Polyfunctional Acids and Bases

Conventions and Examples

Polyfunctional acids and bases are compounds capable of donating or accepting more than one proton. These substances play a significant role in acid-base chemistry due to their stepwise dissociation and multiple equilibria.

• Polyfunctional Acids: Can donate two or more protons. They are often represented as HnA, where n is the number of ionizable protons.

• Polyfunctional Bases: Either have more than one nitrogen-containing functional group (with available lone pairs) or are conjugate bases of polyprotic acids, capable of accepting up to n protons.

• Examples of Polyfunctional Acids:

  • Phosphoric acid (H3PO4)

  • Maleic acid (C2H2(COOH)2)

  • Carbonic acid (H2CO3)

• Examples of Polyfunctional Bases:

  • Ethylenediamine (H2NCH2CH2NH2)

  • Phosphate ion (PO43−)

  • Carbonate ion (CO32−)

Stepwise Dissociation and Ionization Constants

Polyfunctional acids dissociate in water in a stepwise manner, with each step having its own acid dissociation constant (Ka). Typically, the first dissociation constant (Ka1) is much larger than the subsequent ones (Ka2, Ka3, etc.), often by factors of 103 to 105. This is because it is easier to remove a proton from a neutral molecule than from an anion, and even harder from a doubly charged anion.

• Example: Phosphoric Acid (H3PO4)

  • First dissociation:

  • Second dissociation:

  • Third dissociation:

Calculations with Polyprotic Acids

When calculating the pH and concentrations of species in solutions of polyprotic acids, simplifications can be made if the dissociation constants differ by at least a factor of 103. In such cases, the first dissociation dominates the [H+] and pH calculation, and the acid can be treated similarly to a monoprotic acid for practical purposes.

• ICE Table Method: Used to determine equilibrium concentrations of all species.

• Assumption of Negligibility: If the change in concentration (x or y) is much smaller than the initial concentration, it can be neglected to simplify calculations. This assumption should be checked for validity.

Example: Ascorbic Acid (H2C6H6O6)

• Given: 0.20 M ascorbic acid, ,

• Since , only the first dissociation is considered for pH:

• ICE Table setup:

• Solving for x (where x = [H3O+]):

• Percent ionization:

Example: Oxalic Acid (H2C2O4)

• Given: 2.2507 g in 250.0 mL ( M), ,

• Since , first dissociation dominates:

• Quadratic equation required due to high percent ionization:

• M,

Acid Strength and Chemical Structure

Binary Acids

Binary acids consist of hydrogen and one other element (HY, H2Y, etc.). Their acid strength is influenced by the electronegativity of Y and the bond strength between H and Y.

• Across a Period: As the electronegativity of Y increases, acidity increases.

• Down a Group: As the bond strength decreases (Y becomes larger), acidity increases.

Key Point: Bond energy is a crucial factor in determining acid strength for binary acids.

Oxyacids (Oxoacids)

Oxyacids have a hydrogen atom bonded to an oxygen atom, which is bonded to a central non-metal atom. Their acid strength depends on the electronegativity of the central atom and the number of oxygen atoms attached.

• Same Number of Oxygens: The more electronegative the central atom, the stronger the acid.

• Different Number of Oxygens: Acid strength increases with the number of oxygen atoms.

Carboxylic Acids

Carboxylic acids are organic acids with the general formula RCOOH. The strength of a carboxylic acid depends on the nature of the R group. Electronegative substituents (like Cl or F) increase acid strength by pulling electron density away from the carboxyl group, making the O-H bond more polar and the proton easier to ionize.

Example: Chloroacetic acid is stronger than acetic acid due to the electron-withdrawing effect of the chlorine atom.

Summary Table: Common Polyprotic Acids and Ionization Constants

This table summarizes the structures and ionization constants (Ka1, Ka2, Ka3) for several important polyprotic acids, including sulfuric, oxalic, phosphoric, citric, ascorbic, and carbonic acids.