Problem 14 - Chemistry The Central Science

Question

(a) Consider the equilibrium \(\mathrm{B}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons\) \(\mathrm{HB}^{+}(a q)+\mathrm{OH}^{-}(a q) .\) Using Le Châtelier's principle, explain the effect of the presence of a salt of \(\mathrm{HB}^{+}\) on the ionization of B. (b) Give an example of a salt that can decrease the ionization of \(\mathrm{NH}_{3}\) in solution.

Step-by-Step Solution

Answer

Adding a salt of HB+ to the solution increases the concentration of HB+ ions, causing the equilibrium to shift to the left according to Le Châtelier's Principle, and effectively decreasing the ionization of B. An example of a salt that can decrease the ionization of NH3 in solution is Ammonium chloride (NH4Cl), as it increases the concentration of NH4+ ions, shifting the equilibrium to the left and reducing NH3 ionization.

Step 1: Understanding Le Châtelier's Principle

Le Châtelier's Principle states that if a stress is applied to a system at equilibrium, the system will adjust itself to counter the effect of the stress and reach a new equilibrium state. In the context of chemical reactions, this means that if the concentration of reactants or products is changed, the reaction will shift to restore equilibrium.

Step 2: Applying Le Châtelier's Principle to the given equilibrium

Let's first analyze the chemical equilibrium given: \(B(aq) + H_2O(l) \rightleftharpoons HB^+(aq) + OH^-(aq)\) Initially, when there is no salt present, the reaction is at its equilibrium state, which means the ratio of products to reactants is constant. Now, we need to examine the effect of adding a salt of HB+ on this equilibrium.

Step 3: Effect of adding HB+ salt

When we introduce a salt containing HB+ ions into the solution, the concentration of HB+ ions increases. According to Le Châtelier's Principle, the equilibrium will shift to counter this change and restore the original equilibrium state. In this case, the reaction will shift to the left, consuming HB+ and OH- ions and producing more B and H2O. This effectively decreases the ionization of B as fewer B ions will turn into HB+ ions.

Step 4: Finding a salt to decrease ionization of NH3

To find a salt that decreases the ionization of NH3 in solution, we can follow a similar approach. For NH3, the ionization equilibrium can be represented by: \(NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)\) We need to find a salt that can increase the concentration of either NH4+ or OH- ions. One such example is Ammonium chloride (NH4Cl). When dissolved in water, NH4Cl dissociates into NH4+ and Cl- ions. According to Le Châtelier's Principle, the increased concentration of NH4+ ions will cause the equilibrium to shift to the left, reducing the ionization of NH3 in solution.

Key Concepts

Chemical Equilibrium

Chemical equilibrium is a fundamental concept in chemistry that occurs when the rates of the forward and reverse reactions in a chemical process are equal, leading to a constant concentration of reactants and products over time. In a dynamic equilibrium, reactions continue to occur, but since they do so at identical rates, the overall concentrations remain unchanged.

For example, in a reversible reaction like the ionization of a base in water, represented as \(B(aq) + H_2O(l) \rightleftharpoons HB^+(aq) + OH^-(aq)\), both the forward reaction (ionization) and the reverse reaction (reassociation) are happening simultaneously. At equilibrium, the rate of ionization of B equals the rate at which HB^+ and OH^- recombine to form B and H_2O.

Effect of Concentration on Equilibrium

The effect of concentration on chemical equilibrium is a direct application of Le Châtelier's Principle. When the concentration of a reactant or product in a reaction at equilibrium changes, the system will adjust to counteract the imposed change and restore a new equilibrium state.

This adjustment involves shifting the equilibrium position either to the right (favoring products) or to the left (favoring reactants). In the context of our original question, adding a salt containing \(HB^+\) ions to the solution increases the concentration of these ions. According to Le Châtelier's Principle, the system counteracts this by shifting the equilibrium to the left, thus reducing the ionization of B.

Ionization in Solution

Ionization in solution refers to the process by which a molecule turns into ions when it dissolves in a solvent. This process is essential to many reactions in aqueous solutions, including acid-base reactions.

Consider the ionization of ammonia \(NH_3\) in water: \(NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)\). When a salt like ammonium chloride (NH4Cl) is dissolved in water, it dissociates completely producing \(NH_4^+\) ions, which will reduce the ionization of \(NH_3\). This decrease in ionization is due to the equilibrium shift caused by the increased concentration of \(NH_4^+\) ion.

Application of Le Châtelier's Principle

The application of Le Châtelier's Principle helps us determine how systems at equilibrium respond to external changes. It's a powerful tool in predicting the direction in which a reaction will shift to re-establish equilibrium upon distorting the existing balance.

For instance, when the concentration of a product is artificially increased, like adding a salt that contains \(HB^+\) to our equilibrium mixture, the reaction shifts to reduce the concentration of that product, favoring the formation of reactants. In the classroom or laboratory, understanding and applying Le Châtelier's Principle allows students to control reactions, such as manipulating the degree of ionization of substances in solution, to achieve the desired outcome.