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Dissolution vs. Precipitation

DO NOW: What is dissolution? What is precipitation? How are they related? What does the term saturated mean?. Dissolution vs. Precipitation. These opposing processes, dissolution and precipitation will continue until …. The system reaches solubility equilibrium

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Dissolution vs. Precipitation

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  1. DO NOW:What is dissolution?What is precipitation?How are they related?What does the term saturated mean?

  2. Dissolution vs. Precipitation

  3. These opposing processes, dissolution and precipitation will continue until … • The system reaches solubility equilibrium • At this point the rate of precipitation equals the rate of dissolution • No net change in concentration results • The solution is saturated

  4. What happens when a chunk of AgCl is added to water? • Dissolution begins. Ag+ and Cl- concentration increases as more and more ions escape from the solid • For every Ag+ in solution, there is one Cl- in solution, and these amounts are dependent on the solubility of AgCl • At some point, no additional AgCl(s) can dissolve (It’s saturated!) • Although process of dissolution continues, precipitation occurs at the same rate (It’s at equilibrium!) • Saturated solution = equilibrium has been reached

  5. K = [Ag+][Cl-] =1.6 x 10-10 sp Since this reaction reaches equilibrium, apply equilibrium concepts AgCl (s) ↔ Ag+(aq) + Cl-(aq) Solubility product constant

  6. K = [Ag+][Cl-] =1.78 x 10-10 sp • What is the size of K? • What does that tell you is really present at equilibrium? • If a saturated solution of AgCl(aq) is prepared what is the concentration of Ag+ and Cl- ions?

  7. What are the concentrations of silver and chromate ions in a saturated solution of silver chromate at 25ºC? Saturated = equilibrium!!! Write the expression and get the constant (p. 610)

  8. In a saturated solution of magnesium carbonate, the concentration of Mg2+ is 0.000187 M. What is the concentration of carbonate ion and the solubility constant product, Ksp? • In a saturated solution of magnesium hydroxide, the concentration of Mg2+ ions is 0.000165 M. What is the concentration of hydroxide ion and the Ksp?

  9. A solution is prepared by • Dissolving 0.34 moles of calcium sulfate in 2.0 L of hot water. The solution is cooled to 25ºC. At this temperature, Ksp of CaSO4 is 6.1 x10-5. Will a precipitate form? What kind of problem is this? You have concentrations… you have Ksp… what are you trying to find????

  10. Find, then compare the Ksp of • Silver bromide • Silver chloride • Which solid is more soluble at 25ºC? If you are comparing species that break down into the same number of ions, the Ksp can directly be used to compare solubility. How does silver carbonate’s solubility compare?

  11. How can we get more calcium sulfate to precipitate? • CaSO4(s) <-> Ca2+(aq) + SO42-(aq) • Common ion effect • Presence of common ion shifts equilibrium to left to consume ion ( precipitation occurs) • Common ion lowers the solubility of a slightly soluble substance

  12. Specifically, how can the solubility of Mg(OH)2 be increased? • Mg(OH)2(s) <-> Mg2+(aq) + 2OH-(aq)

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