Chapter 19

PRECIPITATION REACTIONS

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Solubility of Ionic Solids

• Depends on the balance of two forces: – Attraction between H 2 O molecules and ions of solid.

– Force of attraction between oppositely charged ions within solid.

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Solubility Rules

• Use in predicting results of precipitation reactions. MEMORIZE THE SOLUBILITY RULES!!!!!

– Determine ions present and possible products.

– Use solubility rules to determine if any are insoluble.

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Example 1

• Ba(NO 3 ) 2 (aq) + Na 2 CO 3 (aq) WOLPA/AP CHEMISTRY/CDO

Solubility Rules

• • • • 1. Salts containing

Group I elements are soluble

(Li + , Na + , K + , Cs + , Rb + ). Exceptions to this rule are rare. Salts containing the ammonium ion (NH 4 + ) are also soluble. 2. Salts containing

nitrate ion (NO 3 ) are generally soluble

. 3. Salts containing

Cl , Br , I are generally soluble

. Important exceptions to this rule are halide salts of Ag + , Pb 2+ , and (Hg 2 ) 2+ . Thus, AgCl, PbBr 2 , and Hg 2 Cl 2 are all insoluble. WOLPA/AP CHEMISTRY/CDO

Solubility Rules Continued

• • 4. Most

silver salts are insoluble

. AgNO 3 and Ag(C 2 H 3 O 2 ) are common soluble salts of silver; virtually anything else is insoluble. 5. Most

sulfate salts are soluble

. Important exceptions to this rule include BaSO 4 , PbSO 4 , Ag 2 SO 4 and SrSO 4 . • 6. Most

hydroxide salts are only slightly soluble

. Hydroxide salts of Group I elements are soluble. Hydroxide salts of Group II elements (Ca, Sr, and Ba) are slightly soluble. Hydroxide salts of transition metals and Al 3+ are insoluble. Thus, Fe(OH) 3 , Al(OH) 3 , Co(OH) 2 are not soluble. WOLPA/AP CHEMISTRY/CDO

Solubility Rules Continued

• • • • 7. Most

sulfides of transition metals are highly insoluble

. Thus, CdS, FeS, ZnS, Ag 2 S are all insoluble. Arsenic, antimony, bismuth, and lead sulfides are also insoluble. 8.

Carbonates are frequently insoluble

FeCO 3 and PbCO 3 . . Group II carbonates (Ca, Sr, and Ba) are insoluble. Some other insoluble carbonates include 9.

Chromates are frequently insoluble

. Examples: PbCrO 4 , BaCrO 4 10.

Phosphates are frequently insoluble

. Examples: Ca 3 (PO 4 ) 2 , Ag 2 PO 4 • 11.

Fluorides are frequently insoluble

. Examples: BaF 2 , MgF 2 PbF 2 .

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Example 1 Continued

• Ba(NO 3 ) 2 (aq) + Na 2 CO 3 (aq) WOLPA/AP CHEMISTRY/CDO

Stoichiometry

• Mole Relations – Coefficients in the net ionic equation can be used in the usual way to relate the moles of reactants and products.

– Moles of ions can be deduced from solute concentrations.

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Example 2

• What is the molar concentration of Ba 2+ and F in a solution containing 0.0075 M BaF 2 WOLPA/AP CHEMISTRY/CDO

Precipitation Titrations

• Used to determine the concentration of species in solution or in a solid mixture.

• Indicator shows, usually by color change, when the species being analyzed for has been consumed WOLPA/AP CHEMISTRY/CDO

General Principles

• Involves formation of a precipitate • Must determine the volume of a standardized titrant needed to just precipitate all of the ion.

• Need an indicator or electrode to determine when the

precipitation

is complete WOLPA/AP CHEMISTRY/CDO

Solubility Equilibria

• Solubility Product Constant, Ksp – Precipitation reactions like all reactions, reach a position of equilibrium.

– Expression for Ksp M a X b <----------> aM Ksp = [M +b ] a [X -a ] b +b + bX -a WOLPA/AP CHEMISTRY/CDO

Solubility Product Principle

• In any water solution in equilibrium with a slightly soluble ionic compound, the product of the concentrations of its ions, each raised to a power equal to its coefficient in the solubility equation is a constant. This constant, Ksp, has a fixed value at a given temperature, independent of the concentrations of the individual ions.

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• AgCl

Two Ion Compound

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• PbCl 2

Three Ion Compound

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• Al(OH) 3

Four Ion Compound

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Calculation of Ksp

• Calculated from measured solubility – AgCl Ksp = (s) (s) = s 2 – PbCl 2 • – Al(OH) 3 Ksp = (s) (2s) 2 = 4s 3 Ksp = (s) (3s) 3 = 27s 4 WOLPA/AP CHEMISTRY/CDO

Example 3

• At 20 o C, a saturated aqueous solution of silver acetate, AgC 2 H 3 O 2 , contains 1.0 g dissolved in 100.0 mL of solution. Calculate the Ksp for AgC 2 H 3 O 2.

