Common-Ion Effect

Introduction

Adding an ion already produced by an equilibrium reaction changes its reaction quotient. For a weak acid, a soluble salt of its conjugate base supplies a common ion and usually suppresses additional acid ionization. The effect follows directly from Q versus Ka rather than from a separate rule to memorize.

Core explanation

For HA + H₂O ⇌ H₃O⁺ + A⁻, the dilute quotient is Q ≈ [H₃O⁺][A⁻]/[HA]. Add a soluble salt containing A⁻ without changing temperature. [A⁻] rises immediately, increasing Q above Ka if the initial mixture was at equilibrium. Net reverse proton transfer consumes some A⁻ and H₃O⁺ and reforms HA until Q returns to Ka. Acid ionization is therefore suppressed relative to the salt-free case.

The acid does not stop dissociating. Forward and reverse proton-transfer processes continue, and a new dynamic equilibrium forms. The final [A⁻] may remain much larger than before because the externally added salt supplies it. The fraction of HA newly dissociated can be much smaller, while total conjugate-base concentration can be larger. Distinguish these quantities.

The same logic applies to weak bases. For NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, adding ammonium salt raises [NH₄⁺], making Q exceed Kb and favoring net reverse change. This can reduce hydroxide compared with ammonia alone. The resulting ammonia/ammonium pair can form a buffer.

Common ions also affect sparingly soluble salts. For AgCl(s) ⇌ Ag⁺ + Cl⁻, adding chloride raises the ion product Qsp and tends to precipitate AgCl, lowering its molar solubility under suitable conditions. The mechanism is another Q-versus-K comparison, though numerical treatment differs because the solid activity is one.

Real solutions require activities when ionic strength changes significantly. Adding a salt changes both the concentration of the common ion and activity coefficients. In basic classroom problems, concentration changes dominate under the assumed dilute model. State those assumptions when quantifying the effect.

Step-by-step reasoning

1. Write the weak-electrolyte or solubility equilibrium and quotient. 2. Identify the ion supplied by the added salt. 3. Determine its immediate effect on Q at fixed K. 4. Predict net reverse or forward adjustment and distinguish final from immediate concentration.

Visual explanation

Draw HA dissociation with A⁻ on the product side. Add an A⁻ arrow from a salt and show Q jumping above Ka, followed by a reverse arrow toward HA.

Real-world analogy

If one outlet of a production line is suddenly flooded with finished goods, the balance may shift toward returning some goods to inventory. It does not erase every externally added item.

Real-world example

An acetate salt added to acetic acid supplies CH₃COO⁻ and suppresses further acetic-acid dissociation. This shared-ion mixture is a common buffer formulation in laboratory work.

Why?

Why does added A⁻ suppress HA dissociation? It immediately raises the product-side quotient above Ka, so net reverse proton transfer restores the equilibrium ratio.

Common misconception

“Adding common ion changes Ka.” At fixed temperature Ka remains the same; species concentrations readjust to satisfy it. The equilibrium position changes, not the constant.

Worked example

Consider HA with Ka = 1.0 × 10⁻⁵ and analytical HA near 0.10 M. Without added A⁻, a simple estimate gives [H₃O⁺] ≈ 0.0010 M. With about 0.10 M A⁻ from a soluble salt, Ka ≈ H₃O⁺/(0.10), so [H₃O⁺] is roughly 1.0 × 10⁻⁵ M in the simplified buffer limit. The common ion strongly reduces acid-derived hydronium, subject to full charge and activity assumptions.

Quick check

1. Does adding acetate to acetic acid make Ka larger? Answer: No. It changes Q and equilibrium composition; Ka stays fixed at the same temperature.

Exam focus

Use Q to state the immediate shift, and avoid claiming complete suppression. In numerical work, include the salt's initial ion concentration in the ICE table.

Advanced insight

Common-ion addition also changes ionic strength, which can alter activity coefficients. A rigorous calculation may use thermodynamic Ka with activities rather than concentration ratios alone.

Summary

A common product ion raises Q and drives net reverse adjustment at fixed K, suppressing weak-electrolyte ionization or sparingly soluble salt dissolution under appropriate conditions.

Practice questions

1. Which ion is common when sodium acetate is added to acetic acid? Answer: Acetate, CH₃COO⁻. 2. What happens to NH₃ ionization when NH₄⁺ is added at fixed temperature? Answer: It is suppressed; Q rises and net reverse change is favored. 3. Is a shared ion effect always a change in equilibrium constant? Answer: No. It changes composition at the same temperature-specific constant.