What is Acid-Base Equilibrium?
Acid-base equilibrium describes the balance between acids, bases, and their conjugate species in solution. At equilibrium, the rates of ionization and neutralization are equal, and the pH remains constant.
At equilibrium, [H⁺][A⁻]/[HA] = Ka for weak acids. Equilibrium position depends on acid strength (Ka) and concentration.
- 1↓Initial: HA onlyPure weak acid
- 2↓Ionization beginsHA → H⁺ + A⁻
- 3Equilibrium reachedBoth HA and ions present
Step-by-step worked examples
A 0.1 M acetic acid (CH₃COOH) solution has Ka = 1.8 × 10⁻⁵. Find [H⁺] at equilibrium.
Ka = [H⁺][A⁻]/[HA] 1.8 × 10⁻⁵ = x²/(0.1 − x) ≈ x²/0.1 (if x << 0.1) x² = 1.8 × 10⁻⁶, x = 1.34 × 10⁻³ M [H⁺] ≈ 1.34 × 10⁻³ M
A 0.05 M ammonia (NH₃) solution, Kb = 1.8 × 10⁻⁵. Find [OH⁻].
Kb = [NH₄⁺][OH⁻]/[NH₃] 1.8 × 10⁻⁵ = x²/(0.05 − x) ≈ x²/0.05 x² = 9 × 10⁻⁷, x = 9.49 × 10⁻⁴ M [OH⁻] ≈ 9.49 × 10⁻⁴ M
At 25 °C, Kw = 1.0 × 10⁻¹⁴. If pH = 5, find pOH.
pH + pOH = 14 pOH = 14 − 5 = 9 [OH⁻] = 10⁻⁹ = 1 × 10⁻⁹ M
Flashcards
Quick quiz
Q1.Weak acid at equilibrium: strong presence of…
Q2.If Ka increases, acidity…
Q3.At equilibrium, which rate equals which?
Q4.Neutral solution at 25 °C: [H⁺] = ?
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Common mistakes
Weak acids don't reach equilibrium. — Correct: All reactions reach equilibrium; weak acids ionize only partially.
Kw changes with temperature. — Correct: Kw = 1.0 × 10⁻¹⁴ only at 25 °C; it changes at other T.
Strong acid has small Ka. — Correct: Strong acids have very large Ka (effectively complete ionization).
Neutral means no H⁺ or OH⁻. — Correct: Neutral: [H⁺] = [OH⁻] = 1 × 10⁻⁷ M at 25 °C.
FAQ
What is acid-base equilibrium?
It's the dynamic balance between ionization and recombination; forward and reverse rates are equal at equilibrium.
Why do weak acids reach equilibrium?
They only partially ionize. The reverse reaction (recombination) becomes significant, balancing the forward reaction.
How does concentration affect Ka?
Ka is constant at a given temperature; concentration affects the position of equilibrium but not Ka itself.
What is the relationship between Ka and pKa?
pKa = −log(Ka). Smaller pKa = stronger acid; pKa + pOH = 14 for conjugate pairs.