Equilibria 6 Common Ion Effect Feb 12, 2020 Agenda 2/12/2020 All Pick up a Chromebook Please

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Equilibria 6 Common Ion Effect Feb 12, 2020 Agenda 2/12/2020 All pick up a chromebook please. Credit Available for Big Idea 6 Notes packet - turn in Respond to Scheduling a full AP Practice Exam

AP Classroom 7.11 Introduction to solubility equilibria (8 qu) 7.12 Common Ion effect (Quiz) 7.13 pH and solubility (Quiz) 2 FRQs Note: CAST Practice Thursday Feb 20.All of us. Common Ion Effect What happens when we dissolve an ionic solid in water that already contains an ion in common with the dissolving salt? Common Ion Effect What happens when we dissolve an ionic solid in water that already contains an ion in common with the dissolving salt?

-12 Ag2CrO4 Ksp = 9.0 x 10 being dissolved in 0.100 M solution of AgNO3. Before any silver chromate dissolves the major species in the solution are Common Ion Effect What happens when we dissolve an ionic solid in water that already contains an ion in common with the dissolving salt?

-12 Ag2CrO4 Ksp = 9.0 x 10 being dissolved in 0.100 M solution of AgNO3. Before any silver chromate dissolves the major species in the solution are

- Ag+ NO3 and H2O Common Ion Effect -12 Ag2CrO4 Ksp = 9.0 x 10 0.100 M solution of AgNO3. - Ag+ NO3 and H2O

[Ag+]0 = 0.100 M 2- [CrO4 ]0 = 0 System comes to equilibrium as solid silver chromate dissolves ⇋ + 2- -12 Ag2CrO4 (s) 2 Ag (aq) + CrO4 (aq) Ksp = 9.0 x 10 Common Ion Effect ⇋ + - -12 Ag2CrO4 (s) 2 Ag (aq) + CrO4 (aq) Ksp = 9.0 x 10 S 2S S

[Ag+] = [Ag+]0 + 2S = 0.100M + 2S

[CrO4-] = [CrO4-]0 + S = 0 + S = S -12 2 2- 2 Ksp = 9.0 x 10 = [Ag+] [CrO4 ] = (0.100 +2S) S Ksp is small, equilibrium lies to the left, assume S small compared to 0.100M Common Ion Effect Ksp is small, equilibrium lies to the left, assume x small

9.0 x 10-12 = (0.100 +2S)2S S(0.1)2

S = (9.0 x 10-12) ÷ (0.1)2

S = 9.0 x 10-10

Since S is much less than 0.100M by the 5% rule, the approximation is valid.

Thus solubility of silver chromate in 0.100 M silver nitrate is 9.0 x 10-10 mol/L Common Ion Effect Equilibrium concentrations are

[Ag+] = 0.100 + 2S = 0.100 +2(9.0 x 10-10) = 0.100 M

2- -10 [CrO4 ] = S = 9.0 x 10 M Now compare the solubilities of silver chromate in pure water and in 0.100 M silver nitrate.

-4 Solubility of Ag2CrO4 in pure water = 1.3 x 10 mol/L Vs. 9.0 x 10-10 mol/L Common Ion Effect - solubility of a solid is lowered if the solution already contains ions common to the solid.

Now compare the solubilities of silver chromate in pure water and in 0.100 M silver nitrate.

-4 Solubility of Ag2CrO4 in pure water = 1.3 x 10 mol/L Vs. 9.0 x 10-10 mol/L Solubility and Common Ions

-11 Calculate the solubility of solid CaF2 (Ksp = 4.0 x 10 ) in 0.025 M NaF solution.

BEFORE any CaF2 dissolves, the solution contains Solubility and Common Ions

-11 Calculate the solubility of solid CaF2 (Ksp = 4.0 x 10 ) in 0.025 M NaF solution.

BEFORE any CaF2 dissolves, the solution contains + - Na (aq) F (aq) and H2O

The solubility equilibrium for CaF2 is Solubility and Common Ions

-11 Calculate the solubility of solid CaF2 (Ksp = 4.0 x 10 ) in 0.025 M NaF solution.

