Chapter 16 - Solubility and Precipitation Equilibria cus is...

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Unformatted text preview: 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 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 Chateliers principle more AB will dissolve (dissolution) If we allow solvent molecules to evaporate then [A+] and [B-] will increase and by Le Chateliers principle more AB will be formed (precipitation) 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 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 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 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 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: Salt Solutions and Deviations From Ideal Behavior In a fashion analogous to ideal gases, ions in solution behave ideally when they are present at comparatively low concentrations (< 0.1 mol L0....
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This note was uploaded on 01/20/2012 for the course CHEM 030.101 taught by Professor Draper during the Fall '08 term at Johns Hopkins.

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Chapter 16 - Solubility and Precipitation Equilibria cus is...

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