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Chapter 19: Problem 3

What is the common-ion effect? How is it related to Le Ch芒telier's principle? Explain with equations that include HF and NaF.

Short Answer

Expert verified
The common-ion effect reduces the ionization of a weak electrolyte due to the addition of a common ion, explained by Le Ch芒telier's principle shifting equilibrium to counteract the disturbance.

Step by step solution

01

- Understand the Common-Ion Effect

The common-ion effect occurs when a weak electrolyte and a strong electrolyte containing a common ion are both dissolved in the same solution. This typically reduces the ionization of the weak electrolyte.
02

- Identify the Chemicals Involved

In this example, the weak acid HF (hydrofluoric acid) and the salt NaF (sodium fluoride) are used. NaF dissociates completely in water to give Na鈦� and F鈦� ions.
03

- Write the Ionization Equations

Write the ionization equation for HF in water:\(\text{HF} \rightleftharpoons \text{H}^+ + \text{F}^-\). For NaF, write: \(\text{NaF} \rightarrow \text{Na}^+ + \text{F}^-\).
04

- Apply the Common-Ion Effect to HF

Adding NaF to a solution of HF increases the concentration of the common ion, F鈦�. Due to this, the ionization of HF is suppressed according to the common-ion effect.
05

- Connect to Le Ch芒telier's Principle

Le Ch芒telier's principle states that a system at equilibrium will adjust to counteract a disturbance. Adding F鈦� shifts the equilibrium of HF dissociation:\[\text{HF} \rightleftharpoons \text{H}^+ + \text{F}^-\] to the left, reducing the concentration of H鈦� and F鈦� ions.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Le Ch芒telier's principle
To understand Le Ch芒telier's principle, imagine a balance. This principle states that when a system at equilibrium faces a change, it reacts to restore balance by counteracting that change.
In a chemical context, if you add more of a product or reactant to a reaction at equilibrium, the system will shift to consume the added substance.
For example, with the reaction involving HF (hydrofluoric acid) and its dissociation into ions: \[ \text{HF} \rightleftharpoons \text{H}^+ + \text{F}^- \]If you add more F鈦� ions by dissolving NaF (sodium fluoride), the system shifts left to reduce the added F鈦� by forming more HF. This is Le Ch芒telier's principle in action, maintaining balance.
Weak Electrolyte
A weak electrolyte is a substance that only partially ionizes in solution.
This means that in water, most of the substance stays in its original form, and only a small fraction breaks apart into ions.
Hydrofluoric acid (HF) is an example of a weak electrolyte. When dissolved in water, it does not completely dissociate into H鈦� and F鈦� ions.
This partial ionization can be represented by the equilibrium: \[ \text{HF} \rightleftharpoons \text{H}^+ + \text{F}^- \]The presence of the common ion, F鈦� from NaF, reduces the ionization of HF even further.
Ionization Equilibrium
Ionization equilibrium occurs when the rate of ionization of a substance is equal to the rate of recombination of ions.
For weak electrolytes like HF, this equilibrium is essential since only a portion of the substance ionizes: \[ \text{HF} \rightleftharpoons \text{H}^+ + \text{F}^- \]When you add more F鈦� ions from NaF, this equilibrium shifts in response to Le Ch芒telier's principle.
The increased concentration of F鈦� causes the equilibrium to shift left, leading to fewer H鈦� and F鈦� ions, and more undissociated HF.
This helps maintain the balance in the system, illustrating the delicate nature of ionization equilibrium in solutions.

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Most popular questions from this chapter

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