Acid dissociation constant

Acid Dissociation Constant

The acid dissociation constant, denoted as K_a, 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 K_a, the more the acid dissociates, and the stronger the acid.

Definition and Expression

The acid dissociation constant is defined by the equation:

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

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

pK_a

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

pK_a = -log₁₀K_a

A lower pK_a 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 pK_a 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

• Base dissociation constant
• pH
• Buffer solution
• Titration

Related Pages

• Acid-base reaction
• Chemical equilibrium
• Le Chatelier's principle

Gallery

• 
  pK_a of acetic acid
• 
  Weak acid speciation
• 
  Carboxylic acid dimers
• 
  Acetic acid pK_a in dioxane-water
• 
  Chloroacetic acid pK_a
• 
  Fumaric acid structure
• 
  Maleic acid structure
• 
  Proton sponge