Problem 78 - Chemistry Matter and Change

Question

Use Le Chatelier's principle to explain what happens to the equilibrium \(\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{H}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})\) when a few drops of HCl are added to pure water.

Step-by-Step Solution

Answer

Adding HCl shifts the equilibrium to the left, decreasing \(\mathrm{OH}^{-}\) concentration.

Step 1: Understand Le Chatelier's Principle

Le Chatelier's principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium will shift to counteract the change and re-establish equilibrium.

Step 2: Identify the Disturbance

In this case, adding a few drops of HCl introduces additional H⁺ ions into the solution. This increases the concentration of H⁺ ions in the equilibrium reaction.

Step 3: Analyze the Equilibrium Shift

The equilibrium reaction is \( \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{H}^{+}(\mathrm{aq}) + \mathrm{OH}^{-}(\mathrm{aq}) \). An increase in H⁺ ion concentration will shift the equilibrium to the left to reduce the concentration of H⁺ ions and re-establish equilibrium.

Step 4: Predict the Effect on OH⁻ Concentration

As the equilibrium shifts to the left, the concentration of \( \mathrm{OH}^{-}(\mathrm{aq}) \) decreases since hydroxide ions will combine with the excess H⁺ to form more water.

Key Concepts

Equilibrium

Equilibrium in chemical reactions refers to a state where the forward and backward reactions occur at the same rate. This means the concentrations of the reactants and products remain constant over time. In the context of the given exercise, the equilibrium involves water dissociating into hydrogen ions (\({H}^{+}\) ions) and hydroxide ions (\({OH}^{-}\) ions). Although the reactants and products continue to interconvert, their concentrations do not change because these processes are balanced. Understanding equilibrium is key when discussing the effects of disturbances, as it determines how a reaction system will respond to changes.

Acid-Base Reaction

An acid-base reaction is a type of chemical reaction that involves the transfer of protons (\({H}^{+}\) ions). In this scenario, adding hydrochloric acid (HCl) to water causes an acid-base reaction.

• HCl dissociates fully in water, contributing more \({H}^{+}\) ions.
• These additional \({H}^{+}\) ions participate in the equilibrium between water and its ions.

This reaction process is governed by how acids donate \({H}^{+}\) ions and how bases accept them. Understanding this interaction helps explain the changes in equilibrium caused by the added HCl.

Disturbance

A disturbance in a chemical equilibrium arises when an external change affects the system. In the context of the exercise, the addition of HCl acts as such a disturbance. When HCl is added:

• The concentration of \({H}^{+}\) ions increases rapidly.
• The equilibrium must then adjust to this sudden increase.

According to Le Chatelier's principle, the system will attempt to counteract this change by shifting the equilibrium to the left, consuming some of the added \({H}^{+}\) ions. This is a classic example of how disturbances alter equilibria in chemical systems.

HCl Addition

When a few drops of HCl are added to pure water, it significantly increases the concentration of \({H}^{+}\) ions. This addition triggers a response from the equilibrium of the water dissociation reaction:\[\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{H}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})\]Due to the Le Chatelier’s principle, the system compensates by shifting the reaction direction to the left. As a result:

• More water is formed from \({H}^{+}\) ions and \({OH}^{-}\) ions.
• The concentration of hydroxide ions decreases as they react with excess \({H}^{+}\) ions.

This shift helps to partially balance the effect of the HCl addition, reducing the disturbance's impact on the system.