CHE 141 1st Edition Lecture 19 Outline- Solubility Equilibria - The Solubility-Product Constant, Ksp- Ksp and Molar Solubility- The Effect of a Common Ion on Solubility - Effect of pH on Solubility- Precipitation- Selective PrecipitationSolubility Equilibria - Soluble a substance that dissolves in water- Insoluble a substance that does not dissolve in water- Solubility: the maximum amount of a substance that dissolves in a given quantity of solvent at a given temperature, expressed in grams of solute per liter of solution, gL-1- Molar solubility: the molar amount of a substance that dissolves in a given quantity of solvent at a given temperature, expressed in moles of solute per liter of solution, molL-1The Solubility-Product Constant, Ksp- We can apply the concept of equilibrium to the process of dissolution of an ionic solid in water: CaF2(s) reversible Ca2+(aq)+2F-(aq)These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- As CaF2(s) dissolves, it breaks up into Ca2+ cations and F- anions- Saturation: the point at which a solution of a substance cant dissolve any more so additional amounts form a solid precipitate- In a saturated solution, the last bit of undissolved CaF2(s) will be in equilibrium with its ions- We can describe an equilibrium constant expression: CaF2(s) reversible Ca2+(aq) +2F-(aq), Ksp=[Ca2+][F-]^2- Remember our rules for heterogeneous equilibrium CaF2 is a solid so we don’t include it- The subscript sp stands for solubility product- The solubility-product constant Ksp is an equilibrium constant for the equilibrium constant between an ionic solid and its ions in a saturated aqueous solutionKsp and Molar Solubility- Ksp like other equilibrium constants has only one value at a given temperature and is known from experimental data collected on common ionic compounds - We can use ICE table to calculate molar solubility from Ksp- We can make the substitution that x=the concentration of ionic solid that dissolves(the molar solubility)- Units of molar solubility are mol L-1=MThe Effect of a Common Ion on Solubility - We’ve already seen that the presence of a common ion suppresses the ionization of an acid (or base) according to Le Chateliers Principle- In a similar way, the presence of a common ion in a solubility equilibrium would also shift the equilibrium to the left- Adding F-(aq) to CaF2(s) reversible Ca2+(aq)+2F-(aq) would cause the equilibrium to shift to the left- In general, the solubility of an ionic compound is lower in a solution containing a common ion than in pure waterEffect of pH on Solubility- The pH of a solution can affect solubility - Consider the weak base Mg(OH)2: Mg(OH)2(s) reversible Mg2+(aq)+2OH-(aq)- If pH is high, concentration of OH- is higho Equilibrium shifts to the lefto Solubility of Mg(OH)2 is decreased- If pH is low, concentration of H3O+ is higho H3O+ reacts with OH-o Equilibrium shifts to the righto Solubility of Mg(OH)2 is increased- In general the solubility of an ionic compound with a strongly basic or weakly basic anion increases with increasing acidity (decreasing pH)- Common basic anions are OH-, S2-, and CO32-, thus hydroxides, sulfides, and carbonates are more soluble in acidic water- Between 1751-1994 ocean pH decreased from 8.25 to 8.14 due to CO2 dissolving in the ocean and forming carbonic acid- This lowering of pH impacts on oceanic calcifying organisms as it increases the solubilityof CaCO3Precipitation- If Q=Ksp solution is at equilibrium (saturated solution)o Saturated solution the dissolved solute (ions) is in dynamic equilibrium with the undissolved solute (solid)- If Q<Ksp the solution is unsaturated, so if additional solid is added it will dissolveo Unsaturated solution contains less than the equilibrium amount of solute- If Q>Ksp the solution is supersaturated, so solid precipitates outo Supersaturated solution contains more than the equilibrium amount of soluteSelective Precipitation- Selective precipitation involves adding a reagent that forms a precipitate with one of the dissolved cations but not the other- A common ex of this is using a soluble hydroxide such as KOH or NaOH to separate Mg2+ from Ca2+ in seawater as Mg(OH)2 has a lower Ksp than Ca(OH)2- For selective precipitation to work, Ksp values must differ by at least a factor of
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