3.4 Common Ion Effect

In the previous sections, we explored the equilibria of weak acids and bases, and the solubility of slightly soluble salts. A common thread in all these equilibria is that they can be disturbed by adding more of one of the species involved. When that added species is an ion that is already part of the equilibrium, the result is a shift in the equilibrium position. This phenomenon is called the common ion effect. It is a direct consequence of Le Chatelier's Principle and is fundamental to understanding buffer solutions and the reduced solubility of salts in solutions containing a common ion.

Definition

The common ion effect is the suppression of the ionization of a weak electrolyte (or the dissolution of a slightly soluble salt) by the addition of a strong electrolyte that has an ion in common with the weak electrolyte (or the salt).

Simple Explanation: If you have an equilibrium involving an ion, and you add more of that same ion from another source, the system will try to remove the excess ion by shifting the equilibrium to the left. This reduces the ionization of the original substance.

Core Concept

The core concept is Le Chatelier's Principle. When a system at equilibrium is subjected to a change (such as an increase in the concentration of one of the species), the system will shift its equilibrium position to counteract the change. In the case of the common ion effect, adding more of a common ion increases its concentration on one side of the equilibrium, causing the equilibrium to shift in the opposite direction to consume some of the added ion.

Detailed Explanation

The textbook uses the example of a solution containing a weak acid, hydrofluoric acid (HF), and its salt, sodium fluoride (NaF), to explain the common ion effect.

The Equilibrium of a Weak Acid

Hydrofluoric acid is a weak acid, so it only partially ionizes in water: HF_{(aq)} \rightleftharpoons H^+_{(aq)} + F^-_{(aq)} This equilibrium lies far to the left, meaning most of the HF remains as undissociated molecules.

Adding a Common Ion

Sodium fluoride (NaF) is a strong electrolyte, so it dissociates completely in water: NaF_{(s)} \xrightarrow{H_2O} Na^+_{(aq)} + F^-_{(aq)} When NaF is added to the HF solution, it introduces a large amount of fluoride ions (F^-), which are the common ion.

The Shift in Equilibrium

• The increase in the concentration of F^- ions (from NaF) upsets the equilibrium of the HF ionization.
• To counteract this increase, the equilibrium shifts to the left (towards the reactants).
• This shift causes some of the F^- ions to combine with H^+ ions to form more undissociated HF.
• As a result, the dissociation of HF decreases in the presence of dissolved NaF.

• The degree of ionization of the weak acid (HF) is suppressed.
• The concentration of the weak acid (HF) increases.
• The concentration of the common ion (F^-) increases, but not as much as it would have if the weak acid had not been present.

Summary

The common ion effect is the reduction in the degree of ionization of a weak electrolyte caused by the addition of a strong electrolyte that has an ion in common with the weak electrolyte.

Intermediate Level

The common ion effect is an application of Le Chatelier's Principle. It occurs when a weak electrolyte (like a weak acid or a slightly soluble salt) is in equilibrium with its ions, and a strong electrolyte containing one of those ions is added. The added common ion increases the concentration of that ion, shifting the equilibrium towards the undissociated form of the weak electrolyte. This results in a decrease in the degree of ionization (or the solubility) of the weak electrolyte. It is a key principle in the functioning of buffer solutions, where the presence of the conjugate base suppresses the ionization of the weak acid.

Important Points

• - Common Ion Effect: Suppression of ionization of a weak electrolyte by adding a common ion.
• - Cause: A shift in equilibrium according to Le Chatelier's Principle.
• - Effect: Decreases the ionization of a weak acid or weak base, or the solubility of a slightly soluble salt.
• - Example: Adding NaF to HF decreases the dissociation of HF.
• HF_{(aq)} \rightleftharpoons H^+_{(aq)} + F^-_{(aq)}
• - Example: Adding HCl to a saturated solution of AgCl decreases the solubility of AgCl.
• - The common ion effect is the principle behind buffer solutions.

Remember

• - Common ion effect: Suppresses ionization or solubility.
• - Cause: Le Chatelier's Principle.
• - Adding a common ion shifts the equilibrium to the left.
• - Example: HF + NaF. The F^- from NaF suppresses the ionization of HF.
• - Example: AgCl + NaCl. The Cl^- from NaCl decreases the solubility of AgCl.
• - Application: Buffers and precipitation reactions.

