9. Polyprotic Acid-Base Equilibria
Polyprotic Acids and Bases
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How are hydrogens removed from polyprotic acids during dissociation in aqueous solution?
A
All at once, in a single step
B
One at a time, with each step having its own equilibrium constant
C
Hydrogens are never removed from polyprotic acids
D
Only the first hydrogen is removed; the others remain bound
Verified step by step guidance1
Understand the concept of polyprotic acids: Polyprotic acids are acids that can donate more than one proton (H⁺) during dissociation. Examples include sulfuric acid (H₂SO₄) and phosphoric acid (H₃PO₄).
Recognize that dissociation occurs stepwise: In aqueous solution, polyprotic acids dissociate one proton at a time, and each dissociation step has its own equilibrium constant, denoted as Kₐ₁, Kₐ₂, etc.
Analyze the equilibrium constants: The first dissociation step (Kₐ₁) typically has the highest value, meaning it is the most favorable. Subsequent dissociation steps (Kₐ₂, Kₐ₃, etc.) have progressively smaller equilibrium constants, indicating that removing additional protons becomes less favorable.
Write the dissociation reactions: For example, for phosphoric acid (H₃PO₄), the dissociation steps are:
1. H₃PO₄ ⇌ H⁺ + H₂PO₄⁻ (Kₐ₁)
2. H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻ (Kₐ₂)
3. HPO₄²⁻ ⇌ H⁺ + PO₄³⁻ (Kₐ₃).
Conclude that hydrogens are removed one at a time: Each proton is removed in a separate step, and the process is governed by its respective equilibrium constant. This stepwise dissociation is a key characteristic of polyprotic acids.
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