Solubility Lesson 6 Changing solubility/Common Ion Effect

Some quick review on Ksp and TIP calculations

Remember to check where your source is for the ion in the precipitate eqn. 1. The molar solubility of Ba(OH) 2 is2. Equal volumes of M Ba(NO 3 ) 2 and M. Ksp = ? M NaOH are mixed. Will a precipitate occur? Multiply by 2 because of Ba(OH) 2(s) Do not multiply by 2 cuz it comes from NaOH TIP < Ksp No ppt.Ksp=4.0 x Ksp= 4[0.0010] 3 TIP=1.25 x Ksp=4s 3 Ksp=[s][2s] 2 Ksp=[Ba 2+ ][OH - ] 2 TIP=[Ba 2+ ][OH - ] 2 TIP=[ ][ ] M M M 1 2 Ba(OH) 2(s) ⇌ Ba OH - s s 2s

3. AgNO 3(s) is added to 1.0 L of a solution containing Cl - in order to precipitate out the Cl - as AgCl (s). After the AgCl (s) has formed, what is the concentration of Cl - ? AgCl (s) ⇌ Ag + +Cl - s s s Ksp=s x =s 2 s=[Cl - ]=1.3 x M

Can you change solubility? Up to now, we have only considered pure water as the solvent for our solutions. –as long as your solution uses water as a solvent, there is no change in the solubility of the salt. However, using a solvent other than water, definitely has an effect on the solubility of the solute (solid).

Factors that Increase or Decrease Solubility The solubility of a substance depends on the presence of other ions (caused by a solvent other than water) AND the temperature. However, only temperature can change Ksp value.

energy + PbCl 2(s) ⇌ Pb 2+ +2Cl - The molar solubility is the amount of PbCl 2(s) in moles per litre that dissolves into ions to form a saturated solution. increases the molar solubility decreases the molar solubility Only changing the temperature changes the value of Ksp. Adding some “substances” will however shift the equilibrium just like we learnt in Unit 2 until the system re-gains balance again. Solid equilibriums are usually endothermic.

For your info. If you go to the right (where ions are), you are increasing solubility. If you go to the left (where the solid is), you are decreasing solubility.

DECREASING the Solubility of a Salt AgNO 3 Substance Added Molar SolubilityKsp Effect energy + AgCl (s) ⇄ Ag + + Cl - NaCl Decrease none What we just added were soluble salts that dissolve into their respective ions. Notice that these soluble salts had one common ion with the equilibrium (Ag, Cl). This is called the COMMON ION EFFECT.

INCREASING the Solubility of a Salt Substance Added Molar SolubilityKsp Effect energy + AgCl (s) ⇄ Ag + + Cl - Na 2 S Pb(NO 3 ) 2 Increase none 2Ag + + S 2- → Ag 2 S (s) Pb Cl - → PbCl 2(s) Forming a precipitate will always drive the equilibrium to make more ions, therefore increasing solubility.

Changing the Solubility of a Salt Substance Added Molar SolubilityKsp Effect energy + AgCl (s) ⇄ Ag + + Cl - H2OH2O AgCl(s) Increase Temperature Decrease Temperature none Increase Decrease Increase Decrease none moles/L does not change solid

Changing the Solubility of a Salt Energy + Ca(OH) 2(s) ⇄ Ca OH - Ca(NO 3 ) 2 Decrease IncreaseNa 2 SO 4 NaOH NaCl Ca(OH) 2(s) Increase Temperature Decrease Temperature Decrease none Increase Decrease Increase Decrease none Substance Added Effect on Molar SolubilityEffect on Ksp

1.Mg(OH) 2 will have the greatest solubility in: Mg(OH) 2(s) ⇌ Mg 2+ +2OH - A.NaOH B.Mg(NO 3 ) 2 C.H 2 O OH - lowers solubility Mg 2+ lowers solubility No effect on solubility Ag + increases solubility by forming a ppt with OH - driving the equilibrium to make more ions. D.AgNO 3

Summary Increase solubility –Add solvent that would precipitate your existing ions. Decrease solubility –Add solvent with a common ion with your existing ions.

Homework Hebden: Pg 108 #81-86 We did not cover sections 3.8 and 3.9- Not responsible for them.