Common Mistakes

• - Confusing the common ion effect with a reaction. The common ion effect is just a shift in an existing equilibrium; it is not a new chemical reaction. The common ion does not react with the weak electrolyte, it just shifts the equilibrium.
• - Thinking the common ion effect increases solubility. It always decreases the solubility of a slightly soluble salt. It suppresses ionization.
• - Believing that the common ion must be from a strong electrolyte. The added substance must be a strong electrolyte to provide a significant concentration of the common ion. If you add a weak electrolyte, the effect is not as pronounced.
• - Forgetting that the common ion effect applies to weak electrolytes. Strong electrolytes are fully ionized already, so adding a common ion cannot suppress their ionization further.
• - Ignoring the effect of dilution. The common ion effect is most pronounced in concentrated solutions. In very dilute solutions, the effect is smaller.

Quick Revision

The common ion effect is the suppression of the ionization of a weak electrolyte (or the solubility of a slightly soluble salt) by the addition of a strong electrolyte that has an ion in common with it. This occurs because the added common ion shifts the equilibrium to the left, according to Le Chatelier's Principle. The textbook example is adding sodium fluoride (NaF) to a solution of hydrofluoric acid (HF). The F^- ions from NaF suppress the ionization of HF, decreasing the concentration of H^+ and increasing the concentration of undissociated HF. The common ion effect is the basis for buffer solutions and is used to decrease the solubility of precipitates in chemical analysis.

60-Second Revision

The common ion effect is the suppression of ionization of a weak electrolyte by adding a strong electrolyte with a common ion. According to Le Chatelier's Principle, the added ion shifts the equilibrium to the left, reducing ionization. For example, adding NaF to HF suppresses the ionization of HF. This effect decreases the solubility of slightly soluble salts and is the principle behind buffer solutions.

Frequently Asked Questions (FAQs)

Q1. Explain the common ion effect with a suitable example. Answer: The common ion effect is the suppression of the ionization of a weak electrolyte by the addition of a strong electrolyte that has an ion in common with it. For example, consider a solution of weak acid, HF, in equilibrium: HF \rightleftharpoons H^+ + F^- If you add NaF (a strong electrolyte that dissociates completely into Na^+ and F^-), the concentration of F^- increases. According to Le Chatelier's Principle, the equilibrium shifts to the left to reduce the F^- concentration by forming more HF. This suppresses the ionization of HF. This is the common ion effect.

Q2. What is the common ion effect, and how does it relate to Le Chatelier's Principle? Answer: The common ion effect is the suppression of the ionization of a weak electrolyte by adding a common ion. It is a direct consequence of Le Chatelier's Principle, which states that if a change is imposed on a system at equilibrium, the system will shift to counteract the change. Adding a common ion increases its concentration, so the system shifts to the left to consume some of that ion, thereby suppressing the ionization of the weak electrolyte.

Q3. What is the effect of the common ion on the solubility of a slightly soluble salt? Answer: The common ion effect decreases the solubility of a slightly soluble salt. For example, the solubility of AgCl is lower in a solution of NaCl (which contains the common ion Cl^-) than in pure water. The added Cl^- shifts the dissolution equilibrium to the left, causing more AgCl to precipitate out of solution.

Q4. Give an example of the common ion effect in a solubility equilibrium. Answer: Consider the equilibrium for the dissolution of silver chloride: AgCl_{(s)} \rightleftharpoons Ag^+_{(aq)} + Cl^-_{(aq)} If NaCl is added to a saturated solution of AgCl, the concentration of Cl^- increases. According to Le Chatelier's Principle, the equilibrium shifts to the left to remove some of the added Cl^-, causing more AgCl to precipitate. The solubility of AgCl is thus decreased by the presence of the common ion (Cl^-).

Q5. Does the common ion effect apply to strong electrolytes? Answer: No, the common ion effect primarily applies to weak electrolytes. Strong electrolytes are already fully ionized, so adding a common ion cannot suppress their ionization further. For example, if you add NaCl to a solution of HCl (a strong acid), the H^+ and Cl^- ions are already fully dissociated, and no significant shift in equilibrium occurs.

Q6. How is the common ion effect related to buffer solutions? Answer: The common ion effect is the fundamental principle behind how buffer solutions work. In a buffer, a weak acid (e.g., acetic acid) is mixed with its conjugate base (e.g., sodium acetate). The common ion (CH_3COO^-) from the salt suppresses the ionization of the weak acid, ensuring that there is a large reservoir of both the weak acid and its conjugate base. This is what allows the buffer to effectively neutralize added acid or base.

