Question

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

Short answer

The buffer neutralizes added acid through the reaction NH\(_3\) + H\(^+\) → NH\(_4$$^+\), and neutralizes added base through the reaction NH\(_4$$^+\) + OH\(^-\) → NH\(_3\) + H\(_2\)O.

Step-by-step solution

Identify the Buffer Components

The buffer is comprised of weak base ammonia (NH\(_3\)) and its conjugate acid ammonium chloride (NH\(_4\)Cl). Ammonia reacts with protons (H\(^+\)) to form ammonium ions (NH\(_4$$^+\)), while ammonium ions (NH\(_4$$^+\)) can release protons (H\(^+\)) to become ammonia (NH\(_3\)).

Neutralizing Added Acid

When an acid is added to the buffer, it releases protons (H\(^+\)) into the solution. The ammonia (NH\(_3\)) present in the buffer reacts with the protons to form ammonium ions (NH\(_4$$^+\)), thereby removing the extra H\(^+\) from the solution. The equation for this reaction is: NH\(_3\) + H\(^+\) → NH\(_4$$^+\).

Neutralizing Added Base

When a base is added to the buffer, it provides hydroxide ions (OH\(^-\)) in the solution. The ammonium ions (NH\(_4$$^+\)) present in the buffer react with the hydroxide ions to form water (H\(_2\)O) and ammonia (NH\(_3\)), which removes OH\(^-\) from the solution. The equation for this reaction is: NH\(_4$$^+\) + OH\(^-\) → NH\(_3\) + H\(_2\)O.

Key concepts

Ammonia and Ammonium Chloride Buffer

Understanding buffer solutions is essential in chemistry, particularly when dealing with solutions that must maintain a stable pH. An excellent example of such a buffer is a mixture of ammonia ((NH_3)) and ammonium chloride ((NH_4Cl)). In this buffer, ammonia acts as a weak base, while ammonium chloride acts as its conjugate acid. Here's how the equilibrium works: when the pH drops due to the addition of an acid, ammonia molecules in the buffer react with the excess hydrogen ions ((H^+)) to form ammonium ions ((NH_4^+)), which reduces the acidity.

Conversely, if the pH increases when a base is added, the ammonium ions in the buffer react with hydroxide ions ((OH^-)) to form water ((H_2O)) and release ammonia. Thanks to this push-and-pull dynamic, this ammonia-ammonium chloride buffer maintains a relatively stable pH in the solution, preventing large shifts in acidity or basicity that can disrupt biological or chemical processes.

Neutralization Reactions

Neutralization reactions are a type of chemical reaction where an acid and a base react to form water and a salt. This process is crucial in buffer systems, including our textbook example of the ammonia-ammonium chloride buffer. In the context of this buffer, the neutralization reactions would involve the weak base (ammonia) or the conjugate acid (ammonium ions) reacting with added acids or bases.

When an acid is introduced, ammonia neutralizes the additional hydrogen ions ((H^+)), forming ammonium ions ((NH_4^+)). On the other hand, when a base is added, the ammonium ions neutralize hydroxide ions ((OH^-)), producing water and freeing up ammonia molecules. These reactions exemplify how buffers neutralize added acidic or basic substances to keep the pH of the solution within a narrow range.

Acid-Base Equilibrium

Acid-base equilibrium is the balance between acid and base concentrations in a solution. It is an important concept in buffer systems, which rely on the presence of a weak acid and its conjugate base (or vice versa) to resist changes in pH. The principle at work is Le Chatelier’s principle, which predicts that a system will react to minimize the change caused by a disturbance.

In our ammonia-ammonium chloride buffer, the equilibrium shifts to minimize the impact of added acids or bases. When excess (H^+) ions are introduced, the reaction proceeds to form more (NH_4^+) ions, while additional (OH^-) ions cause a shift that generates more (NH_3) and water. These shifts in equilibrium allow the solution to maintain a fairly constant pH, illustrating the effectiveness of buffer systems in regulating acid-base balance.