Ефект спільного йону (Лє Шательє)

To discuss the Common Ion Effect, lets first define what a common ion is.

Common Ion

A common ion is an ion that enters a solution from two different sources, or in other words, is common to both salts present in a salt solution.

For example, say you have a solution of zinc hydroxide, and you added zinc ions to the solution. The zinc ions would be considered common ions since they are sourced both from the solution and the salt that is added.

Another example where the common ion is found in both salts present in a solution would be if you had a solution of $NaCl$ (or $Na^{+}$ and $CL^{-}$ ions) and you added $AgCl$ . Since the $Cl^{-}$ ions are common to both salts in solution, they are common ions.

What is the Common Ion Effect?

The common ion effect is the shift in the solubility equilibrium that results from the addition of a common ion in solution. The solubility of an ionic compound will decrease with the addition of a common ion to its solution.

The common ion effect is a result of Le Chatelier's principle-- if an equilibrium were to become unbalanced, the reaction will shift to restore the balance. Therefore, if a common ion is added to either a weak acid or weak base equilibrium, the equilibrium will shift towards the left (reactants) to relieve the stress of excess product. This is often seen when analyzing the solubility of weak electrolytes, such as salts.

To better understand how to predict the direction of the reaction, explore this link!

When a common ion is added to a salt solution, the salt will precipitate, which will decrease the concentration of the individual ions. The concentration of individual ions will continue to decrease until the solubility equilibrium is once again reached.

For example, calcium ions are added to a saturated solution of calcium sulfate. The excess $Ca^{2+}$ ions causes additional $CaSO_{4}$ to precipitate from solution. To restore the balance, the equilibrium is shifted back to the left and solubility is lowered as a result.

Let's look at the previous example in more detail.

Example

In the saturated solution of calcium sulfate, there exists an equilibrium between the solid calcium sulfate, $CaSO_{4}$ , and its ions, $Ca^{2+}$ and $SO_{4}^{2-}$ . In this solution, most of the calcium sulfate exists as molecules, but a small amount will dissociate into ions.

$CaSO_{4(s)} \Leftrightarrow Ca^{2+}_{(aq)} + SO^{2-}_{4(aq)}$

Equilibrium expression: $K_{sp}= \frac{[Ca^{2+}][SO_{4}^{2-}]}{[CaSO_{4}]}= 2.4x10^{-5}$

Remember: $K_{sp}$ is the solubility product constant.

Now, say that you add calcium chloride, $CaCl_{2}$ , to the saturated solution. The addition of the common ion causes the concentration of calcium ions in the solution to increase.

If we look at our equilibrium expression, we can see that this increase will also cause the whole numerator ( $[Ca^{2+}]$ times $[SO_{4}^{2-}]$ to also increase, thus being made momentarily greater than the $K_{sp}$ value. Here, the equilibrium is shifted to the left. Therefore, to rebalance itself, more $CaSO_{4}$ will precipitate out of the solution until the ion product is once again equal to the $K_{sp}$ . The additional calcium sulfate now in solution causes the solubility to decrease.

It is important to note that when equilibrium is regained, the concentrations of the calcium and sulfate ions are no longer equal to one another. There is now a much greater concentration of calcium ions in solution. Since the concentration of calcium sulfate has also been increased, the product of the concentrations will still be equal to the $K_{sp}$ .

Overall As a result of the common ion effect, any sparingly soluble substance will be even less soluble in a solution that contains any ion that it shares (common ion).

Ефект спільного йону (Лє Шательє)

(Video) 17.5 The Common Ion Effect and Precipitation

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