Solubility Equilibrium - Lecture 9

Definition of Solution

• Solution: Homogeneous mixture of substances.
  • Solute: The substance that is dissolved.
  • Solvent: The dissolving agent.
• Aqueous solution: Solution where the solvent is water.
  • Electrolyte: Forms conductive solution with ions.
  • Non-electrolyte: Forms non-conductive solution with neutral molecules.

Introduction to Solubility

• Solubility: The maximum amount of a substance that can dissolve in a given amount of solvent at a specific temperature.
  • Expressed as mass of solute per 100 mL solvent (g/100 mL) or molar concentration (mol/L).
  • $Mass\ Solubility = \frac{Mass \ of \ Solute}{Volume \ of \ Solution} (g/L \ or \ g/mL)$
  • $Molar\ Solubility = \frac{Moles \ of \ Solute}{Volume \ of \ Solution} (mol/L)$

Solubility of Salts

• Ionic solids (salts) are held in lattices by electrical attractions.
• Precipitation: Ion exchange reaction with an insoluble product.
• Insoluble compounds: Less than 0.01 mol/L dissolves in H2O.
• Soluble Compounds
  • Almost all salts of $Na^+$, $K^+$, $NH_4 ^+$
  • Salts of nitrate $NO<em>3 ^-$, chlorate $ClO</em>3 ^-$, perchlorate $ClO<em>4 ^-$, acetate $CH</em>3COO^-$
  • Compounds containing $F^-$
  • Salts of sulfate, $SO_4 ^{2-}$
  • Almost all salts of $Cl^-$, $Br^-$, $l^-$
• Insoluble Compounds
  • All salts of carbonate,$CO_3 ^{2-}$
  • phosphate, $PO_4 ^{3-}$
  • oxalate,$C<em>2O</em>4 ^{2-}$
  • chromate, $CrO_4 ^{2-}$
  • Most metal sulfides, $S^{2-}$
  • Most metal hydroxides and oxides
• Solubility usually increases with increasing temperature.

Solubility Equilibrium

• Saturated solution: Dynamic equilibrium between solid and its ions; rate of dissolving equals to the rate of precipitation.
• Represented by a double arrow equilibrium equation.
• Equilibrium constant (Keq) for dissolution is the solubility product constant (Ksp).
• $Keq = Ksp$
• The Solubility Product Constant, Ksp
  • Ksp < 10^{-5}, insoluble compound
  • 10^{-2} < Ksp < 10^{-5}, slightly soluble compound
  • Ksp > 10^{-2}, soluble compound
• Larger the value of Ksp => The greater number of ions in the solution => The greater the solubility of a compound

The Solubility Product Expression

• General dissolution equilibrium reaction: $aA(s) \rightleftharpoons bB(aq) + dD(aq)$
• $K_{sp} = [B]^b[D]^d$

The Ion Product, Q

• Ion product (Q): Product of ion concentrations at any instant.
• $Q = [B]^b[D]^d$
• If Q < K_{sp}, the solution is unsaturated.
• If Q > K_{sp}, the solution is supersaturated.
• If $Q = K_{sp}$, the solution is saturated and at equilibrium.

Relating Solubilities to the Solubility Product Constants

• Molar solubility: Concentration of "dissolved solid" in a saturated solution.
• For $AgCl(s) \rightleftharpoons Ag^+(aq) + Cl^-(aq)$, solubility equals $[Ag^+]$ or $[Cl^-]$.
• For $Ca<em>3(PO</em>4)<em>2(s) \rightleftharpoons 3Ca^{2+}(aq) + 2PO</em>4^{3-}(aq)$, solubility is one-third of $[Ca^{2+}]$ or one-half of $[PO_4^{3-}]$.

Common Ion Effect

• Shift in equilibrium upon addition of a common ion (Le Châtelier’s principle).
• e.g., Adding $AgNO<em>3$ to a saturated solution of $AgCH</em>3COO$
• Usually decreases the solubility of a slightly soluble salt.
• A reaction with a common ion present has a lower solubility​, and the reaction without the common ion has a greater solubility.

The Effects of pH on Solubility

• Solubility depends on pH.
• $CaCO<em>3(s) \rightleftharpoons Ca^{2+}(aq) + CO</em>3^{2-}(aq)$
• $CO<em>3^{2-}(aq) + H</em>2O(l) \rightleftharpoons HCO_3^{-}(aq) + OH^{-}(aq)$
• Adding acid shifts equilibrium to the right, increasing solubility.