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1 PRECIPITATION REACTIONS Solubility of Salts Section 18.4
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2 Analysis of Silver Group All salts formed in this experiment are said to be INSOLUBLE and form when mixing moderately concentrated solutions of the metal ion with chloride ions.
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3 Analysis of Silver Group Although all salts formed in this experiment are said to be insoluble, they do dissolve to some SLIGHT extent. AgCl(s) Ag + (aq) + Cl - (aq) When equilibrium has been established, no more AgCl dissolves and the solution is SATURATED.
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4 Analysis of Silver Group AgCl(s) Ag + (aq) + Cl - (aq) When solution is SATURATED, expt. shows that [Ag + ] = 1.67 x 10 -5 M. This is equivalent to the SOLUBILITY of AgCl. What is [Cl - ]? [Cl - ] is equivalent to the AgCl solubility.
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5 Analysis of Silver Group AgCl(s) Ag + (aq) + Cl - (aq) Saturated solution has [Ag + ] = [Cl - ] = 1.67 x 10 -5 M Use this to calculate K c K c = [Ag + ] [Cl - ] = (1.67 x 10 -5 )(1.67 x 10 -5 ) = 2.79 x 10 -10
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6 Analysis of Silver Group AgCl(s) Ag + (aq) + Cl - (aq) K c = [Ag + ] [Cl - ] = 2.79 x 10 -10 Because this is the product of “solubilities”, we call it K sp = solubility product constant See Table 18.2 and Appendix J
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8 Lead(II) Chloride PbCl 2 (s) Pb 2+ (aq) + 2 Cl - (aq) K sp = 1.9 x 10 -5
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9 Solution 1. 1. Solubility = [Pb 2+ ] = 1.30 x 10 -3 M [I - ] = ? [I - ] = 2 x [Pb 2+ ] = 2.60 x 10 -3 M Solubility of Lead(II) Iodide Consider PbI 2 dissolving in water PbI 2 (s) Pb 2+ (aq) + 2 I - (aq) Calculate K sp if solubility = 0.00130 M
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10 Solution 2. K sp = [Pb 2+ ] [I - ] 2 = [Pb 2+ ] [2Pb 2+ ] 2 K sp = 4 [Pb 2+ ] 3 Solubility of Lead(II) Iodide = 4 (solubility) 3 K sp = 4 (1.30 x 10 -3 ) 3 = 8.8 x 10 -9 PbI 2 (s) Pb 2+ (aq) + 2 I - (aq)
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11 Precipitating an Insoluble Salt Hg 2 Cl 2 (s) Hg 2 2+ (aq) + 2 Cl - (aq) K sp = 1.1 x 10 -18 = [Hg 2 2+ ] [Cl - ] 2 If [Hg 2 2+ ] = 0.010 M, what [Cl - ] is req’d to just begin the precipitation of Hg 2 Cl 2 ? That is, what is the maximum [Cl - ] that can be in solution with 0.010 M Hg 2 2+ without forming Hg 2 Cl 2 ?
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12 Precipitating an Insoluble Salt Hg 2 Cl 2 (s) Hg 2 2+ (aq) + 2 Cl - (aq) K sp = 1.1 x 10 -18 = [Hg 2 2+ ] [Cl - ] 2 Recognize that K sp = maximum ion concs. Precip. begins when product of. EXCEEDS the K sp.
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13 Precipitating an Insoluble Salt Hg 2 Cl 2 (s) Hg 2 2+ (aq) + 2 Cl - (aq) K sp = 1.1 x 10 -18 = [Hg 2 2+ ] [Cl - ] 2 Solution [Cl - ] that can exist when [Hg 2 2+ ] = 0.010 M, If this conc. of Cl - is just exceeded, Hg 2 Cl 2 begins to precipitate.
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14 Precipitating an Insoluble Salt Hg 2 Cl 2 (s) Hg 2 2+ (aq) + 2 Cl - (aq) K sp = 1.1 x 10 -18 Now raise [Cl - ] to 1.0 M. What is the value of [Hg 2 2+ ] [Hg 2 2+ ] = K sp / [Cl - ] 2 = K sp / (1.0) 2 = 1.1 x 10 -18 M The concentration of Hg 2 2+ has been reduced by 10 16 !
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15 The Common Ion Effect Adding an ion “common” to an equilibrium causes the equilibrium to shift back to reactant.
