Common Ion Effect — AP Chemistry
1. Definition and Core Concept ★☆☆☆☆ ⏱ 2 min
The common ion effect describes the suppression of ionization or dissolution of a weak electrolyte (weak acid, weak base, or sparingly soluble ionic salt) when a soluble compound containing an ion already present in the equilibrium (the "common ion") is added to the solution. It is a direct application of Le Chatelier’s principle to equilibrium systems. Unlike minor ionic strength effects from adding non-common ion salts (which are not tested on AP Chemistry), the common ion effect produces a large, easily calculable change in equilibrium concentrations that is explicitly tested on the exam. This topic accounts for 7-9% of the total AP exam score, appears in both MCQ and FRQ, and is the foundation of buffer solutions.
2. Effect on Weak Acid/Base Ionization ★★☆☆☆ ⏱ 4 min
When a weak acid $HA$ ionizes in solution, it establishes the equilibrium:
HA(aq) \rightleftharpoons H^+(aq) + A^-(aq)
If a soluble salt of the conjugate base $NaA$ is added, it dissociates completely to release $A^-$, the common ion. Adding $A^-$ increases product concentration, shifting equilibrium left toward undissociated $HA$. This reduces percent ionization of $HA$, lowers $[H^+]$, and increases pH compared to a solution of $HA$ alone. The same logic applies to weak bases: adding a common conjugate acid suppresses ionization, lowers $[OH^-]$, and decreases pH.
We use the small x (5%) approximation, which assumes dissociation of the weak acid is negligible compared to initial concentrations of $HA$ and added $A^-$:
K_a = \frac{[H^+][A^-]}{[HA]} \approx \frac{[H^+][A^-]_0}{[HA]_0}
Rearranging gives the Henderson-Hasselbalch equation, a direct result of the common ion effect:
pH = pK_a + \log\left(\frac{[A^-]_0}{[HA]_0}\right)
Exam tip: On the AP exam, you can directly use the Henderson-Hasselbalch equation for common ion weak acid/base systems to save time, as the 5% approximation almost always holds.
3. Effect on Solubility of Sparingly Soluble Salts ★★★☆☆ ⏱ 5 min
The common ion effect significantly reduces the molar solubility ($s$, moles of solid that dissolve per liter of solution) of sparingly soluble ionic compounds. For the general dissolution equilibrium:
M_xX_y(s) \rightleftharpoons xM^{y+}(aq) + yX^{x-}(aq)
If a common ion is added, equilibrium shifts left toward the solid, reducing solubility. For sparingly soluble salts, $s$ is already very small, so the contribution of dissolved salt to the common ion concentration is negligible, so we approximate the common ion concentration as equal to its initial added concentration.
Exam tip: Always write the balanced dissolution reaction before plugging concentrations into $K_{sp}$; do not assume all salts are 1:1.
4. Qualitative Predictions ★★☆☆☆ ⏱ 3 min
AP exams commonly ask for qualitative predictions of how adding a common ion changes pH, percent ionization, or solubility, with no calculation required. These rely on core rules:
- Adding a common product ion always shifts equilibrium toward the reactant side.
- No common ion = no common ion effect (ionic strength effects are not tested).
- For weak acids: adding common conjugate base → lower $[H^+]$ → higher pH → lower percent ionization.
- For weak bases: adding common conjugate acid → lower $[OH^-]$ → lower pH → lower percent ionization.
- For sparingly soluble salts: adding any common ion → lower solubility.
Exam tip: Always write the balanced equilibrium reaction in your justification; AP graders require explicit reference to shift direction for full credit.
5. AP Style Concept Check ★★★☆☆ ⏱ 2 min
Common Pitfalls
Why: Students forget that dissociation/dissolution is already suppressed, so the contribution from the weak electrolyte is negligible
Why: Students confuse lower $[H^+]$ with lower pH, or mix up the direction of equilibrium shift
Why: Students memorize the 1:1 salt formula and apply it to all salts regardless of stoichiometry
Why: Students assume all salts contain a common ion, but only salts with one ion matching the equilibrium produce the effect
Why: Students misremember the order of terms in the formula