Chapter 13: Problem 92

Consider the reaction $$ \mathrm{Fe}^{3+}(a q)+\mathrm{SCN}^{-}(a q) \rightleftharpoons \mathrm{FeSCN}^{2+}(a q) $$ How will the equilibrium position shift if a. water is added, doubling the volume? b. $\mathrm{AgNO}_{3}(a q)$ is added? (AgSCN is insoluble.) c. $\mathrm{NaOH}(a q)$ is added? $\left[\mathrm{Fe}(\mathrm{OH})_{3}\right.$ is insoluble.] d. $\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3}(a q)$ is added?

Short Answer

Expert verified

a. The equilibrium shifts to the reactants side due to the dilution of all species in the reaction. b. The equilibrium shifts to the left upon adding AgNO3(aq) to compensate for the loss of SCN⁻(aq) forming insoluble AgSCN. c. The equilibrium shifts to the left as NaOH(aq) reacts with Fe³⁺(aq) forming insoluble Fe(OH)₃. d. The equilibrium shifts to the right upon adding Fe(NO₃)₃(aq) as it increases the concentration of Fe³⁺(aq).

Step by step solution

Le Chatelier's Principle states that when the equilibrium system is disturbed by a change in concentration, pressure, or temperature, the system will counteract the change. In this case, the volume of the solution is doubled, causing a dilution of all the species in the reaction. The reaction will adjust by shifting to the side with more ions, which is the reactants side, since the total number of ions is higher on that side of the equation. #b. $\mathrm{AgNO}_{3}(a q)$ is added. (AgSCN is insoluble.)

When $\mathrm{AgNO}_{3}(a q)$ is added to the solution, it reacts with $\mathrm{SCN}^{-}(a q)$ to form insoluble AgSCN(s), which removes $\mathrm{SCN}^{-}(a q)$ from the equilibrium. According to Le Chatelier's Principle, the equilibrium will shift to counteract this change; in this case, the equilibrium will shift to the left to produce more $\mathrm{SCN}^{-}(a q)$. #c. $\mathrm{NaOH}(a q)$ is added. $\left[\mathrm{Fe}(\mathrm{OH})_{3}\right.$ is insoluble.]

When $\mathrm{NaOH}(a q)$ is added to the solution, it reacts with $\mathrm{Fe}^{3+}(a q)$ to form insoluble $\mathrm{Fe(OH)}_{3}(s)$, which removes $\mathrm{Fe}^{3+}(a q)$ from the equilibrium. According to Le Chatelier's Principle, the equilibrium will shift to counteract this change; in this case, the equilibrium will shift to the left to produce more $\mathrm{Fe}^{3+}(a q)$. #d. $\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3}(a q)$ is added.

When $\mathrm{Fe(NO}_{3})_{3}(a q)$ is added to the solution, it increases the concentration of $\mathrm{Fe}^{3+}(a q)$. According to Le Chatelier's Principle, the equilibrium will shift to counteract this change; in this case, the equilibrium will shift to the right to consume the added $\mathrm{Fe}^{3+}(a q)$ and produce more $\mathrm{FeSCN}^{2+}(a q)$.