Introduction to Solubility Equilibria
Chemistry: Cambridge International A Level, topic 1.7.7
Introduction to Solubility Equilibria
Solubility equilibria are established when the dissolution and precipitation of a solute species occur at equal rates.
Solubility product
- The solubility of a substance can vary from essentially zero (insoluble or sparingly soluble) to infinity (miscible).
- A solute with finite solubility can yield a saturated solution when it is added to a solvent in an amount exceeding its solubility.
- This results in a heterogeneous mixture of the saturated solution and the excess, undissolved solute.
Example
- For example, a saturated solution of silver chloride is one in which the equilibrium shown above has been established.
- In this solution, an excess of solid AgCl dissolves and dissociates to produce aqueous Ag+ and Cl– ions at the same rate that these aqueous ions combine and precipitate to form solid AgCl.
- Because silver chloride is a sparingly soluble salt, the equilibrium concentration of its dissolved ions in the solution is relatively low.
𝐾sp
- The equilibrium constant for solubility equilibria such as this one is called the solubility product constant, 𝐾sp, in this case:
- AgCl(s)⇌Ag+(aq)+Cl−(aq)
- 𝐾sp=[Ag+(aq)][Cl−(aq)]
- Recall that only gasses and solutes are represented in equilibrium constant expressions, so the 𝐾sp does not include a term for the undissolved AgCl.
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Interpreting 𝐾sp
- A 𝐾sp value of greater than one means a salt is soluble.
- Calculating 𝐾sp is covered in the next section.
Test yourself on Introduction to Solubility Equilibria
What is the correct 𝐾sp expression for the below reaction?
NaCl(s)⇌ Na+(aq)+Cl−(aq)What is the correct 𝐾sp expression for the below reaction?
CaF2(s) ⇌ Ca2+(aq)+2F−(aq)