Chem 26: Solubility Equilibria

• Chem 26 covers Solubility Equilibria

Learning Objectives

• Determine the solubility product expression for partially soluble salts
• Predict the effect of factors that impact solubility equilibrium of substances
• Solve problems on selective precipitation

The Solubility Product Constant

• Denoted as Ksp
• Defined as the equilibrium constant for equilibrium established between a solid solute and its ions in a saturated solution
• In aqueous solution, CaSO4 solubilization to its ions is written as: CaSO4 (s) ⇄ Ca2+ (aq) + SO42–(aq)
• The solubility product constant is expressed as: Ksp = [Ca2+][SO42–] = 9.10 x 10-6 at 25°C
• Solids are not included in the equilibrium expression

The Relationship Between Solubility and Ksp

• In the expression Ksp = [Ca2+][SO42–] = 9.10 x 10-6 at 25 °C, [Ca2+] and [SO42–] are molar solubilities for the respective ions
• Molar solubility is the molarity of dissolved ions in a saturated aqueous solution
• Molar solubility of one ion can be calculated with Ksp, and the molar solubility of the other ion

Limitations of the Ksp Concept

• Ksp is usually limited to slightly soluble solutes
• For more soluble solutes, ion activities must be used
• Activities are smaller than the measured concentrations
• Ksp does not account for ion-pairs
• Assumes that all dissolved solutes are separated cations and anions
• Ion pairs are known to exist in solution

Writing Solubility Product Expressions

• Follow the steps to write Ksp expression for Pbl2
• Write the equilibrium reaction for the balanced chemical equation: Pbl2 (s) ⇄ Pb2+ (aq) + 2 I- (aq)
• Write the Ksp expression, molar concentration exponent of the products is equal to the coefficient of the ion in the reaction. Ksp = [Pb2+][I-]2
• [I-] is raised to 2, as there are 2 moles of dissolved I- per mole of Pbl2 in saturated solution.

Calculating Ksp from Solubility

• A sparingly soluble salt, CaSO4, was found to have a concentration of 0.015 M in a saturated aqueous solution.
• Solubility product constant of salt is determined
• The Ksp expression can be set up -CaSO4(s) ⇄ Ca2+(aq) + SO42-(aq)
• Substitute known concentrations where -Ksp = [Ca2+][SO42–]
• Ksp = (0.015)² = 2.3 x 10-4

Factors Affecting Solubility

• Temperature
• Common ion effect
• pH
• Amphoterism
• Complex ion formation

The Common-Ion Effect in Solubility Equilibria

• If a few drops of K2CrO4 solution is added to a saturated solution of Ag2CrO4, is defined by the equation
• Ag2CrO4 (s) ⇄ 2Ag+(aq) + CrO4-(aq)
• The reaction shifts to the left to reestablish equilibirum
• The ions become less soluble as the common ion, CrO42-, is added
• Ionic interactions are important even when an ion is not participating in the equilibrium Ag2CrO4 (s) ⇄ 2Ag+(aq) + CrO4-(aq)
• When a salt with no common ion is added to a saturated salt solution, the ions become more soluble
• Increased overall ionic charge of the solution
• Noncommon ions tend to increase solubility
• If KNO3 is added to above equilibria, the reaction shifts to the right

Simultaneous Equilibria

• Can be defined as other equilibria that are usually present in a solution
• Auto-ionization of water
• Reactions between solute ions and other species
• Acid base reactions
• Complex-ion formation
• These reactions will have an effect on the equilibrium

Solubility and pH

• Solution pH can affect the solubility of a salt
• When the anion of the salt is the conjugate base of a weak acid (i.e., Mg(OH)2) -Mg(OH)2 (s) ⇄ Mg2+ (aq) + 2OH- (aq) -OH-(aq) + H3O+(aq) ⇄ 2 H2O(aq)
• When pH is high, high pH favors left equilibrium shift
• At acidic pH, Mg(OH)2 becomes more soluble as it is consumed in the second equilibrium
• Solubilty of ionic compound with a strongly basic-weakly basic anion increases with increasing [H+]

Amphoterism and Solubility

• Metal hydroxides (Al+3, Zn+2, Cr+3, Sn+2) are soluble in strongly basic and acidic conditions
• Amphoteric molecules act as both acid and base
• At low pH, metal hydroxides behave like Mg(OH)2
• At high pH, Al(OH)3 forms complex ion by acting as a Lewis acid -Al(OH)3 (s) + OH-(aq) ⇄ Al(OH)4 -(aq)

Amphoterism – Purification of Aluminum Ore

• As not all metal hydroxides are amphoteric, the concept of amphoterism is used to separate Al from Fe
• Soluble Al(OH)4 + Fe(OH)3 Solid precipitate can be found in Bauxite (Al2O3 + Fe2O3)

Factors Affecting Solubility

• Complex Formation:
  • A polyatomic cation or anion is composed of a central metal ion and a ligand.
  • Ligands- Lewis base that can donate electron pair -Coordination compounds- Substance which contain complex ions.
• This leads to a higher solubility in presence of NH3 (aq)