Diprotic acids, such as H?SO?, are able to donate two protons to a solution. However, the dissociation constant for the second proton is usually much lower than for the first. This is because:

Question

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The 2nd proton dissociates slowly because of two factors: the presence of hydrogen ions already in solution from the 1st proton, and the fact that the 2nd ionization of the conjugate base requires more energy. After the first dissociation, the conjugate base is negatively charged (-1). To remove a second proton, thus making the charge -2, requires more energy.

Diprotic Acids, Such as H?SO?, Are Able to Donate Two