16.11 : 溶解性に影響する因子

The solubility of an ionic compound can vary depending on other solutes present in the solution. The solute may be an ion common to the compound, or an acid or base. Thus, two major factors affecting the solubility are the common ion effect and the pH of a solution.

If lead(II) chloride is added to a solution of sodium chloride, both salts will ionize in the water, producing sodium and lead cations and the common anion, chloride.

Because the chloride ions in solution come from both the complete ionization of sodium chloride and the partial ionization of lead(II) chloride, their concentration would be higher than that of either the sodium ions or the lead ions.

To compensate, the equilibrium between the solid lead(II) chloride and its ions in solution shifts towards the undissociated salt, causing more lead(II) chloride to remain undissolved.

Thus, the presence of a common ion decreases the solubility of a sparingly soluble substance.

For instance, the molar solubility, x, of lead(II) chloride in a 0.100 molar sodium chloride solution can be calculated from the ICE table.

The initial concentration of lead(II) ions in the solution is zero, while chloride is 0.100 molar.

Each molecule of lead(II) chloride will dissociate into one lead and two chloride ions. So, the change in concentration of lead ions will be +x and that of chloride will be +2x.

The equilibrium concentration of lead ions will be x, but for the chloride ions, it will be the sum of 2x and 0.100.

At 25 °C, the value of K_sp for lead(II) chloride is 1.17 × 10⁻⁵, and the equilibrium expression is the product of the equilibrium concentrations of lead and chloride ions, which is x(2x + 0.100)².

Because the K_sp is small, 2x is much smaller than 0.100 molar and their sum can be approximated to 0.100 molar.

Substituting into the expression yields 1.17 × 10⁻⁵ = x(0.100)².

Solving for x, the molar solubility of lead(II) chloride in 0.100 molar sodium chloride solution is 1.17 × 10⁻³ molar.

Compared to the molar solubility of lead(II) chloride in water, which is 1.43 × 10⁻² molar, the common ion decreases the solubility of lead(II) chloride 12 fold.

Solubility of a compound can also be affected by the pH of the solution.

Consider the partial dissociation of calcium hydroxide into calcium ions and hydroxide. If the pH is increased, by adding potassium hydroxide for example, the common hydroxide ion will lower the solubility of calcium hydroxide.

Conversely, if the pH is decreased, for example by adding hydrochloric acid, the protons will combine with hydroxide ions, lowering the concentration of hydroxide ions in solution.

The solubility equilibrium of calcium hydroxide would then shift towards the products, leading to an increased solubility in an acidic solution.