Question

(a) Consider the equilibrium $\mathrm{B}(aq)+{\mathrm{H}}_2\mathrm{O}(l)\rightleftharpoons$ ${\mathrm{HB}}^{+}(aq)+{\mathrm{OH}}^{-}(aq).$ 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.

Short answer

The addition of a salt of HB+ to the equilibrium system will increase the concentration of HB+ ions. According to Le Châtelier's principle, the system will respond by shifting the equilibrium to the left, favoring the reactants over the products and decreasing the ionization of B. An example of a salt that can decrease the ionization of NH3 is ammonium chloride (NH4Cl), as the added NH4+ ions will form a complex with NH3, reducing the concentration of free NH3 and thus decreasing its ionization in the solution.

Step-by-step solution

The given equilibrium reaction is: $$B(aq)+H_{2}O(l)\rightleftharpoons H{B}^{+}(aq)+O{H}^{-}(aq)$$ This reaction represents the ionization of a weak base (B) in water. In this process, the weak base reacts with water to form its conjugate acid (HB+) and hydroxide ions (OH-). #Step 2: Le Châtelier's principle refresher#

Le Châtelier's principle states that if a stress is applied to a system at equilibrium, the system will respond by shifting the equilibrium position to counteract the stress. This means that, if we add a reactant or a product to the system, the equilibrium will shift to maintain the balance between reactants and products. #Step 3: Effect of adding a salt of HB+ on the ionization of B#

If we add a salt which contains HB+ ions to the system, we are effectively increasing the concentration of HB+ in the system. According to Le Châtelier's principle, the system will respond by shifting the equilibrium to reduce the concentration of HB+ ions. This means the equilibrium will shift to the left, favoring the reactants over the products. As a result, the ionization of B decreases, which means fewer B molecules react with water to form HB+ and OH- ions. #Step 4: Example of a salt that can decrease the ionization of NH3#

We need to find a salt that contains a cation that can form a complex with NH3. One such example is a salt of ammonium ion (NH4+) which can decrease the ionization of NH3 in a solution. A common example of such a salt is ammonium chloride (NH4Cl). When NH4Cl is added to a solution containing NH3, the following reaction occurs: $$N{H}_4^{+}(aq)+N{H}_3(aq)\rightleftharpoons N{H}_{4}N{H}_3^{+}(aq)$$ As the equilibrium shifts to the right, the concentration of free NH3 decreases, which will subsequently decrease its ionization in an aqueous solution.

Key concepts

Chemical Equilibrium

Chemical equilibrium occurs when a chemical reaction proceeds such that the concentrations of reactants and products remain constant over time. At this point, the rate of the forward reaction equals the rate of the reverse reaction. In the reaction $B(aq)+H_{2}O(l)\rightleftharpoons H{B}^{+}(aq)+O{H}^{-}(aq)$, the system has reached equilibrium when the formation of $H{B}^{+}$ and $O{H}^{-}$ ceases to change measurably.

• The forward reaction involves the ionization of weak base $B$.
• The reverse reaction involves recombining $H{B}^{+}$ and $O{H}^{-}$ back to $B$.

Le Châtelier's Principle plays a crucial role in predicting how the equilibrium position will shift in response to changes in concentration, temperature, or pressure. If a change occurs in the condition of an equilibrium, the system will adjust to counteract that change.

Ionization

Ionization is the process by which a neutral molecule gains or loses electrons, thus forming ions. In the context of weak bases, ionization involves the reaction of the base with water to form a conjugate acid and a hydroxide ion. For example:$$B(aq)+H_{2}O(l)\rightleftharpoons H{B}^{+}(aq)+O{H}^{-}(aq)$$This equation shows the ionization of a weak base $B$, resulting in the production of $H{B}^{+}$, a conjugate acid, and $O{H}^{-}$. In general, the degree of ionization determines how many ions of the weak base and its conjugate acid are present in the solution.

• A higher level of ionization results in more ions and a stronger basic character.
• A lower level of ionization means fewer ions and a weaker basic character.

Weak Bases

Weak bases are substances that only partially ionize in aqueous solution. Unlike strong bases which completely dissociate in water, weak bases establish an equilibrium between the undissociated base and its ions. The equilibrium equation $B(aq)+H_{2}O(l)\rightleftharpoons H{B}^{+}(aq)+O{H}^{-}(aq)$ illustrates a weak base reacting with water.
Characteristics of weak bases include:

• Low degree of ionization in water.
• Presence of an equilibrium state.
• Formation of a conjugate acid and hydroxide ions.

Common examples of weak bases include ammonia ($N{H}_3$) and other amines. They are contrasted with strong bases like $NaOH$ which ionize completely.

Ammonium Chloride

Ammonium chloride ($N{H}_{4}Cl$) is an example of a salt that can influence the ionization of weak bases such as ammonia ($N{H}_3$). When dissolved in water, it gives $N{H}_4^{+}$ and $C{l}^{-}$ ions. The presence of $N{H}_4^{+}$ can shift the equilibrium of ammonia's ionization as seen in:$$N{H}_3(aq)+H_{2}O(l)\rightleftharpoons N{H}_4^{+}(aq)+O{H}^{-}(aq)$$If $N{H}_{4}Cl$ is added, it increases the $N{H}_4^{+}$ concentration, pushing the equilibrium to the left and reducing $N{H}_3$ ionization. This means fewer $O{H}^{-}$ ions are produced, slightly reducing the basicity of the solution.
Ammonium chloride is thus useful in moderating the basic characteristics of ammonia-containing solutions.

Conjugate Acid-Base Pairs

Conjugate acid-base pairs refer to a set of two species that transform into each other by the gain or loss of a proton. For example:- A base $B$ can accept a proton to become its conjugate acid $H{B}^{+}$.- Conversely, $H{B}^{+}$ can donate a proton to revert to $B$.In the given equilibrium reaction:$$B(aq)+H_{2}O(l)\rightleftharpoons H{B}^{+}(aq)+O{H}^{-}(aq)$$$B$ and $H{B}^{+}$ form a conjugate acid-base pair.
Conjugate pairs are integral in understanding reaction dynamics, especially concerning ionization. They illustrate how the presence of related ions affects equilibrium positions, as seen with buffer solutions and acidity/basicity adjustments.