Q19.65P - Chemistry: Molecular Nature Of Matter And Change

Question

In a gaseous equilibrium, the reverse reaction occurs when ${\mathbf{\text{Q}}}_{\mathbf{\text{c}}}{\mathbf{\text{ > K}}}_{\mathbf{\text{c}}}$. What occurs in aqueous solution when ${\mathbf{\text{Q}}}_{\mathbf{\text{sp}}}{\mathbf{\text{ > K}}}_{\mathbf{\text{sp}}}$?

Step-by-Step Solution

Answer

When the Qsp value is greater than Ksp value, that means that we have a higher concentration of ${\text{Pb}}^2+$  ions in the solution, than we would have in the equilibrium state. This means that the direction of the dissolution equation will be shifted from right to left and more of PbF 2 (s) will be formed.

Step 1: Concept Introduction

The $\mathbf{\text{Qsp}}$ -ion-product expression is obtained by multiplying the concentrations of ions generated by the dissolution of a chemical. When a solution is saturated, the $\mathbf{\text{ Ksp }}$ value is referred to as the value (solubility-product constant).

$$\begin{gathered} {\mathbf{\text{MX}}}_{\mathbf{\text{2}}}{\mathbf{\text{ > > M}}}^{\mathbf{\text{2 + }}}{\mathbf{\text{ + 2X}}}^{\mathbf{\text{ - }}} \\ \mathbf{\text{Ksp = }}\mathbf{\left[}{\mathbf{\text{M}}}^{\mathbf{\text{2 + }}}\mathbf{\right]}{\mathbf{\left[}{\mathbf{\text{X}}}^{\mathbf{\text{ - }}}\mathbf{\right]}}^{\mathbf{\text{2}}} \end{gathered}$$

Step 2: Explanation

To tackle this problem, we can formulate the dissolution equation for ${\text{ PbF}}_{\text{2}}$ as follows: 

$$\begin{gathered} {\text{PbF}}_{\text{2}}{\text{(s) > > Pb}}^{\text{2 + }}{\text{(aq) + 2F}}^{\text{ - }}\text{(aq)} \\ \text{Qsp = }\left[{\text{Pb}}^{\text{2 + }}\right]{\left[{\text{F}}^{\text{ - }}\right]}^{\text{2}} \end{gathered}$$ 

 

When the $\text{Qsp}$ value is greater than the $\text{Ksp}$ value, the concentration of ${\text{Pb}}^{\text{2 + }}{\text{ and F}}^{\text{ - }}$ ions in the solution is higher than it would be in the equilibrium state.

This indicates that the dissolution equation will be moved from right to left, resulting in more ${\mathrm{PbF}}_2( \mathrm{s})$ being created according to the Le Chatelier principle, and we will be able to extract this precipitate from the solution.