Q7. Can the common ion effect be reversed? Answer: The common ion effect is a shift in equilibrium. It can be reversed by removing the common ion (e.g., by precipitation or dilution) or by adding a different ion that removes the common ion from the solution. The equilibrium will then shift back to the right, increasing ionization or solubility.

Q8. What is the difference between the common ion effect and a buffer solution? Answer: The common ion effect is a general phenomenon in which the ionization of a weak electrolyte is suppressed by adding a common ion. A buffer solution is a specific application of the common ion effect, in which a weak acid (or base) and its conjugate base (or acid) are present in significant amounts to resist changes in pH. All buffers rely on the common ion effect, but not all common ion effects create buffers.

Q9. Why is the common ion effect important in analytical chemistry? Answer: In analytical chemistry, the common ion effect is used to reduce the solubility of precipitates and ensure complete precipitation. For example, in the gravimetric determination of chloride, a slight excess of silver nitrate is added to precipitate AgCl. The excess Ag^+ ions act as a common ion, reducing the solubility of AgCl and ensuring that as much chloride as possible is recovered.

Multiple Choice Questions (MCQs)

Q1. Which phenomenon describes a shift in equilibrium due to the addition of an ion already involved in the equilibrium? A. Le Chatelier's Principle B. Common Ion Effect C. Nernst's Law D. Avogadro's Principle Correct Answer: B Explanation: The common ion effect is the suppression of the ionization of a weak electrolyte by the addition of a strong electrolyte that has an ion in common with it.

Q2. In a solution of hydrofluoric acid (HF) and sodium fluoride (NaF), the addition of NaF causes: A. The ionization of HF to increase. B. The ionization of HF to decrease. C. The pH to increase. D. The pH to decrease. Correct Answer: B Explanation: The common ion effect: adding the common ion (F^-) suppresses the ionization of the weak acid (HF), shifting the equilibrium to the left. This decreases the ionization of HF and increases the pH (because H^+ is reduced).

• Q3. What is the effect of adding a common ion to a saturated solution of a slightly soluble salt?
• A. It increases the solubility of the salt.
• B. It decreases the solubility of the salt.
• C. It has no effect on solubility.
• D. It causes the salt to become more soluble.
• Correct Answer: B
• Explanation: According to Le Chatelier's Principle, adding a common ion shifts the dissolution equilibrium to the left, decreasing the solubility of the salt.

Q4. The common ion effect is a direct consequence of: A. Le Chatelier's Principle. B. The Nernst equation. C. The Arrhenius equation. D. The Boltzmann distribution. Correct Answer: A Explanation: The common ion effect is a shift in equilibrium caused by the addition of a common ion. According to Le Chatelier's Principle, the system shifts to counteract the change, thereby suppressing ionization.

Q5. In the equilibrium HF \rightleftharpoons H^+ + F^-, what happens when NaF is added? A. The equilibrium shifts to the right. B. The equilibrium shifts to the left. C. The pH decreases. D. The ionization of HF increases. Correct Answer: B Explanation: NaF provides additional F^- ions. The equilibrium shifts to the left to consume some of the added F^-, forming more HF. This is the common ion effect.

Q6. The common ion effect is most significant for: A. Strong electrolytes. B. Weak electrolytes. C. Non-electrolytes. D. Gases. Correct Answer: B Explanation: Strong electrolytes are already fully ionized, so adding a common ion cannot suppress their ionization. The common ion effect applies primarily to weak electrolytes, which are only partially ionized.

Q7. What happens to the solubility of AgCl when NaCl is added to a saturated solution of AgCl? A. It increases. B. It decreases. C. It remains the same. D. It becomes zero. Correct Answer: B Explanation: Adding NaCl provides the common ion Cl^-. This shifts the equilibrium AgCl \rightleftharpoons Ag^+ + Cl^- to the left, decreasing the solubility of AgCl.

• Q8. Which of the following is an application of the common ion effect?
• A. Buffer solutions
• B. Precipitation reactions
• C. Both A and B
• D. Neither A nor B
• Correct Answer: C
• Explanation: The common ion effect is the basis for buffer solutions and is used in precipitation reactions to reduce the solubility of a slightly soluble salt.

