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Chapter 16 - Solubility and Precipitation Equilibria...

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Solubility and Precipitation Equilibria cus is on the dissolution and precipitation of ionic solids (salts) in wa Precipitation of sodium acetate Precipitation of K 2 PtCl 4 as solvent evaporates
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General Concepts Dissolution and precipitation are both controlled by thermodynamics and the concepts of equilibrium The nature of salt dissolution and precipitation: + - Dissolution: AB(s) A (aq) + B (aq) + - Precipitation: A (aq) + B (aq) AB(s) → ¬ + - Equilibrium: A (aq) + B (aq) AB(s) If we add more solvent then [A+] and [B-] will decrease and by Le Chatelier’s principle more AB will dissolve (dissolution) If we allow solvent molecules to evaporate then [A+] and [B-] will increase and by Le Chatelier’s principle more AB will be formed (precipitation)
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A saturated solution - a solution where equilibrium has been established between the solid and its dissolved form Unsaturated solution - all of the solid has dissolved Solvation – process of solid dissolution by the solvent Supersaturation – situation where the concentration of the dissolved solid exceeds its equilibrium value. The system is then under kinetic and not thermodynamic control General Concepts
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Solubility Solubility is defined as the greatest amount (either in g or in moles) that will dissolve in equilibrium in a specified volume of solvent (usually water) at a specific temperature (usually 25 o C). Insoluble: Solubility is less than 0.1 g L -1 The solubilities of salts vary widely and are classified into three types: Slightly soluble: Solubility is > 0.1 g L -1 but < 10.0 g L -1 Soluble: Solubility is > 10.0 g L -1
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Solubility + - Dissolution: AB(s) A (aq) + B (aq) H = +(ve) For most salts the enthalpy of dissolution is endothermic (heat is taken in from the surroundings) Question: So why do salts dissolve at all given that H is positive? Because H is positive most salts become more soluble at higher temperatures For some salts H is negative (exothermic) and these salts become less soluble at higher temperatures Temperature
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Solubility The solubility of a particular ionic solid can typically be predicted based on knowledge of the anions (and the metal cation). For example, almost all nitrates are soluble and most carbonates are insoluble
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Equilibria Between Solids and Solutions → ¬ + - CsCl(s) Cs (aq) + Cl (aq) When an ionic solid dissolves the solubility equilibrium is established. For example, in the case of CsCl the equilibrium can be written as: For a highly soluble salt the equilibria lies exclusively to the right At a molecular level it is useful to understand the process of dissolution which can be represented for CsCl by the following picture:
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Salt Solutions and Deviations From Ideal Behavior
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