lecture 15sf

lecture 15sf - pH and Solubility Fe(OH)3(s) Fe3+(aq) + 3OH...

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pH and Solubility Fe(OH) 3 (s) Fe 3+ (aq) + 3OH - (aq) K sp = 4 x 10 -38 What can be done to make this compound more soluble in water (shift the equilibrium to the right)? Adding more solid won’t work. Solid have constant composition, and are not part of the K expression. Removing an ion from the right side will shift the reaction to the right, making the solid more soluble. Hydroxide, OH - is a base. Adding H + ions will attract OH - to form water. Fe(OH) 3 (s) Fe 3+ (aq) + 3OH - (aq) 3H + (aq) + 3OH - (aq) 3H 2 O Adding: Fe(OH) 3 + 3H + 3H 2 O + Fe 3+ Or, alternatively: Fe(OH) 3 + 3H 3 O + → 6H 2 O + Fe 3+ Most hydroxides are insoluble in water, but their solubility greatly increases in acid solution. The H + ions attract the OH - away from the metal ion, and the hydroxide dissolves. CaCO 3 (s) Ca 2+ (aq) + CO 3 2- (aq) K sp = 8.7 x 10 -9 What can be done to make this compound more soluble in water (shift the equilibrium to the right)? Like OH - , carbonate, CO 3 2- is also a base. It is the conjugate base of the weak acid, HCO 3 - . Adding H + ions will attract CO 3 2- to form HCO 3 - . Adding 2H + ions will attract CO 3 2- to form H 2 CO 3 which breaks down to H 2 O + CO 2 CaCO 3 (s) Ca 2+ (aq) + CO 3 2- (aq) 2H + (aq) + CO 3 2- (aq) CO 2 + H 2 O Adding: CaCO 3 (s) + 2H + Ca 2+ + CO 2 + H 2 O Alternatively: CaCO 3 + 2H 3 O + → Ca 2+ + CO 2 + 3H 2 O Most carbonates are insoluble in water, but their solubility greatly increases in acid solution. The H + ions attract the CO 3 2- away from the metal ion, and carbon dioxide is released.
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Why are limestone statues eaten away by acid rain? Acid rain contains H + ions. The pH is < 7. Limestone is a form of calcium carbonate, which is much more soluble in acid than water. CaCO 3 (s) + 2H + Ca 2+ + CO 2 + H 2 O Alternatively: CaCO 3 + 2H 3 O + → Ca 2+ + CO 2 + 3H 2 O AgCl(s) Ag + (aq) + Cl - (aq) K sp = 1.8 x 10 -10 Will adding acid to AgCl increase its solubility (push equilibrium to right)? Why or why not? Cl - is the conjugate “base” of HCl, a strong acid. Since HCl is 100% ionized, Cl - has 0% ability to attract H + ions. It is a “nominal base” (base in name only). Its ability to act as a base in water is negligible. Since H + ions do not attract Cl
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lecture 15sf - pH and Solubility Fe(OH)3(s) Fe3+(aq) + 3OH...

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