarrow \end{array}$

Subpart c)

The addition of $\mathrm{NaOH}$ will remove ${\mathrm{Fe}}^{3+}$ ions because the added hydroxide ions react with ${\mathrm{Fe}}^{3+}$ ions to form a precipitate, $\mathrm{Fe}{\left(\mathrm{OH}\right)}_3$. Therefore, it will shift the reaction to the left.

Subpart d)

When $\mathrm{Fe}{\left({\mathrm{NO}}_3\right)}_3$is added to the reaction, the concentration of${\mathrm{Fe}}^{3+}$is increased. So, the reaction will produce more${\mathrm{Fe}}^{3+}$ ions to reach the equilibrium again. Hence, the reaction will shift to the right.