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ksp - Solomons Study Notes General Chemistry CHE 132 Spring...

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S olomon’s Study Notes General Chemistry CHE 132 Spring 2009 Midterm 3 Solomon Weiskop PhD [ Solubility Equilibria (Ksp) ] These Study Notes cover material of Ch. 17 that relates to the dissolving of ionic compounds.
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Study Notes & Practice Problems are available to print out by registering at www.solomonlinetutor.com Solomon Weiskop PhD Solomon’s CHE 1 32 Tutoring © Copyright 2009
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1 1. Solubility Equilibria These Study Notes are all about dissolving ionic compounds in water. An ionic compound consists of cations and anions held together by ionic bonds. Ionic compounds are solids at room temperature. The two most familiar examples of ionic compounds are salts (e.g. NaCl, BaSO 4 etc.) and metal hydroxides (NaOH etc.). They both consist of cations and anions held together by ionic bonds. The only difference is that for a metal hydroxide, the anion is hydroxide OH - . (Another name for a metal hydroxide is an Arrhenius base.) When the solid ionic compound dissolves in water, ionic bonds break and the ions separate (dissociate). As this happens, the ions become surrounded by water molecules. In fact, it is the water molecules that are actually responsible for teasing the ions apart in the first place: Water is a polar molecule i.e. it has a dipole. The negative ends of the water dipoles get close to the positively-charged cations and stabilize them. The positive ends of the water dipoles get close to the negatively-charged anions and stabilize them. The stabilizing interaction between water’s dipole and the charge of an ion is referred to as a Dipole-Ion ( DI ) Intermolecular Force. We call this process Solvation. In the case of aqueous solutions, it is also called Hydration .
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2 Many ionic compounds are highly soluble in water. They dissolve in water 100%: NaCl s 100% Na +1 aq + Cl 1 aq NH 4 NO 3 s 100% NH 4 +1 aq + NO 3 1 aq K 2 SO 4 s 100% 2K +1 aq + SO 4 2 aq Many other ionic compounds, however, are only slightly soluble in water. For such slightly-soluble ionic compounds we can determine how much will dissolve in water by writing down an Equilibrium Expression for the Solubility Equilibrium and setting up a concentration table (similar to what we did in the Study Notes on Equilibrium and as will be discussed soon with specific examples). The equilibrium constant for the solubility of an ionic compound in water is called 𝐊 ?? . Here are a few Solubility Equilibria , for slightly-soluble ionic compounds, along with their corresponding Equilibrium Expressions and numerical values for 𝐊 ?? : PbCl 2 s Pb 2+ aq + 2Cl 1 aq 𝐊 ?? = Pb 2+ eq Cl 1 eq 2 𝐊 ?? = 1.7 × 10 5 CuBr s Cu 1+ aq + Br 1 aq 𝐊 ?? = Cu 1+ eq Br 1 eq 𝐊 ?? = 5.3 × 10 9 BaSO 4 s Ba 2+ aq + SO 4 2 aq 𝐊 ?? = Ba 2+ eq SO 4 2 eq 𝐊 ?? = 1.1 × 10 10 AgBr s Ag 1+ aq + Br 1 aq 𝐊 ??
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