Question

Given the chemical equation for the ionization of hydrofluoric acid $$ \mathrm{HF}(a q) \rightleftarrows \mathrm{H}^{+}(a q)+\mathrm{F}^{-}(a q) $$ Predict the direction of equilibrium shift for each of the following stresses: (a) increase [HF] (b) increase \(\left[\mathrm{H}^{+}\right]\) (c) decrease [HF] (d) decrease \(\left[\mathrm{F}^{-}\right]\) (e) add solid NaF (f) add gaseous HCl (g) add solid \(\mathrm{NaOH}\) (h) increase \(\mathrm{pH}\)

Short answer

(a) Right, (b) Left, (c) Left, (d) Right, (e) Left, (f) Left, (g) Right, (h) Right.

Step-by-step solution

Le Chatelier's Principle Overview

According to Le Chatelier's Principle, a system at equilibrium will shift in a direction that counteracts any change imposed on it. In chemical equilibrium, this can result in shifts to increase or decrease concentrations of reactants or products to restore balance.

Analyzing Increased [HF]

When you increase the concentration of HF, the equilibrium will shift to the right to use up the added HF. This results in the formation of more products, \(\mathrm{H}^{+}\) and \(\mathrm{F}^{-}\).

Analyzing Increase in [H+]

Increasing the concentration of \(\mathrm{H}^{+}\) ions will shift the equilibrium to the left, towards the reactants, to reduce the added \(\mathrm{H}^{+}\) ions.

Analyzing Decrease in [HF]

Decreasing the concentration of HF will cause the system to shift to the left, towards the reactants, to produce more HF.

Analyzing Decrease in [F-]

Decreasing \(\left[\mathrm{F}^{-}\right]\) will cause the equilibrium to shift to the right, towards the products, to replenish \(\mathrm{F}^{-}\) ions.

Analyzing Addition of Solid NaF

Adding solid NaF provides more \(\mathrm{F}^{-}\) ions. This causes the equilibrium to shift to the left, towards the reactants, to reduce the excess \(\mathrm{F}^{-}\) ions.

Analyzing Addition of Gaseous HCl

Adding gaseous HCl increases the \(\mathrm{H}^{+}\) ion concentration, shifting the equilibrium to the left, towards the reactants, to decrease the excess \(\mathrm{H}^{+}\) ions.

Analyzing Addition of Solid NaOH

Adding solid NaOH will remove \(\mathrm{H}^{+}\) ions by forming water, causing the equilibrium to shift to the right to replace the \(\mathrm{H}^{+}\) ions.

Analyzing Increase in pH

Increasing the pH means decreasing \(\mathrm{H}^{+}\) concentration, which causes the equilibrium to shift to the right, towards the products, to increase the \(\mathrm{H}^{+}\) concentration.

Key concepts

Chemical Equilibrium

Chemical equilibrium is a fundamental concept in chemistry where a reaction has reached a state where the concentrations of reactants and products remain constant over time. This does not mean that the reactants and products are equal in concentration, but rather that their rates of formation and consumption are equal. Thus, they balance each other out.

In the case of the ionization of hydrofluoric acid, represented by the equation: \[ \mathrm{HF}(aq) \rightleftarrows \mathrm{H}^{+}(aq)+\mathrm{F}^{-}(aq) \]The system is at equilibrium when the rate of ionization of HF is equal to the rate of reformation of HF from \(\mathrm{H}^{+}\) and \(\mathrm{F}^{-}\).

This balance can be disturbed by changing concentrations, temperature, or pressure, prompting the system to shift to regain equilibrium as per Le Chatelier’s Principle. Such shifts can lead to an increased concentration of either the products or the reactants.

Ionization of Acids

Ionization of acids refers to the process where an acid dissociates into its ions in solution. For example, when hydrofluoric acid (HF) dissolves in water, it ionizes to produce hydrogen ions \(\mathrm{H}^{+}\) and fluoride ions \(\mathrm{F}^{-}\). This is a reversible reaction, which can reach equilibrium.

The strength of an acid is determined by its tendency to donate protons. Strong acids ionize completely, while weak acids, like HF, ionize only partially. This partial ionization is dependent on the equilibrium constant, which can be influenced by factors such as concentration and temperature.

• Strong acids have large ionization constants and a high concentration of \(\mathrm{H}^{+}\) ions.
• Weak acids have smaller ionization constants, indicating less ionization and, therefore, a lower concentration of \(\mathrm{H}^{+}\) ions.

Understanding ionization is essential for predicting the behavior of acids in various chemical reactions and how they respond to changes in their environment.

Equilibrium Shift

An equilibrium shift occurs when the conditions of a reaction at equilibrium are changed, leading the system to adjust itself in order to re-establish equilibrium. According to Le Chatelier's Principle, a system under stress will move in the direction that opposes the stress.

For the ionization of hydrofluoric acid:

• Increase in @@[HF]@@: The equilibrium will shift to the right, increasing the concentration of \(\mathrm{H}^{+}\) and \(\mathrm{F}^{-}\) to balance the added HF.
• Increase in @@[H^+]@@: The reaction will shift to the left, trying to consume the extra \(\mathrm{H}^{+}\) ions.
• Decrease in @@[HF]@@: The system shifts to the left to produce more HF.
• Decrease in @@[F^-]@@: Causes a shift to the right to replenish \(\mathrm{F}^{-}\) ions.

Such shifts help in understanding how reactions respond to changes and are crucial for processes in industrial chemistry, biological systems, and environmental science.