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Determination of Solubility

• In pure water – Ksp = s 2 s = (Ksp) 1/2 – Ksp = 4s 3 s = (Ksp/4) 1/3 – Ksp = 27s 4 s = (Ksp/27) 1/4 WOLPA/AP CHEMISTRY/CDO

Example 4

• Estimate the solubility of lead (II) bromide in (a) moles per liter and (b) grams per liter of pure water. Ksp = 6.3 x 10 -6 WOLPA/AP CHEMISTRY/CDO

Uses of Ksp

• Calculation of concentration of one ion, knowing that of the other WOLPA/AP CHEMISTRY/CDO

Example 5

• You have a solution that has a lead (II) concentration of 0.0012 M. What is the maximum concentration of chloride ions that would be present? Ksp = 1.7 x 10 -5 WOLPA/AP CHEMISTRY/CDO

Uses of Ksp

• Determination of whether a precipitate will form • Compare original concentration product, P, to Ksp – if P < Ksp, no precipitate will form – if P > Ksp, precipitate forms until P becomes equal to Ksp WOLPA/AP CHEMISTRY/CDO

Example 6

• You have 100.0 m of a solution that has a lead (II) concentration of 0.0012 M. Does PbCl 2 precipitate when 1.20 g of solid NaCl is added?

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Determination of Solubility

• In solution containing a common ion – Solubility is much less than in pure water WOLPA/AP CHEMISTRY/CDO

Example 7

• Calculate the solubility of silver carbonate, Ag 2 CO 3 , in moles per liter, in pure water. Compare this with the molar solublity of Ag 2 CO 3 in 225 mL of water to which 0.15 g of Na 2 CO 3 has been added.

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Simultaneous Equilibria

• Two or more reactions occur at the same time is a solution, all of them being described as equilibrium processes.

• The equilibrium for the overall reaction is the product of the equilibrium constants for the summed reactions. That is K net = K 1 x K 2 WOLPA/AP CHEMISTRY/CDO

Solubility and pH

• Any salt containing an anion that is the conjugate base of a weak acid dissolves in water to a greater extent than that given by Ksp because the ions undergo a hydrolysis reaction.

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Example 8

PbS • PbS(s) <----------> Pb 2+ (aq) + S 2 (aq) Ksp = 8.4 x 10 -28 • S 2 (aq) + H 2 O(l) <----------> HS (aq) + OH (aq) Kb = 1 x 10 5 • Overall • PbS(s) + H 2 O(l) <---> Pb 2+ (aq) + HS (aq) + OH (aq) Knet = 8.4 x 10 -23 WOLPA/AP CHEMISTRY/CDO

• In general, the solubility of a salt containing the conjugate base of a weak acid is increased by addition of a stronger acid to the solution. In contrast, the salts are not soluble in strong acid if the anion is the conjugate base of a strong acid.

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• CaCO 3 <--------> Ca 2+ (aq)+ CO 3 2 (aq) K = Ksp = 3.8 x 10 -9 • CO 3 2 (aq) + H 2 O(l) <--------> HCO 3 (aq) + OH (aq) K = Kb = 2.1 x 10 -4 • OH (aq) + H 3 O + (aq) <--------> 2H 2 O(l) K = 1/Kw = 1 x 10 14 WOLPA/AP CHEMISTRY/CDO

• NET • CaCO 3 (s) + H 3 O + (aq) <------> Ca 2+ (aq) + HCO 3 (aq) + H 2 O(l) • Knet = (Ksp) (Kb)/(Kw) = 79.8

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Solubility and Complex Ions

• Examples of complex ions: AgCl 2 , Ag(S 2 O 3 ) 2 3 , Ag(CN) 2 • The solubility of certain “insoluble “ compounds can be increased when a complex ion is formed. Complex ions usually refer to cations in which surrounding water molecules have been replaced by some other electron pair donor. The equilibrium constant will equal the solubility constant times the formation constant for the complex ions. (Note - Chapter 23 in your textbook covers complex ions - their formation and nomenclature) WOLPA/AP CHEMISTRY/CDO

Solubility and Complex ions

• AgCl(s) + 2NH 3 • K = (Ksp) (Kf)  Ag(NH 3 ) 2 + + Cl • = (1.8 x 10 -10 ) (1.6 x 10 7 ) = 0.00288 = 0.0029 WOLPA/AP CHEMISTRY/CDO

Example 9

• Solid gold (I) chloride AuCl, is dissolved when excess cyanide ions, CN , are added to give a water soluble complex ion.

• AuCl(s) + 2CN (aq)  Au(CN) (aq) + Cl • Show that this equation is the sum of two other equations and calculate the Knet.

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Solubility, Ion Separations, and Qualitative Analysis

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