Question

In Hydrogen chloride is added to a buffer solution of ammonia, \(\mathrm{NH}_{3}\), and ammonium chloride, \(\mathrm{NH}_{4} \mathrm{Cl}\). What is the effect on the concentration of ammonia? On the concentration of ammonium chloride?

Short answer

The concentration of ammonia decreases, while ammonium ions increase.

Step-by-step solution

Understand the Buffer System

The buffer solution consists of ammonia ( NH_3 ) and ammonium chloride ( NH_4Cl ). Ammonia acts as a weak base, while ammonium chloride is its salt, providing ammonium ions ( NH_4^+ ) that help maintain the pH.

Determine the Reaction with HCl

When hydrogen chloride (HCl) is added to the buffer, it dissociates completely into H^+ and Cl^− . The H^+ ions will react with the base component of the buffer, which is ammonia ( NH_3 ), to form ammonium ions ( NH_4^+ ).

Effect on Ammonia Concentration

The addition of H^+ ions decreases the concentration of ammonia ( NH_3 ) as it is converted into ammonium ions ( NH_4^+ ) through the reaction: NH_3 + H^+ ightarrow NH_4^+.

Effect on Ammonium Ion Concentration

As ammonia is consumed to form more NH_4^+ ions, the concentration of ammonium ions increases. This is due to the chemical equilibrium shifting to produce more ammonium ions.

Key concepts

Ammonia

Ammonia, also known by its chemical formula \( \mathrm{NH}_{3} \), is a colorless gas with a distinct, pungent smell. It is a common weak base in chemistry.

• Weakly basic: Ammonia is considered a weak base because it does not fully dissociate in water. Instead, it partially ionizes, producing hydroxide ions \( \mathrm{OH}^{-} \) and ammonium ions \( \mathrm{NH}_{4}^{+} \).
• Poor conductor of electricity: Due to its weak ionization, ammonia solutions are poor conductors compared to strong bases.
• Role in buffers: As part of a buffer solution, ammonia reacts with acids to maintain a stable pH. When hydrogen ions (\( \mathrm{H}^{+} \)) are introduced, ammonia neutralizes them by forming ammonium ions.

Controlling the pH with ammonia makes it valuable in many industrial and laboratory settings where pH stability is crucial.

Ammonium Chloride

Ammonium chloride, with the chemical formula \( \mathrm{NH}_{4} \mathrm{Cl} \), is the salt of ammonia and hydrochloric acid. It plays an important role in buffer solutions.

• Source of ammonium ions: \( \mathrm{NH}_{4} \mathrm{Cl} \) dissociates in solution to release ammonium ions (\( \mathrm{NH}_{4}^{+} \)) and chloride ions (\( \mathrm{Cl}^{-} \)).
• Buffer component: In combination with ammonia, it helps stabilize pH. The ammonium ions can recombine with hydroxide ions to moderate changes in pH caused by added acids or bases.
• Solubility: It is highly soluble in water, which ensures a good supply of \( \mathrm{NH}_{4}^{+} \) ions in the solution.

Because of its role in buffer systems, ammonium chloride is often utilized in chemical and pharmaceutical applications to regulate acidity levels.

Chemical Equilibrium

Chemical equilibrium is a state where the concentration of reactants and products remains constant over time because the forward and reverse reactions occur at the same rate.

• Dynamic process: Even though the concentrations do not change, both reactions continue to happen simultaneously.
• Le Châtelier’s principle: This principle describes how a system at equilibrium responds to disturbances, such as changes in concentration, pressure, or temperature. For instance, adding \( \mathrm{HCl} \) to the ammonia buffer increases the \( \mathrm{H}^{+} \) concentration, shifting the equilibrium to favor the formation of more \( \mathrm{NH}_{4}^{+} \) ions.
• Buffer equilibrium: In a buffer, equilibrium helps resist drastic pH changes. As the ammonia becomes ammonium, the overall pH remains relatively constant despite the addition of acids.

Understanding chemical equilibrium is vital for predicting and controlling how different chemical systems will respond to changes.

Weak Bases

Weak bases are substances that do not completely dissociate in water. Ammonia is a classic example.

• Partial ionization: Weak bases, like ammonia, only partially ionize in solution, leading to a mixture of undissociated base molecules and ions.
• Moderate pH: Solutions of weak bases tend to have a relatively higher pH than neutral water but are not as alkaline as solutions of strong bases.
• pH buffering capacity: By reacting slowly with acids, weak bases can moderate changes in pH, making them essential components of buffers.

These characteristics make weak bases very useful in chemical processes where gradual pH adjustments are essential or desired.