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Predicting

Predicting. Precipitation. Challenge!. Consider the reaction: Na 2 SO 4(s)  2Na 2+ (aq) + SO 4 2- (aq) You have a 0.15 mol/L solution of sodium sulphate. Only 4.3% of the solution dissolves into the ions and the rest remains un-dissolved.

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Predicting

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  1. Predicting Precipitation

  2. Challenge! Consider the reaction: Na2SO4(s) 2Na2+(aq) + SO42-(aq) You have a 0.15 mol/L solution of sodium sulphate. Only 4.3% of the solution dissolves into the ions and the rest remains un-dissolved. Calculate the concentration of all ions in the solution once it has reached saturation!

  3. NaCl Na + Cl Na + Cl NaCl Back To Basics Crystallizing Dissolving

  4. Dissolving =Rforward Crystallizing = Rreverse When Rforward = Rreverse …. EQUILIBRIUM!!! (Saturated) Ksp = [Na+ ][Cl- ] [NaCl] But I wanna use the concentrations I have at any time! WARNING: EQUILIBRIUM CONCENTRATIONS ONLY WHEN CALCULATING K

  5. Q is for concentrations at any time during the reaction what K is for equilibrium concentrations. The equation for Q is exactly the same as the equation for K, only you get to use any concentration. By comparing Q to K we can find out if something is under, at, or over what is necessary for equilibrium. A solution at equilibrium is called SATURATED and the water can no longer handle any more solute. What if you add MORE solute?! It doesn’t dissolve. The water can’t handle it, so it stays solid and undissolved.

  6. NaCl Na + Cl Consider the following: NaOH + HCl NaCl + H2O We’ve chemically formed a solute! But will it dissolve?! It will if it can, but if it can't, then it won't.

  7. WILL IT PRECIPITATE? • Find the concentration of dissolved solute as soon as it’s produced • Get the Ksp value for the dissociation of the solute (Appendix C!!!!) • Calculate the Q value for the dissolved solute • Compare!!!

  8. IF: Q < Ksp The solution is not at equilibrium. The water can handle dissolving the solute and thus, the solute dissolves. (no precipitate) Q = Ksp The solution just at equilibrium. The water can just barely handle dissolving the solute and thus, the solute dissolves. (no precipitate) Q > Ksp The solution is not at equilibrium because there is too much solute. The water simply can’t handle keeping it dissolved and so…. IT PRECIPITATES!

  9. Solutes with a high Ksp value are less likely to precipitate Solutes with a low Ksp value are more likely to precipitate But everything can precipitate, you just need to get enough of it! http://www.youtube.com/watch?v=RjBjwQF276A&feature=related

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