Predicting
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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. - PowerPoint PPT Presentation
Transcript of Predicting
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!
NaCl Na + Cl
Na + Cl NaCl
Dissolving =Rforward
Crystallizing = Rreverse
When Rforward = Rreverse ….
Ksp = [Na+ ][Cl- ]
[NaCl]
WARNING: EQUILIBRIUM CONCENTRATIONS ONLY WHEN CALCULATING K
But I wanna use the
concentrations I have at any
time!
Q is for concentrations at any time during the reaction what K is for equilibrium concentrations.
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.
The equation for Q is exactly the same as the equation for K, only you get to use any concentration.
Consider the following:
NaOH + HCl NaCl + H2O
We’ve chemically formed a solute! But will it dissolve?!
NaCl Na + Cl
1) Find the concentration of dissolved solute as soon as it’s produced
2) Get the Ksp value for the dissociation of the solute
(Appendix C!!!!)
3) Calculate the Q value for the dissolved solute
4) Compare!!!
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….
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
