Solubility equilibrium. Factors affecting solubility

Transcription

1 Chapter 17 Continued equilibrium K sp Factors affecting solubility 1 SOLUBILITY is the quantity of a substance that dissolves to form a saturated t solution : g/l Molar solubility: mole/l Some salts are very soluble (> 0.1 M). Recall solubility rules (Chapter 9). Some salts are sparingly soluble (< 0.1 M) sometimes referred ee edto oas insoluble. soube We will now consider precipitation and solubility of ionic salts and their equilibria in water. MA(s) M + (aq) + A (aq) 2

2 SOLUBILITY EQUILIBRIA: A saturated solution has a solid in equilibrium with the dissolved ions. Precipitation Dissolution When forward rate = backward rate: equilibrium AgCl(s) Ag + (aq) + Cl (aq) The concentration of solid does not change at equilibrium 3 Product K sp is the name of the equilibrium constant for the dissolution of slightly soluble salts. Product: K sp g y AgCl AgBr AgI CdS ZnS Mg(OH) Ca(OH) CaF BaCO BaSO

3 The quantity of solute that dissolves (the solubility) can be determined quantitatively using K sp Calculate [Ca 2+ ] and [F ] for a saturated CaF 2 solution. K sp = at 25 o C What is the solubility of CaF 2? solubility = amount of CaF 2 dissociatedi d 5 Factors that Affect of slightly soluble salts. The Common-Ion Effect ph Formation of complex ions Amphoterism 6

4 Adding a strong electrolyte that contains a common ion will reduce the solubility of a salt. The Common-Ion Effect BaSO 4 (s) Ba 2+ (aq) + SO 2 4 (aq) NaCl(s) Na + (aq) + Cl (aq) 7 Calculate the solubility of a salt in a solution that contains a common ion. What is the solubility of CaF 2 in 0.01 M NaF solution? K = 39x10 11 sp 3.9 Initial Δ Final CaF 2 (s) Ca 2+ (aq) + 2F (aq) 8

5 Factors Affecting : ph. This is an example of the common ion effect. ph If a substance has a basic anion: Substances with acidic cations: 9 The effect of ph can be calculated quantitatively. What is the solubility of Mg(OH) 2 (in pure water)? K sp (Mg(OH) 2 = ) 10

6 The effect of ph can be calculated quantitatively. What is the solubility of Mg(OH) 2 in a solution with a ph of 9? 11 Qualitatively: try to predict the effect of ph. 1. Milk of Magnesia Mg(OH) 2 MgCl 2 + 2NaOH Mg(OH) 2 +2NaCl 2. Barium salts BaCl 2 + 2NaOH Ba(OH) 2 +2NaCl 12

7 Qualitatively: try Effect to predict of ph the effect of ph. 2. Barium salts (continued) BaCl 2 + 2Na 2 CO 3 BaCO 3 (s) + 2NaCl Net Ionic Equation Adding Acid: 13 Qualitatively: try Effect to predict of phthe effect of ph. 2. Barium salts (continued) BaCl 2 + 2Na 2 SO 4 BaSO 4 (s) + 2NaCl Net Ionic Equation Adding Acid: 14

8 Summary: Effect of ph on common ions If either the anion or the cation of a salt is involved in an acid base equilibrium, then addition of acid or base will affect the solubility: it is a common ion problem. USE LeCHATELIER S PRINCIPLE Basic Metal hydroxides Example Mg(OH) 2 Salts of weakly basic anion Example: NaF 15 Factors Affecting : formation of complex ions will increase the solubility of a salt. Complex Ions Metal ions can act as Lewis acids and form complex ions with Lewis bases in the solvent. The formation of these complex ions increases the solubility of these salts. Complex Ion K f Equilibrium Equation [Cu(NH 3 ) 4 ] Cu 2+ (aq) + 4 NH 3 (aq) [Cu(NH 3 ) 4 ] 2+ (aq) [Ag(NH ) 2 ] Ag (aq) NH 3 (aq) [Ag(NH 3 ) 2 ] (aq) [Ag(CN) 2 ] Ag + (aq) + 2 CN (aq) [Ag(CN) 2 ] (aq) [CdBr 4 ] Cd 2+ (aq) + 4 Br (aq) [CdBr 4 ] 2 (aq) [Cr(OH) 4 ] Cr 3+ (aq) + 4 OH (aq) [Cr(OH) 4 ] (aq) [Ni(NH 3 ) 6 ] Ni 2+ (aq) + 6 NH 3 (aq) [Ni(NH 3 ) 6 ] 2+ (aq) [Cu(CN) 4 ] Cu 2+ (aq) + 4 CN (aq) [Cu(CN) 4 ] 2 (aq) [Fe(CN) 6 ] Fe 2+ (aq) + 6 CN (aq) [Fe(CN) 6 ] 4 (aq) 16