Q9. In a buffer solution of acetic acid and sodium acetate, the common ion is: A. CH_3COOH B. CH_3COO^- C. Na^+ D. H^+ Correct Answer: B Explanation: The common ion in this buffer is the acetate ion (CH_3COO^-), which is provided by the sodium acetate. It suppresses the ionization of the weak acid (CH_3COOH).

• Q10. What is the common ion effect primarily used for in analytical chemistry?
• A. To increase the solubility of salts.
• B. To decrease the solubility of salts and ensure complete precipitation.
• C. To increase the ionization of weak acids.
• D. To change the color of solutions.
• Correct Answer: B
• Explanation: The common ion effect is used to reduce the solubility of precipitates, ensuring that a particular ion is completely removed from solution.

Multiple Choice Questions (MCQs)

Q1. Which phenomenon is the suppression of the ionization of a weak electrolyte by adding a strong electrolyte with an ion in common? A. Buffer action B. Common ion effect C. Hydrolysis D. Neutralization Correct Answer: B Explanation: The common ion effect is the suppression of ionization of a weak electrolyte by the addition of a strong electrolyte that has an ion in common with it.

Q2. What is the common ion in a solution of HF and NaF? A. H^+ B. Na^+ C. F^- D. HF Correct Answer: C Explanation: The common ion is the ion that is present in both the weak electrolyte (HF) and the strong electrolyte (NaF), which is the fluoride ion (F^-).

• Q3. What effect does adding a common ion have on the ionization of a weak acid?
• A. It increases the ionization.
• B. It decreases the ionization.
• C. It has no effect.
• D. It increases the pH.
• Correct Answer: B
• Explanation: The common ion effect suppresses the ionization of a weak acid, shifting the equilibrium to the left (towards the undissociated acid).

Q4. According to Le Chatelier's Principle, what happens when a common ion is added to an equilibrium mixture? A. The equilibrium shifts to consume the added ion. B. The equilibrium shifts to produce more of the added ion. C. The equilibrium constant changes. D. The equilibrium is unaffected. Correct Answer: A Explanation: When a common ion is added, its concentration increases. The system shifts the equilibrium to counteract this change by consuming some of the added ion.

• Q5. What is the effect of the common ion effect on the solubility of a slightly soluble salt?
• A. It increases the solubility.
• B. It decreases the solubility.
• C. It has no effect on solubility.
• D. It changes the color of the solution.
• Correct Answer: B
• Explanation: The common ion effect decreases the solubility of a slightly soluble salt by shifting the dissolution equilibrium to the left.

Q6. Which of the following is a direct application of the common ion effect? A. The functioning of buffer solutions. B. The precipitation of a salt from a solution. C. The determination of pH. D. The melting point of a solid. Correct Answer: A Explanation: Buffer solutions rely on the common ion effect. The common ion from the salt suppresses the ionization of the weak acid (or base), allowing the buffer to maintain a stable pH.

• Q7. In a buffer solution of acetic acid and sodium acetate, which ion suppresses the ionization of acetic acid?
• A. Na^+
• B. CH_3COO^-
• C. H^+
• D. OH^-
• Correct Answer: B
• Explanation: The acetate ion (CH_3COO^-) is the common ion that suppresses the ionization of acetic acid (CH_3COOH).

Q8. What happens to the concentration of undissociated HF when NaF is added to a solution of HF? A. It decreases. B. It increases. C. It remains the same. D. It becomes zero. Correct Answer: B Explanation: The equilibrium shifts to the left, meaning more H^+ and F^- combine to form undissociated HF. Therefore, the concentration of HF increases.

Q9. Why does the common ion effect not apply to strong electrolytes? A. Because they are not ionized. B. Because they are already fully ionized. C. Because they are insoluble. D. Because they have no common ions. Correct Answer: B Explanation: Strong electrolytes are fully ionized in solution, so adding a common ion cannot suppress their ionization further. The common ion effect is significant only for weak electrolytes, which are only partially ionized.

Q10. In a saturated solution of CaF_2, adding NaF will cause: A. More CaF_2 to dissolve. B. Less CaF_2 to dissolve. C. The pH to decrease. D. The solution to become unsaturated. Correct Answer: B Explanation: The added F^- (from NaF) is a common ion. It shifts the dissolution equilibrium of CaF_2 to the left, causing more CaF_2 to precipitate and less to dissolve.