Solubility Equilibrium Essay

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AP* Chemistry Solubility Equilibrium SOLUBILITY EQUILIBRIA (The Solubiliy-Product Constant, Ksp) We’ve got good news and we’ve got bad news… The good news: Solubility equilibrium is really simple. The bad news: You know all those solubility rules that state a substance is insoluble? They are actually a bit soluble after all. Only the future attorneys among you read the fine print. Soluble is often defined as greater than 3 grams dissolving in 100 mL of water. So, there is a lot of wiggle room for solubility up to 3 grams! This type of equilibria deals with that wiggle room. If you can actually see that a salt is insoluble, then the solution is actually saturated. Saturated solutions of salts present yet another type of chemical equilibria. Slightly soluble salts establish a dynamic equilibrium with the hydrated cations and anions in solution. • When the solid is first added to water, no ions are initially present. • As dissolution proceeds, the concentration of ions increases until equilibrium is established. This occurs when the solution is saturated. • The equilibrium constant, the Ksp, is no more than the product of the ions in solution. (Remember, solids do not appear in equilibrium expressions.) • For a saturated solution of AgCl, the equation would be: AgCl(s) Ag+(aq) + Cl−(aq) • • The solubility product expression would be: Ksp = [Ag+][Cl−] The AgCl(s) does not appear in the equilibrium expression since solids are left out. Why? The concentration of the solid remains relatively constant. A table of Ksp values follows on the next page. *AP is a registered trademark of the College Board, which was not involved in the production of, and does not endorse, this product.© 2008 by René McCormick. All rights reserved Write the Ksp expression for each of the following reactions and find its value in the table above. CaF2(s) Ag2SO4(s) Bi2S3(s) Ca+2 + 2 F−

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