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Complex Ion Equilibria K form. Fractional Precipitation Fractional precipitation is the technique of separating two or more ions from a solution by adding.

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Presentation on theme: "Complex Ion Equilibria K form. Fractional Precipitation Fractional precipitation is the technique of separating two or more ions from a solution by adding."— Presentation transcript:

1 Complex Ion Equilibria K form

2 Fractional Precipitation Fractional precipitation is the technique of separating two or more ions from a solution by adding a reactant that precipitates first one ion, then another, and so forth. For example, when you slowly add potassium chromate, K 2 CrO 4, to a solution containing Ba 2+ and Sr 2+, barium chromate precipitates first.

3 After most of the Ba 2+ ion has precipitated, strontium chromate begins to precipitate. Fractional Precipitation Fractional precipitation is the technique of separating two or more ions from a solution by adding a reactant that precipitates first one ion, then another, and so forth. It is therefore possible to separate Ba 2+ from Sr 2+ by fractional precipitation using K 2 CrO 4.

4 Effect of pH on Solubility Sometimes it is necessary to account for other reactions aqueous ions might undergo. For example, if the anion is the conjugate base of a weak acid, it will react with H 3 O +. You should expect the solubility to be affected by pH.

5 Effect of pH on Solubility Sometimes it is necessary to account for other reactions aqueous ions might undergo. Consider the following equilibrium. H2OH2O Because the oxalate ion is conjugate to a weak acid (HC 2 O 4 - ), it will react with H 3 O +. H2OH2O

6 Effect of pH on Solubility Sometimes it is necessary to account for other reactions aqueous ions might undergo. According to Le Chateliers principle, as C 2 O 4 2- ion is removed by the reaction with H 3 O +, more calcium oxalate dissolves. Therefore, you expect calcium oxalate to be more soluble in acidic solution (low pH) than in pure water.

7 Complex-Ion Equilibria Many metal ions, especially transition metals, form coordinate covalent bonds with molecules or anions having a lone pair of electrons. This type of bond formation is essentially a Lewis acid-base reaction

8 Complex-Ion Equilibria Many metal ions, especially transition metals, form coordinate covalent bonds with molecules or anions having a lone pair of electrons. For example, the silver ion, Ag+, can react with ammonia to form the Ag(NH 3 ) 2 + ion.

9 Complex-Ion Equilibria A complex ion is an ion formed from a metal ion with a Lewis base attached to it by a coordinate covalent bond. A complex is defined as a compound containing complex ions. A ligand is a Lewis base (an electron pair donor) that bonds to a metal ion to form a complex ion.

10 Complex-Ion Formation The aqueous silver ion forms a complex ion with ammonia in steps. When you add these equations, you get the overall equation for the formation of Ag(NH 3 ) 2 +.

11 Complex-Ion Formation The formation constant, K f, is the equilibrium constant for the formation of a complex ion from the aqueous metal ion and the ligands. The formation constant for Ag(NH 3 ) 2 + is: The value of K f for Ag(NH 3 ) 2 + is 1.7 x 10 7.

12 Complex-Ion Formation The formation constant, K f, is the equilibrium constant for the formation of a complex ion from the aqueous metal ion and the ligands. The large value means that the complex ion is quite stable. When a large amount of NH 3 is added to a solution of Ag +, you expect most of the Ag + ion to react to form the complex ion.

13 Complex-Ion Formation The dissociation constant, K d, is the reciprocal, or inverse, value of K f. The equation for the dissociation of Ag(NH 3 ) 2 + is The equilibrium constant equation is

14 Equilibrium Calculations with K f What is the concentration of Ag + (aq) ion in M AgNO 3 that is also 1.00 M NH 3 ? The K f for Ag(NH 3 ) 2 + is 1.7 x In 1.0 L of solution, you initially have mol Ag + (aq) from AgNO 3. This reacts to give mol Ag(NH 3 ) 2 +, leaving (1.00- (2 x 0.010)) = 0.98 mol NH 3. You now look at the dissociation of Ag(NH 3 ) 2 +.

15 Equilibrium Calculations with K f What is the concentration of Ag + (aq) ion in M AgNO 3 that is also 1.00 M NH 3 ? The K f for Ag(NH 3 ) 2 + is 1.7 x The following table summarizes. Starting Change -x +x+2x Equilibrium x x0.98+2x

16 Equilibrium Calculations with K f What is the concentration of Ag + (aq) ion in M AgNO 3 that is also 1.00 M NH 3 ? The K f for Ag(NH 3 ) 2 + is 1.7 x The dissociation constant equation is:

17 Equilibrium Calculations with K f What is the concentration of Ag + (aq) ion in M AgNO 3 that is also 1.00 M NH 3 ? The K f for Ag(NH 3 ) 2 + is 1.7 x Substituting into this equation gives:

18 Equilibrium Calculations with K f What is the concentration of Ag + (aq) ion in M AgNO 3 that is also 1.00 M NH 3 ? The K f for Ag(NH 3 ) 2 + is 1.7 x If we assume x is small compared with and 0.98, then

19 Equilibrium Calculations with K f What is the concentration of Ag + (aq) ion in M AgNO 3 that is also 1.00 M NH 3 ? The K f for Ag(NH 3 ) 2 + is 1.7 x and The silver ion concentration is 6.1 x M.

20 Amphoteric Hydroxides An amphoteric hydroxide is a metal hydroxide that reacts with both acids and bases. For example, zinc hydroxide, Zn(OH) 2, reacts with a strong acid and the metal hydroxide dissolves.

21 Amphoteric Hydroxides An amphoteric hydroxide is a metal hydroxide that reacts with both acids and bases. With a base however, Zn(OH) 2 reacts to form the complex ion Zn(OH) 4 2-.

22 Amphoteric Hydroxides An amphoteric hydroxide is a metal hydroxide that reacts with both acids and bases. When a strong base is slowly added to a solution of ZnCl 2, a white precipitate of Zn(OH) 2 first forms.

23 Amphoteric Hydroxides An amphoteric hydroxide is a metal hydroxide that reacts with both acids and bases. But as more base is added, the white preciptate dissolves, forming the complex ion Zn(OH) Other common amphoteric hydroxides are those of aluminum, chromium(III), lead(II), tin(II), and tin(IV). Zn(OH) 2 (s) + OH - --> Zn(OH) 4 2- (aq) Al(OH) 3 (s) + OH - --> Al(OH) 4 - (aq)

24 Solubility of Complex Ions The solubility of a slightly soluble salt increase when one of its ions can be changed into a complex ion. AgBr (s) Ag + + Br - k sp = 5.0 x Ag+ 2NH 3 Ag (NH 3 ) 2 + K form = 1.6 x 10 7 AgBr + 2NH 3 Ag (NH 3 ) Br - K c = 8.0 x The NH 3 ligand remove Ag+ and shifts the equilibrium to the right, increasing the solubility of AgBr. K c = K form x k sp

25 Example How many moles of AgBr can dissolve in 1.0 L of 1.0 M NH 3 ? AgBr (s) + 2NH 3 Ag (NH 3 ) Br – 1.0 M0 0 -2X+X +X 1.0-2XX X Kc = X 2 /1.0 2 = 8.0 x x = 2.8 x x mol of AgBr dissolves in 1L of NH 3


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