RI 2023 Solubility Equilibria

Raffles Institution Year 6 H2 Chemistry 2023 Lecture Notes 20 - Solubility Equilibria A Contents (a) Solubility product (b) Common ion effect (c) Complex ion formation C Lecture Outline 1 . Solubility 2. Solubility product 3. Ionic product and precipitation 4. Common ion effect 5. Formation of complex ions 6. Effect of pH 7. Limitations to solubility product concept 1 I solubility 1.1 Defining solubility B Learning Outcomes At the end of the lectures , you should be able to: (a) show understanding of, and apply, the concept of solubility product, Ksp (b) calculate l<sp from concentrations and vice versa (c) discuss the effects on the solubility of ionic salts by the following: • common ion effect • formation of complex ions, as exemplified by the reactions of halide ions with aqueous silver ions followed by aqueous ammonia D References 1. Chemistry in Context by Hill & Holman 2. A level Chemistry by Ramsden 3. Chemistry : The Molecular Nature of Matter and Change by Martin S. Silberberg • The maximum amount of solute which can dissolve in a given amount of solvent at a particular temperature is called the solubility of the solute in that solvent. • A solution containing the maximum amount of solute that can be dissolved in the given amount of solvent is called a saturated solution. • The solubility of a salt at a stated temperature can be expressed in various units, e.g. Solubility may be expressed as: Units number of moles of solute dissolved in 1 dm3 of solution mol dm-3 mass of solute dissolved in 1 dm3 of solution g dm-3 mass of solute dissolved in 100 g of solvent g per 100 g solvent mass of solute dissolved in 106 g of solution ppm (parts per million) WorkeJ{ e~-~mpJicJi If the solubility of Ag2CrO4 is 6.05 x 10-5 mol dm-3, what is the concentration of each of its constituent ions in a saturated solution? Ag2CrQ4(s) .= 2Ag+(aq) + Cro/-(aq) In a saturated solution of Ag2CrQ4, [Ag+)= (2)(6.05 x 10-5) = 1.21 x 10-4 mol dm-3 [Cro/-J = 6.05 x 10-5 mol dm-3 -1-

1.2 Dissolution as an Equilibrium Process Soluble salts Soluble salts dissociate fully in solution into their constituent ions. Example: NaC/(s)~ Na•(aq) + C,(aq) Sparingly soluble salts • When a small amount of a sparingly soluble ionic solid, MX, dissolves in water at a given temperature, an aqueous solution containing M• and x- ions is formed. • As more solid MX is added to the solution, the concentrations of the ions increase. • Eventually, the solution becomes saturated, i.e. it contains the maximum amount of dissolved solute at that particular temperature in the presence of undissolved solute. • At this point, The ions in the saturated solution are in dvnamic equilibrium with the excess undissolved solid: ~-----------------------------, , In a saturated solution, : : dissolution and precipitation : : occur at the same rate. , ~------------------------------· The rate of the forward reaction equals rate of the backw