For polyprotic acids, ionization constants for successive ionization steps decrease rapidly. That is,K_a1 ≫ K_a2 ≫ K_a3. The ionization constants for the first two steps in the ionization of [Fe(H₂O)₆]³⁺ (reactions 24.10 and 24.11) are more nearly equal in magnitude. Why does this multistep ionization seem not to follow the pattern for polyprotic acids?

Reaction 24.10

Reaction 24.11

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[Fe(H₂O)6]³+ + H₂O = [FeOH(H₂O)5]2+ + H3O+ K₁₁ =9 x 104 (24.10)