BEFORE any CaF2 dissolves, the solution contains + - Na (aq) F (aq) and H2O

The solubility equilibrium for CaF2 is ⇋ 2+ - -11 CaF2 (s) Ca (aq) + 2F (aq) Ksp = 4.0 x 10 = [Ca2+][F-]2 ⇋ 2+ - -11 2+ - 2 CaF2 (s) Ca (aq) + 2F (aq) Ksp = 4.0 x 10 = [Ca ][F ] 2+ Initial Conc. [Ca ]0 = 0 - [F ]o = 0.025 M (from the NaF) ⇋ 2+ - -11 2+ - 2 CaF2 (s) Ca (aq) + 2F (aq) Ksp = 4.0 x 10 = [Ca ][F ] 2+ Initial Conc. [Ca ]0 = 0 - [F ]o = 0.025 M (from the NaF) 2+ Equil. Conc [Ca ] = S solubility of CaF2 - [F ] = 0.025 + 2S (1mol CaF2 dissolves to give 2 moles of F-) ⇋ 2+ - -11 2+ - 2 CaF2 (s) Ca (aq) + 2F (aq) Ksp = 4.0 x 10 = [Ca ][F ] 2+ Initial Conc. [Ca ]0 = 0 - [F ]o = 0.025 M (from the NaF) 2+ Equil. Conc [Ca ] = S solubility of CaF2 - [F ] = 0.025 + 2S (1mol CaF2 dissolves to give 2 moles of F-)

-11 2+ - 2 2 Ksp = 4.0 x 10 = [Ca ][F ] = S(0.025 + 2S) Assuming 2S is negligible compared with 0.025 (since Ksp is small) gives -11 2+ - 2 2 Ksp = 4.0 x 10 = [Ca ][F ] = S(0.025 + 2S) Assuming 2S is negligible compared with 0.025 (since Ksp is small) gives

4.0 x 10-11 〰 S(0.025)2

S 〰 6.4 x 10-8 Approximation is valid by 5% rule

Solubility is 6.4 x 10-8 mol/L. 7.12 common ion effect 4 Questions in AP Classroom 7.13 pH and Solubility

Change in pH can greatly affect a salt’s solubility. ⇋ 2+ - Mg(OH)2 (s) Mg (aq) + 2 OH (aq) Addition of OH- ions (increase pH) will, by the ______effect, force the equilibrium to the ______, ______the solubility of magnesium hydroxide. pH and Solubility

Change in pH can greatly affect a salt’s solubility. ⇋ 2+ - Mg(OH)2 (s) Mg (aq) + 2 OH (aq) Addition of OH- ions (increase pH) will, by the common ion effect, force the equilibrium to the left, decreasing the solubility of magnesium hydroxide.

On the other hand, an addition of H+ ions ( a decrease in pH) ______the solubility, because ….. pH and Solubility ⇋ 2+ - Mg(OH)2 (s) Mg (aq) + 2 OH (aq) On the other hand, an addition of H+ ions ( a decrease in pH) increases the solubility, because hydroxide ions are removed from solution by reacting with the added hydrogen ions. In response to the lower concentration of hydroxide ions, the equilibrium position moves to the right.

** This is why a suspension of solid magnesium hydroxide or milk of magnesia dissolves as required in the stomach to combat excess acidity. pH and Solubility + 3- ⇋ 2- H (aq) + PO4 (aq) HPO4 (aq) The solubility of silver phosphate ( ) is greater in acid 3- than in pure water because the PO4 ion is a strong base that 2- reacts with H+ to form the HPO4 ion. This lowers the 3- concentration of PO4 and shifts the solubility equilibrium to the right. pH and Solubility + 3- ⇋ 2- H (aq) + PO4 (aq) HPO4 (aq)

The solubility of silver phosphate (Ag3PO4) is greater in acid 3- than in pure water because the PO4 ion is a strong base that 2- reacts with H+ to form the HPO4 ion. This lowers the 3- concentration of PO4 and shifts the solubility equilibrium to the right, which corresponds to an increase in solubility of the silver phosphate. ⇋ 3- Ag3PO4(s) 3 Ag+(aq) + PO4 (aq) pH and Solubility Silver Chloride (AgCl)

Has the same solubility in acid as in pure water.

Cl- ion is a very weak base (HCl is a very strong acid) so no HCl molecules are formed. Thus the addition of H+ to a solution containing Cl- ions does not affect [Cl-] and has no effect on the solubility of a chloride salt. pH and Solubility Silver Chloride (AgCl)

Has the same solubility in acid as in pure water.

Cl- ion is a very weak base (HCl is a very strong acid) so no HCl molecules are formed. Thus the addition of H+ to a solution containing Cl- ions does not affect [Cl-] ad has not effect on the solubility of a chloride salt. pH and Solubility Limestone caves

Increased solubility of carbonates in acid

Carbon dioxide dissolved in groundwater makes it acidic, dissolves calcium carbonate, resulting in caves.

Stalactites and stalagmites form - reverse process. 7.13 AP Classroom Quiz - 4 questions

Then

Free Response Questions 2 more - (on paper)