Acid dissociation constant

Acid Dissociation Constant[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

Acetic Acid[edit | edit source]

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[edit | edit source]

Phosphoric acid speciation

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

Citric Acid[edit | edit source]

Citric acid speciation

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

Related Concepts[edit | edit source]

Related Pages[edit | edit source]

Gallery[edit | edit source]

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

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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Acid dissociation constant