what are polyprotic acids? explain with the help of an example
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Polyprotic acids contain more than one mole ionizable hydronium ions per mole of acids. They ionize to give more than one H+ions per molecule. Possible forms of three polyprotic acids are given below after their dissociation into H+ ions.
H2S, HS-, S2- H2SO4, HSO4-, SO42- H3PO4, H2PO4-, HPO42-, PO43- These acids ionize in several stages, giving out one proton at each stage. The acidity constants for these acids may be written as K1, K2, K3, ...
Consider H2S,
H2S = H+ + HS- [H+] [HS-] K1 = ---------- [H2S] and HS- = H+ + S2- [H+] [S2-] K2 = ----------- [HS-] Obviously, for the overall ionization reaction, H2S = 2 H+ + S2- [H+]2 [S2-] Koverall = ----------- [H2S] = K1 K2 Confirm the above obvious result on a sheet of paper to satisfy yourself.
For polyprotic acids, the following is always true:
K1 > K2 > K3 > ...For most acids, K1/K2 = 1E5 or 100000, andK2/K3 = 1E5, but oxalic acid is different. For oxalic acid, K1 = 5.6E-2, and K2 = 5.4E-4. The two acidic groups are separated by a C-C bond in oxalic acid.
H2S, HS-, S2- H2SO4, HSO4-, SO42- H3PO4, H2PO4-, HPO42-, PO43- These acids ionize in several stages, giving out one proton at each stage. The acidity constants for these acids may be written as K1, K2, K3, ...
Consider H2S,
H2S = H+ + HS- [H+] [HS-] K1 = ---------- [H2S] and HS- = H+ + S2- [H+] [S2-] K2 = ----------- [HS-] Obviously, for the overall ionization reaction, H2S = 2 H+ + S2- [H+]2 [S2-] Koverall = ----------- [H2S] = K1 K2 Confirm the above obvious result on a sheet of paper to satisfy yourself.
For polyprotic acids, the following is always true:
K1 > K2 > K3 > ...For most acids, K1/K2 = 1E5 or 100000, andK2/K3 = 1E5, but oxalic acid is different. For oxalic acid, K1 = 5.6E-2, and K2 = 5.4E-4. The two acidic groups are separated by a C-C bond in oxalic acid.
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