medium · MCAT chem-phys

For a polyprotic titration, why does the second pK_a usually occur at a higher pH than the first pK_a?

1. The first proton removed is always part of a distinctly stronger, more covalent bond.
2. The surrounding water becomes progressively more basic as base is added during the titration process.
3. Removing a positively charged proton from an already negative ion is electrostatically unfavorable.
4. The overall concentration of the parent acid simply decreases after the first proton has fully dissociated from it.

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