Question

A buffer contains significant amounts of ammonia and ammonium chloride. Write equations showing how this buffer neutralizes added acid and added base.

Short answer

The buffer neutralizes added acid through the reaction: NH3 + H+ -> NH4+. It neutralizes added base via the reaction: NH4+ + OH- -> NH3 + H2O.

Step-by-step solution

Neutralizing Added Acid

When an acid (H+) is added to the buffer, the ammonia (NH3) present in the buffer reacts with the protons (H+) to form ammonium ions (NH4+). Write the chemical reaction for this process: NH3 + H+ -> NH4+.

Neutralizing Added Base

When a base (OH-) is added, the ammonium ions (NH4+) present in the buffer donate protons (H+) to neutralize the hydroxide ions (OH-), forming water (H2O) and ammonia (NH3). The reaction is: NH4+ + OH- -> NH3 + H2O.

Key concepts

Acid-Base Neutralization

Understanding acid-base neutralization is key to grasping how buffer solutions work. In chemistry, this process refers to the reaction where an acid and a base react to form water and a salt. This is a type of double-displacement reaction represented by the general equation:
\[ \text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{H}_2\text{O} \]
In the context of a buffer solution containing ammonia (\text{NH}_{3}) and ammonium chloride (\text{NH}_{4}\text{Cl}), when an acid is added, it donates protons (\text{H}^+) that are readily accepted by the ammonia molecules, creating ammonium ions (\text{NH}_{4}^+). Conversely, when a base is added, it supplies hydroxide ions (\text{OH}^-) that combine with protons from the ammonium ions in the buffer to form water and regenerate ammonia.

By understanding this, students can see that the role of a buffer is to maintain pH stability by neutralizing any added acids or bases without drastically changing the pH of the solution.

Ammonia and Ammonium Chloride Buffer

The ammonia and ammonium chloride buffer is a classic example of a buffer system. It consists of a weak base, ammonia (\text{NH}_{3}), and its conjugate acid, the ammonium ion (\text{NH}_{4}^+), from ammonium chloride. This combination is crucial because they can react with added acids or bases to prevent significant changes in the pH level of the solution.

Reaction with Added Acid:

When an acid is introduced to the buffer, the equation \[ \text{NH}_{3} + \text{H}^+ \rightarrow \text{NH}_{4}^+ \] shows ammonia acting as a base, accepting protons to form more ammonium ions.

Reaction with Added Base:

In the case of added base, the ammonium ions act as an acid, releasing protons to neutralize the base through the reaction \[ \text{NH}_{4}^+ + \text{OH}^- \rightarrow \text{NH}_{3} + \text{H}_2\text{O} \]
These reactions highlight the dual role of the components in the buffer, which can switch between providing and accepting protons (\text{H}^+) depending on what is added to the solution, thus maintaining a stable pH level.

Chemical Equilibrium

The concept of chemical equilibrium is integral to understanding how buffers operate. Equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of reactants and products over time. It's represented by the dynamic equilibrium expression:
\[ \text{Reactants} \rightleftharpoons \text{Products} \]
In the buffer solution, the ammonia and ammonium ions are constantly converting into each other. The presence of both components means that whether an acid or base is added, the system can shift to counteract the pH change, aligning with Le Chatelier's principle. This principle states that, if a dynamic equilibrium system is disturbed, it will adjust to minimize the disturbance and re-establish a state of equilibrium.

So, when \text{H}^+ is added, more \text{NH}_{3} is converted to \text{NH}_{4}^+. If \text{OH}^- is added, the reaction shifts to produce more \text{NH}_{3} and water. These shifts allow the buffer solution to accommodate changes while maintaining equilibrium and a relatively constant pH. Understanding chemical equilibrium helps students grasp how delicate and dynamic the balance within a buffer solution truly is.