Precipitation, Dissolution, & Lewis A/B

Dissolution of Salts

Solid ionic compounds can dissolve to a certain extent

• Upon dissolution, the ionic compound will dissociate to ions

Solubility Product (Ksp)

Equilibrium constant for the reaction of dissolution

• A solid ionic compound in equilibrium with its ions in solution)

Molar Solubility (S)

Maximum possible concentration of a solute in a solvent

• More of that solid can’t be dissolved at equilibrium

• M of the dissolved solute in a saturated sol’n - solid in equilibrium with is dissolved form in sol’n

Molar Solubility and Ksp

• Solubility product is related to molar solubility

• Cannot compare solubilities of ionic compounds solely based on Ksp

  • Ksp can only be compared when compounds have the same dissociation stoichiometry

Predicting Precipitation

• If one of the dissolved ions is removed, reaction proceeds to the right

• If one of the dissolved ions is added, reactions proceeds to the left

• If more solid is added: no change!

  • The solution is already saturated (solid is not part of eq expression, so it doesn’t affect equilibrium)

Quotient (Q)

Prediction of the reaction direction:

• Q > Ksp → precipitation formation (ions combine to form solid; shift to the left to reestablish equilibrium)

• Q < Ksp → dissolution of the ppt/ions remain dissolved in sol’n (shifting to the right to reach equilibrium)

Ion Concentration and Precipitation

Adding ions to solution to start precipitation

• When Q = Ksp → precipitation begins

Selective Precipitation

If mixing ions can result in formation of several precipitates, the first to precipitate is the one with lowest threshold concentration

• If trying to find what will ppt first with the addition of ions, do K_sp/[ion]

• Example: Q_AgCl = K_sp = [Ag+][Cl-] → [Ag+] = K_sp/[Cl-]

Common Ion and Solubility

Common ion = the ion that is introduced and is also present in the ionic compound

• Solubility changes depending on the solution composition

  • Solubility decreases in the presence of common ions in sol’n

Lewis Definition

Views an acid base reaction as the donation and acceptance of an electron pair to form a covalent bond

Adduct

The connection between the acid and the base in a Lewis pair

Lewis Base

Any species that donates an electron pair to form a bond

• Must have a lone pair of electrons to donate

• Any substances that’s a Bronsted Lowry base is also a Lewis base

Lewis Acid

Any species that accepts an electron pair to form a bond

• Must have a vacant orbital (or be able to rearrange its bonds to form one) to accept a lone pair

• Some substances that are Bronsted Lowry acids are NOT Lewis acids

Electron-Deficient Molecules as Lewis Acids

Example":

B and Al have an unoccupied p orbital that can accept a pair of electrons

Metal Cations as Lewis Acids

Acts as a Lewis acid when it dissolves in water to form a hydrated ion

• The O atom in H2O donates a lone pair to an available orbital on the metal cation

Complex Ion Formation

Lewis A/B reaction: formation of a complex ion

• Formed by combining a cation with several anions or neutral molecules

• A ligan is the ion or molecule that is attached to the central metal cation in the complex ion