142_Ch17_Overview_part2

Other Aspects of Aqueous Equilibria

Outline

  • The Common Ion Effect

  • Buffer Solutions

  • Acid-Base Titrations

  • Solubility of Salts

  • Precipitation Reactions

  • Equilibria involving complex ions

  • Solubility of complex ions

Solubility of Salts

  • Reaction: CaCl2 (aq) + Na2CO3 (aq) ⟶ CaCO3 (s) + 2 NaCl (aq)

  • Previous approaches used solubility rules to classify compounds as soluble or insoluble.

  • Actual solubility is a continuum; this section revisits solubility from an equilibrium perspective.

Saturated Solutions

  • Definition: Contains the maximum solute that can dissolve, along with undissolved solute, both in equilibrium.

  • Elements: Dissolved and undissolved solute coexist.

Solubility Equilibrium

  • For CaF2 (s):

    • Reaction: CaF2 (s) ⇄ Ca2+ (aq) + 2 F- (aq)

    • Equilibrium expression: Ksp = [Ca2+][F-]^2

    • Ksp: Indicates the extent to which the reaction favors products.

Solubility Product Constant, Ksp

  • General expression for substance AxBy:

    • AxBy (s) ⇄ x Ay+ (aq) + y Bx- (aq)

    • Ksp = [Ay+]^x[Bx-]^y

  • Examples: NaCl (s), K3PO4 (s), AlCl3 (s)

Some Ksp Values

  • Table of Common Insoluble Compounds and their Ksp Values:

    • CaCO3: Ksp = 3.4 x 10^-9

    • MnCO3: Ksp = 2.3 x 10^-11

    • FeCO: Ksp = 3.1 x 10^-11

    • CaF2: Ksp = 5.3 x 10^-11

    • AgCl: Ksp = 1.8 x 10^-10

    • BaSO4: Ksp = 1.1 x 10^-10

Relating Solubility and Ksp Values

  • Solubility measures the amount of compound dissolved.

  • Molar solubility: Number of moles dissolved in a liter.

  • Molar solubilities can be derived from Ksp values.

Sample Problems

  1. NaCl solubility at 25 °C: 360 g/L, find Ksp.

  2. Pbl2 saturated solution: [Pb2+] = 1.30 x 10^-3 M, find Ksp.

  3. Calculate molar solubility for MgF2 where Ksp = 5.2 x 10^-11.

  4. Compare solubility of calcium carbonate vs magnesium fluoride based on Ksp values.

  5. Rank solubility of AgBr, AgCl, AgI based on Ksp values.

Solubility and Common Ion Effect

  • Common Ion Effect: The solubility of an ionic compound reduces in solutions with a common ion.

  • Example: High pH reduces solubility of Fe(OH)2 due to increased OH- concentration.

Precipitation Reactions

  • Use Ksp to predict precipitation and calculate ion concentrations pre/post-precipitation.

  • Qsp calculation for assessing precipitation likelihood:

    • Qsp < Ksp: Solution unsaturated.

    • Qsp = Ksp: Solution saturated.

    • Qsp > Ksp: Supersaturated, precipitation occurs.

Complex Ions

  • Hydrated transition metals: E.g., Ag+(aq) is [Ag(H2O)2]+(aq).

  • Ligands: Neutral molecules or ions that bind to central metals, forming complex ions.

  • Lewis bases enhance solubility of certain salts by forming complex ions with metal ions.

Sample Problems with Complex Ions

  1. Calculate equilibrium concentration of Cu2+ when mixed with NH3.

  2. Determine new concentration of Ag+ when solubility of AgCl changes in presence of ligands.