9 A complex ion is formed when a metal ion bonds to Lewis bases. Hydration of metal ions is a Lewis Acid-Base reaction Cu 2+ (aq) + 4 H 2 O(l) [Cu(OH 2 ) 4 ] 2+ (aq) Lewis + Lewis Lewis Acid/Base Adduct = Metal Complex Acid Base Cu 2+ (aq) = [Cu(H 2 O) 4 ] 2+ (aq) Other Lewis bases react with metal ions to form complexes Cu 2+ (aq) + 4 NH 3 (aq) [Cu(NH 3 ) 4 ] 2+ (aq) Cu 2+ (aq) + 4 CN (aq) [Cu(CN) 4 ] 2 (aq) Cu 2+ (aq) + 4 Cl (aq) [Cu(Cl) 4 ] 2 (aq) Ni 2+ (aq) + 6 NH 3 (aq) [Ni(NH 3 ) 6 ] 2+ (aq) 17 METAL COMPLEX IONS are exceptionally stable. [Cu(OH 2 ) 4 ] NH 3 [Cu(NH 3 ) 4 ] H 2 O Cu 2+ (aq) What is K eq? + 4NH 3 [Cu(NH 3 ) 4 ] H 2 O K f VALUES OF SOME COMPLEXES [Ni(NH 3 ) 6 ] x 10 9 [Cu(NH 3 ) 4 ] 2+ 5 x [Cu(CN) 4 ] 2-1x [Ag(NH 3 ) 2 ] x 10 7 [Ag(CN) 2 ] - 1 x [Ag(S 2 O 3 ) 2 ] x

10 Sample problem involving Complex Ion Formation What is the [Cu 2+ ] in 1 L solution that has 1 x10 3 moles of Cu 2+ (aq) ions (free ions + complex ions) and that is 0.1 M in NH 2+ = 5x at equilibrium? K f [Cu(NH 3 ) 4 ] (aq) Cu 2+ (aq) + 4 NH 3 (aq) [Cu(NH 3 ) 4 ] 2+ (aq) 19 How is Complex Ion Formation related to solubility? CuCO =23x 10 3 is a sparingly soluble salt, K sp CuCO How can I get it to dissolve? Demo: CuSO +CO (aq) 32 (aq) CuCO 3 (s) + SO 42 (aq) insoluble What is the equilibrium constant for the following reaction? CuCO 3 (s) + 4NH 3 (aq) CO 3 2 (aq) + [Cu(NH 3 ) 4 ] 2+ (aq) K sp CuCO 3 = 2.3 x K f [Cu(NH 3 ) 4 ] 2+ = 5 x

11 Factors Affecting : Amphoteric metal oxides and hydroxides are soluble in strong acid or base, because they can act either as acids or bases. Amphoterism: ability of a molecule to act as both an acid and base. Examples of such cations are Al 3+, Zn 2+, and Sn Recall AMPHOTERISM: is the ability of a molecule l to act both as an acid and as a base. This results in metal hydroxides which are soluble in both acids and bases. Hydration: Al 3+ (aq) Al(H 2 O) 3+ 6 Al(OH) 3 (s) + 3H 2 O Al(OH) 3 (H 2 O) 3 (s) In acid: Al(OH) +H 3 (H 2 O) 3 + Al(OH) 2 (H 2 O) + 4 (aq) increases. In base: Al(OH) 3 (H 2 O) 3 + OH Al(OH) 4 (H 2 O) 2 (aq) + H 2 O solubility increases. Chem

12 AMPHOTERIC METAL HYDROXIDES There are amphoteric hydroxides of Al 3+ Cr 3+ Zn 2+ Sn 2+ Many transition metal ions These involve formation of complex ions, such as: dissolves in Al(OH) (OH 2 ) 3 (s) H (aq) Al(OH) 4+ 2 (OH 2 ) (aq) acid + OH (aq) Al(OH) 4 (OH 2 ) 2 (aq) base Examples of amphoteric hydroxides: Low and high ph increases solubility Al(OH) 3 Cr(OH) 3 Zn(OH) 3 Sn(OH) 2 23