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Chapter 15: Problem 11

The common ion effect for weak acids is to significantly decrease the dissociation of the acid in water. Explain the common ion effect

Short Answer

Expert verified
The common ion effect occurs when a weak electrolyte solution is combined with a solution containing a common ion, which suppresses the extent of ionization of the weak electrolyte. In the case of weak acids, it decreases their dissociation in water. According to Le Chatelier's principle, when the concentration of the common ion increases, the system shifts to counteract the change, decreasing the amount of H+ and A- produced. This ultimately leads to a significant decrease in the dissociation of the weak acid in water, demonstrating the common ion effect.

Step by step solution

01

Understanding the common ion effect

The common ion effect is a phenomenon that occurs when a weak electrolyte solution, like a solution of a weak acid, is combined with a solution containing an ion that is also present in the weak electrolyte. The presence of this common ion suppresses the extent of ionization of the weak electrolyte because of the established equilibrium in weak electrolyte solutions.
02

Demonstrating weak acids' dissociation

Weak acids partially dissociate in water, resulting in an equilibrium between the undissociated acid, the free hydrogen ions (H+), and the conjugate base (A-), which is produced when the weak acid loses a proton. This equilibrium can be represented by the equation: \(HA \rightleftharpoons H^+ + A^-\) Here, HA represents the weak acid, H+ represents the hydrogen ion, and A- is the conjugate base. The equilibrium constant (Ka) of this reaction is given by: \(K_a = \frac{[H^+][A^-]}{[HA]}\) Where [H+], [A-], and [HA] are the concentrations of the hydrogen ions, conjugate base, and undissociated weak acid, respectively.
03

Introducing the common ion

When a salt containing an ion common with the weak acid solution is introduced, the concentration of the common ion increases. For example, in the case of a weak acid with the conjugate base A- as its ions, if we add a salt like NaA, the concentration of the common ion, A-, will significantly increase. This increase in the [A-] will result in the shift of the equilibrium towards the left, as predicted by Le Chatelier's principle, decreasing the dissociation of the weak acid (HA).
04

Explaining dissociation decrease

Le Chatelier's principle states that when a system in equilibrium is disturbed by a change in concentration of the species involved, the system shifts to counteract the change and re-establish the equilibrium. In this case, when the concentration of the common ion (A-) is increased, the system counteracts this change by reducing the ionization of the weak acid (HA), decreasing the amount of H+ and A- produced. This shift in equilibrium ultimately leads to a significant decrease in the dissociation of the weak acid in water, which is the common ion effect.

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Most popular questions from this chapter

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