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Acid dissociation constant

• The acid dissociation constant is a special case of a dissociation constant, in that it indicates the extent of hydrogen ion (H⁺) release from or dissociation by an acid.
  
  
• Some or most of an acid, when added to water, dissociates into hydrogen ions (H⁺) and its component part (A^-).
  
  
• The extent of dissociation determines the concentration of free hydrogen ions (H⁺) in solution, which is dependent on the strength of the hydrogen bonds, that is acid strength.
  
  
• The dissociation of an acid in solution produce -
  
  
  • acid ions (A^- - also called the conjugate base)
    
    
  • hydrogen ions (H⁺ - also called free hydrogen ions)
    
    
  • and the remaining undisociated acid molecules (AH_(aq))
    
    (aq) = in a water solution
    
    
    
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  AH(aq)		< = >		A-(aq)		+		H+(aq)
  undisociated		yields		dissociated		and		hydrogen
  acid molecule				acid ion				ion
  (acid)				(conjugate base)

  
  
  
  As an example
  
  

  HCl		< = >		Cl-		+		H+
  hydrochloric acid		yields		chlorine ion		and		hydrogen ion

  
  
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  The extent of this dissociation, at equilibrium, is represented by the acid dissociation constant (K_a) -
  
  
  [A^-] [H⁺] …….(product of the molar concentration (M) of the dissociated forms)
  
  K_a = ----------- …....(divided by)
  
  [AH_(aq)] …….(the molar concentration (M) of the undisociated form)
  
  
  i.e.
  
  [CL^-] [H⁺] …….(product of the molar concentration (M) of the dissociated forms)
  
  K_a = ----------- …......(divided by)
  
  [HCl] …….........(the molar concentration (M) of the undisociated form)
  
  
  
• When weak acids are involved, the final K_a is very small,
  
  i.e. very little dissociation or strongly bonded hydrogen ions (H⁺)
  
  i.e. low concentrations of A^- and H⁺
  
  e.g. 10^-5 M.(typical of juice or wine acids)
  
  This is more conveniently represented as pK_a
  
  i.e. -log₁₀ 10^-5 M
  
  = 5 (see pH scale)