key term - Stepwise dissociation

Definition

Stepwise dissociation refers to the process by which a polyprotic acid or base loses its protons (H⁺ ions) in a series of distinct steps, each corresponding to a separate equilibrium. This sequential loss means that each step can have its own unique dissociation constant, and as the process continues, the strength of the acid or base generally decreases with each step. Understanding this term is crucial when analyzing the behavior of polyprotic substances, particularly in acid-base equilibria and titration reactions.

5 Must Know Facts For Your Next Test

1. In stepwise dissociation, each proton is released from the acid or base one at a time, leading to multiple equilibria that can be analyzed independently.
2. The first dissociation step typically has the largest dissociation constant (Ka₁), indicating it is the strongest dissociation, while subsequent steps have progressively smaller constants (Ka₂, Ka₃, etc.).
3. For diprotic acids like sulfuric acid, the first dissociation step is strong (complete), while the second step is weaker and partially dissociated.
4. The concept of stepwise dissociation is important in titration curves for polyprotic acids, where each plateau on the curve corresponds to a proton being neutralized.
5. Stepwise dissociation also influences the pH of solutions containing polyprotic acids, as each equilibrium shifts the balance of H⁺ ions and affects acidity.

Review Questions

• How does stepwise dissociation affect the overall strength of a polyprotic acid?
  • Stepwise dissociation directly influences the overall strength of a polyprotic acid since each proton is released sequentially. The first dissociation usually has the highest strength due to its larger dissociation constant, while subsequent steps yield weaker dissociations with smaller constants. This means that as more protons are lost from the acid, it becomes less acidic overall, affecting its behavior in solution and during reactions.
• What role does stepwise dissociation play in determining the titration curve of a diprotic acid?
  • In the titration of a diprotic acid, stepwise dissociation leads to distinct plateaus on the titration curve for each proton lost. The first plateau corresponds to the complete neutralization of the stronger acidic form, while subsequent plateaus represent partial neutralization of the weaker acidic form. Understanding these changes helps predict pH transitions and allows chemists to determine equivalence points more accurately during titrations.
• Evaluate how the presence of multiple stepwise dissociations impacts buffer solutions involving polyprotic acids.
  • Buffer solutions containing polyprotic acids benefit from multiple stepwise dissociations because they can resist changes in pH over a wider range. Each dissociation equilibrium contributes to buffering capacity at different pH levels. When an acid or base is added to such a buffer, both forms of the polyprotic acid can interact with H⁺ or OH⁻ ions, helping maintain a relatively stable pH. This versatility makes polyprotic acids particularly useful in biological systems where pH stability is critical.

Related terms

polyprotic acids:

Acids that can donate more than one proton per molecule, such as sulfuric acid (H₂SO₄) and phosphoric acid (H₃PO₄).

dissociation constant: A numerical value that represents the extent to which an acid or base dissociates in solution, typically denoted as Ka for acids and Kb for bases.

equilibrium constant: A value that expresses the ratio of concentrations of products to reactants at equilibrium for a reversible reaction, often used in the context of dissociation processes.