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Acids that contain more than one acidic (ionizable) hydrogen (proton) are called polyprotic or polybasic acids. The dissociation of polyprotic acids occurs in a stepwise fashion, one proton lost at a time. For example, the generic triprotic acid will dissociate as shown in Reactions (1) through (2). The equilibrium constants for these reactions are symbolized by Ka.
H2A- + H20 <-----> H3O+ + HA2- Ka2 = [ H3O+] [HA2-]/ [H2A-] (2)
HA2- + H20 <-----> H3O+ + A3- Ka3 = [ H3O+] [A3-] / [HA2-] (3)
Reactions such as those shown in Reactions (4) and (5) are sometimes used as the basis of chemical calculations, but these reactions are generally not considered to be representative of what is actually occurring in solution. These reactions can be useful at times even though they are not rigorously correct chemically. The equilibrium constant for Reaction (4) is the product of Ka1 and Ka2 while the equilibrium constant for Reaction (5) is the product of Ka1, Ka2, and Ka3.
H3A + 3H20 <-----> 3H3O+ + A3- (5)
Remember that the numerical value of an equilibrium constant is an indication of how far to the right a reaction proceeds.
The greater the magnitude of the equilibrium constant, the more complete the reaction. Generally the numerical value of Ka1 is larger than that of Ka2, which is larger than Ka3 This is reasonable when one considers that successive protons are removed from species with progressively higher negative charges.
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Ramesh
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