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16 Common Ion Effect PbCl 2 (s) Pb 2+ (aq) + 2 Cl - (aq) K sp = 1.9 x 10 -5
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17 Calculate the solubility of BaSO 4 in (a) pure water and (b) in 0.010 M Ba(NO 3 ) 2. K sp for BaSO 4 = 1.1 x 10 -10 BaSO 4 (s) Ba 2+ (aq) + SO 4 2- (aq) Solution Solubility in pure water = [Ba 2+ ] = [SO 4 2- ] = x K sp = [Ba 2+ ] [SO 4 2- ] = x 2 x = (K sp ) 1/2 = 1.1 x 10 -5 M Solubility in pure water = 1.1 x 10 -5 mol/L The Common Ion Effect
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18 Solution Solubility in pure water = 1.1 x 10 -5 mol/L. Now dissolve BaSO 4 in water already containing 0.010 M Ba 2+. Which way will the “common ion” shift the equilibrium? ___ Will solubility of BaSO 4 be less than or greater than in pure water?___ The Common Ion Effect BaSO 4 (s) Ba 2+ (aq) + SO 4 2- (aq) LEFT LESS
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19 Solution [Ba 2+ ][SO 4 2- ] initial change equilib. The Common Ion Effect + y 0.0100 + y 0.010 + y y Calculate the solubility of BaSO 4 in (a) pure water and (b) in 0.010 M Ba(NO 3 ) 2. K sp for BaSO 4 = 1.1 x 10 -10 BaSO 4 (s) Ba 2+ (aq) + SO 4 2- (aq)
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20 Solution K sp = [Ba 2+ ] [SO 4 2- ] = (0.010 + y) (y) Because y < 1.1 x 10 -5 M (= x, the solubility in pure water), this means 0.010 + y is about equal to 0.010. Therefore, K sp = 1.1 x 10 -10 = (0.010)(y) y = 1.1 x 10 -8 M = solubility The Common Ion Effect BaSO 4 (s) Ba 2+ (aq) + SO 4 2- (aq)
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21 SUMMARY Solubility in pure water = SO 4 2- (aq) = x = 1.1 x 10 -5 M Solubility in presence of added Ba 2+ SO 4 2- (aq) = 1.1 x 10 -8 M Le Chatelier’s Principle is followed! The Common Ion Effect BaSO 4 (s) Ba 2+ (aq) + SO 4 2- (aq)
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23 + Separating Metal Ions Cu 2+, Ag +, Pb 2+ K sp Values AgCl1.8 x 10 -10 PbCl 2 1.7 x 10 -5 PbCrO 4 1.8 x 10 -14 K sp Values AgCl1.8 x 10 -10 PbCl 2 1.7 x 10 -5 PbCrO 4 1.8 x 10 -14
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24 Separating Salts by Differences in K sp A solution contains 0.020 M Ag + and Pb 2+. Add CrO 4 2-. Which precipitates first? K sp for Ag 2 CrO 4 = 9.0 x 10 -12 K sp for PbCrO 4 = 1.8 x 10 -14Solution The substance whose K sp is first exceeded precipitates first. The ion requiring the lesser amount of CrO 4 2- ppts. first.
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25 Separating Salts by Differences in K sp [CrO 4 2- ] to ppt. PbCrO 4 = K sp / [Pb 2+ ] = 1.8 x 10 -14 / 0.020 = 9.0 x 10 -13 M K sp for Ag 2 CrO 4 = 9.0 x 10 -12 K sp for PbCrO 4 = 1.8 x 10 -1 Calculate [CrO 4 2- ] required by each ion. [CrO 4 2- ] to ppt. Ag 2 CrO 4 = K sp / [Ag + ] 2 = 9.0 x 10 -12 / (0.020) 2 = 2.3 x 10 -8 M PbCrO 4 precipitates first
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26 K sp (Ag 2 CrO 4 )= 9.0 x 10 -12 K sp (PbCrO 4 ) = 1.8 x 10 -14 How much Pb 2+ remains in solution when Ag + begins to precipitate? Separating Salts by Differences in K sp
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27 Separating Salts by Differences in K sp We know that [CrO 4 2- ] = 2.3 x 10 -8 M to begin to ppt. Ag 2 CrO 4. What is the Pb 2+ conc. at this point? [Pb 2+ ] = K sp / [CrO 4 2- ] = 1.8 x 10 -14 / 2.3 x 10 -8 M = 7.8 x 10 -7 M Lead ion has dropped from 0.020 M to < 10 -6 M
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28 Separating Salts by Differences in K sp Add CrO 4 2- to solid PbCl 2. The less soluble salt, PbCrO 4, precipitates PbCl 2 (s) + CrO 4 2- PbCrO 4 + 2 Cl - SaltK sp PbCl 2 1.7 x 10 -5 PbCrO 4 1.8 x 10 -14
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29 Separating Salts by Differences in K sp PbCl 2 (s) + CrO 4 2- PbCrO 4 + 2 Cl - SaltK sp PbCl 2 1.7 x 10 -5 PbCrO 4 1.8 x 10 -14 PbCl 2 (s) Pb 2+ + 2 Cl - K 1 = K sp Pb 2+ + CrO 4 2- PbCrO 4 K 2 = 1/K sp K net = K 1 K 2 = 9.4 x 10 8 Net reaction is product-favored
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31 Formation of complex ions explains why you can dissolve a ppt. by forming a complex ion. Dissolving Precipitates by forming Complex Ions AgCl(s) + 2 NH 3 Ag(NH 3 ) 2 + + Cl -
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32 Examine the solubility of AgCl in ammonia. AgCl(s) Ag + + Cl - K sp = 1.8 x 10 -10 Ag + + 2 NH 3 Ag(NH 3 ) 2 + K form = 1.6 x 10 7 ------------------------------------- AgCl(s) + 2 NH 3 Ag(NH 3 ) 2 + + Cl - K net = K sp K form = 2.9 x 10 -3 By adding excess NH 3, the equilibrium shifts to the right. Dissolving Precipitates by forming Complex Ions
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