Solubility and Complex Ion Equilibria

Transcription

1 Solubility and Complex Ion Equilibria The Solubility Product Constant (K sp ) is the equilibrium constant for the solubility equilibrium of a slightly soluble (or nearly insoluble) ionic compound. CaC 2 O 4(s) H2O K sp = [Ca +2 ] [C 2 O 4 ] Ca +2 + C 2 O 4 Like other equilibrium constants, K sp depends on the temperature.

2 Examples: A liter of a solution saturated at 25ºC with calcium oxalate, CaC 2 O 4, is evaporated to dryness, giving a g residue of CaC 2 O 4. Calculate the solubility product constant for the salt at 25ºC. By experiment it is found that 1.2 x 10-3 mol of lead(ii) iodide, PbI 2, dissolves in 1 L of aqueous solution at 25ºC. What is the solubility product constant at this temperature? The mineral fluorite is calcium fluorite, CaF 2. Calculate the solubility (in grams per liter) of calcium fluorite in water from the solubility product constant (3.4 x ).

3 In the previous examples the hydrolysis of an anion is not important. If the anion is fairly basic, you need to take in to consideration the possibility of hydrolysis. The importance of the K sp becomes apparent when you consider the solubility of one salt in the solution with another salt having the same cation or anion. Example: What is the solubility of calcium oxalate in 0.15 M calcium chloride? Compare this molar solubility with that found in the previous example for CaC 2 O 4 in pure water. CaC 2 O 4(s) H2O Ca +2 + C 2 O 4 Precipitation Calculations Precipitation is another way of looking at a solubility equilibrium. Rather than ask how much of a substance will dissolve in solution, you ask: Will precipitation occur for given starting ion concentrations? Will the reaction go in the forward or reverse direction? To answer this you need to evaluate the reaction quotient, Q c and compare it to the K c. If Q c < K sp, the reaction should go in the forward direction If Q c >K sp, the reaction should go in the reverse direction If Q c = K sp, the reaction mixture is at equilibrium Here the Q c is considered the Ion product because it is the product of the ion concentration in a solution, each concentration raised to a power equal to the number of ions in the formula of the ionic compound.

4 Example: The concentration of calcium ion in blood plasma is M. If the concentration of oxalate ion is 1.0 x 10-7 M, do you expect calcium oxalate to precipitate? A solution of M lead (II nitrate, Pb(NO 3 ) 2 was poured into 456 ml of M sodium sulfate, Na 2 SO 4. Would a precipitate of lead(ii) sulfate be expected to form if 255 ml of the lead nitrate solution were added? Fractional Precipitation is the technique of separating two or more ions from a solution by adding a reactant that precipitates first ion, then another ion and so forth. Suppose a solution is 0.10 M Ba +2 and 0.10 M Sr +2. When you slowly add a concentrated solution of K 2 CrO 4 to the solution the BaCrO 4 precipitates out first. To understand why you need to: 1. Calculate the concentration of CrO 4 to begin to precipitate out the Ba Calculate the concentration of CrO 4 to begin to precipitate out the Sr 2+

5 Effect of ph on Solubility CaC 2 O 4(s) H2O Ca +2 + C 2 O 4 In general: C 2 O 4 + H + HC 2 O H 2 O (l) Salts of weak acids should be expected to be more soluble in acidic solutions Consider two slightly soluble salts, calcium carbonate and calcium sulfate. Which of these would have its solubility affected by the addition of HCl? Would the solubility of the salt increase or decrease? Many metal sulfides are insoluble in water, but dissolve in acidic solutions. Complex Ion Formation, or stability constant, K f, of a complex ion is the equilibrium constant for the formation of the complex ion from the aqueous metal ion and the ligands. Ag + + NH 3 Ag(NH) NH 3 Ag(NH) 3 + Ag(NH 3 ) 2 + [Ag(NH 3 ) + 2 ] K f = [Ag + ] [NH 3 ] 2 The dissociation constant, K d, for a complex ion is the reciprocal, or inverse value of K f K d = = [Ag + ] [NH 3 ] 2 [Ag(NH 3 ) 2 + ] What is the concentration of Ag + ion in M AgNO 3, that is also 1.00 M NH 3?

6 Amphoteric hydroxides are metal hydroxides that react with both bases and acids. Example: Zn is a metal that forms insoluble hydroxides Zinc hydroxide reacts with an acid: Zn(OH) 2(s) + 2H 3 O + But also reacts with a base: Zn(OH) 2(s) + OH - Zn H 2 O (l) Zn(OH) 4 2- Complex ions and Solubility Predicting whether a precipitate will form in the presence of a complex ion Will silver chloride precipitate from a solution that is 0.010M AgNO 3 and 0.010M NaCl? Will silver chloride precipitate from this solution if it is also 1.00 M NH 3?

7 Qualitative Analysis is the determination of the identity of the substances present in a mixture.