Acid Dissociation Constant

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Acid Dissociation Constant
Acid Dissociation Constant
An acid dissociation constant, Ka, (also known as acidity constant, or acid-ionization constant)
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 can be written symbolical y as: HA A- + H+,
where HA is a generic acid that dissociates by splitting into A-, known as the conjugate base
of the acid, and the hydrogen ion or proton, H+, which, in the case of aqueous solutions,
exists as a solvated hydronium ion. In the example shown in the figure, HA represents acetic
acid, and A- the acetate ion.
The chemical species HA, A- and H+ are said to be in equilibrium when their concentrations
do not change with the passing of time. The dissociation constant is usual y written as a
quotient of the equilibrium concentrations (in mol/L), denoted by [HA], [A-] and [H+]:
Due to the many orders of magnitude spanned by Ka values, a logarithmic measure of the
acid dissociation constant is more commonly used in practice. The logarithmic constant, pKa,
which is equal to -log10 Ka, is sometimes also (but incorrectly) referred to as an acid
dissociation constant:
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The larger the value of pKa, the smaller the extent of dissociation. A weak acid has a pKa
value in the approximate range -2 to 12 in water. Acids with a pKa value of less than about -2
are said to be strong acids; a strong acid is almost completely dissociated in aqueous
solution, to the extent that the concentration of the undissociated acid becomes undetectable.
pKa values for strong acids can, however, be estimated by theoretical means or by
extrapolating from measurements in non-aqueous solvents in which the dissociation constant
is smal er, such as acetonitrile and dimethylsulfoxide.
Hydrogen Ion Concentration of Weak Acid :-
Weak acid :- A monobasic weak acid HA it dissociates in aqueous solutions.
HA (aq) <====> H+ (aq) + A-(aq)
Suppose the initial concentration of the acid is `C' mol /dm3. At equilibrium, let its degree of
dissociation be ?. According to Ostwald's dilution law. Dissociation constant of the acid is
given by,
Ka= ?2C/ (1- ?). If ? is smal , 1-? ? 1. Therefore Ka= ?2C
?2 = Ka/C or ? = ?Ka/C....... (1)
HA (aq) <====> H+ (aq) + A-(aq)
Initial number of moles C - -
Number of moles
dissociated at equilibrium C? - -
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No. of moles present
at equilibrium C - C ? C? C?
Thus, the molar concentration of H+ ions in the solution at equilibrium.
[H+] = C?........ (2)
Substituting the value of ? from (1),
[H+] = C ?Ka/C or [H+] = ?KaxC ................. (3)
Example: A weak acid is one which doesn't ionize completely in aqueous solution of water.
Acetic acid is an example of weak acid. It reacts with water to produce a hydroxonium ions
and ethanoate ions, but the back reaction is more successful than the forward one. The ions
react very easily to reform the acid and water.
At any one time, only about 1% of the acetic acid molecules have converted into ions.
CH3COOH (aq) + H2O (l) <====> H3O+ (aq) + C2H3O2-(aq)
Or CH3COOH (aq) <====> H+ (aq) + C2H3O2-(aq)
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