Acid dissociation constant

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Acid Dissociation Constant

The acid dissociation constant, denoted as Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions. The equilibrium is represented as:

HA _ H+ + A-

where HA is a generic acid that dissociates into a proton (H+) and its conjugate base (A-). The larger the value of Ka, the more the acid dissociates, and the stronger the acid.

Definition and Expression

The acid dissociation constant is defined by the equation:

Ka = \( \frac{[\text{H}^+][\text{A}^-]}{[\text{HA}]} \)

where [H+], [A-], and [HA] are the molar concentrations of the hydrogen ion, the conjugate base, and the undissociated acid, respectively.

pKa

The pKa is the negative logarithm (base 10) of the acid dissociation constant:

pKa = -log10Ka

A lower pKa value indicates a stronger acid, which means it more fully dissociates in solution.

Factors Affecting Acid Strength

Several factors influence the strength of an acid, including:

  • Electronegativity: More electronegative atoms can stabilize the negative charge on the conjugate base, increasing acid strength.
  • Resonance: Delocalization of charge through resonance can stabilize the conjugate base.
  • Inductive Effect: Electron-withdrawing groups can stabilize the conjugate base through the inductive effect.
  • Hybridization: The s-character of the hybrid orbitals can affect acidity; more s-character can lead to stronger acids.

Examples of Acid Dissociation Constants

Acetic Acid

Acetic acid dissociation

Acetic acid (CH3COOH) is a weak acid with a pKa of approximately 4.76. It partially dissociates in water to form acetate ions (CH3COO-) and hydrogen ions (H+).

Phosphoric Acid

Phosphoric acid speciation

Phosphoric acid (H3PO4) is a triprotic acid with three dissociation constants, corresponding to the loss of each proton.

Citric Acid

Citric acid speciation

Citric acid (C6H8O7) is a weak organic acid with three carboxyl groups, each with its own dissociation constant.

Related Concepts

Related Pages

Gallery

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  • Weak acid speciation

  • Carboxylic acid dimers

  • Acetic acid pKa in dioxane-water

  • Chloroacetic acid pKa

  • Fumaric acid structure

  • Maleic acid structure

  • Proton sponge

  • Acid dissociation constant

  • Acid dissociation constant

  • Acid dissociation constant

  • Acid dissociation constant

  • Acid dissociation constant

  • Acid dissociation constant

  • Acid dissociation constant

  • Acid dissociation constant

  • Acid dissociation constant

  • Acid dissociation constant

  • Acid dissociation constant

  • Acid dissociation constant

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Contributors: Prab R. Tumpati, MD