Consider the dissolution of$\mathbf{\text{PbS}}$in water:

$\mathbf{PbS}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{+}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\left(}\mathbf{l}\mathbf{\right)}\mathbf{\rightleftharpoons }{\mathbf{Pb}}^{\mathbf{2}\mathbf{+}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{+}{\mathbf{HS}}^{\mathbf{-}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{+}{\mathbf{OH}}^{\mathbf{-}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}$

Adding aqueous$\mathbf{\text{NaOH}}$ causes more$\mathbf{\text{PbS}}$to dissolve. Does this violate Le Chatelier’s principle? Explain.

According to Le Chatelier's principle, if a system in equilibrium is disturbed, it moves in a direction to nullify the effect of the disturbance and re-establish a new equilibrium.

First, the dissolution equation for $\mathrm{PbS}$is given below.

$\mathrm{PbS}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\iff {\mathrm{Pb}}^{2+}\left(\mathrm{aq}\right)+{\mathrm{HS}}^{-}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$

According to Le Chatelier's principle, when the concentration of reactants is increased or the concentration of products is decreased, the reaction goes toward the products (to the right). It is similar the opposite way, if the concentration of reactants is decreased or the concentration of products is increased, the reaction goes toward the reactants (to the left).

If we add aqueous $\mathrm{NaOH}$ to this solution, we would expect the reaction to shift to the left, because of the increased concentration of ${\text{OH}}^{\text{-}}$ions, but this doesn't happen and $\mathrm{PbS}$becomes easier to dissolve. This happens because dissolved ion forms a complex with localid="1663622053740" ${\text{OH}}^{\text{-}}$ions –

${\text{Pb}}^{\text{2 +}}{\text{(aq) + 2OH}}^{\text{-}}\text{(aq)}\iff {\text{Pb(OH)}}_{\text{2}}$

This complex can precipitate and leave the solution, allowing more role="math" localid="1663622147207" $\mathrm{PbS}$ to dissolve, so this does not violates the Le Chatelier's principle.

Therefore, violation of Le Chatelier's principle does not take place.