Question

\(\mathrm{NH}_{4} \mathrm{Cl}\) solution is: (a) Neutral (b) Acidic (c) Basic (d) Amphoteric

Short answer

(b) Acidic

Step-by-step solution

Understanding the Components

First, identify the ions present in the \(\mathrm{NH}_{4} \mathrm{Cl}\) solution. Ammonium chloride dissolves in water to form \(\mathrm{NH}_4^+\) and \(\mathrm{Cl}^-\) ions.

Determine the Nature of Ions

- \(\mathrm{NH}_4^+\) is the ammonium ion, which can donate a proton to water, making the solution acidic.- \(\mathrm{Cl}^-\) is the chloride ion, which is a weak conjugate base of \(\mathrm{HCl}\) and does not affect the pH significantly.

Assess the Overall Solution

The \(\mathrm{NH}_4^+\) ion makes the solution slightly acidic as it contributes protons to the solution, causing a decrease in pH.

Key concepts

Ammonium Ion

The ammonium ion, denoted as \( \mathrm{NH}_4^+ \), is a fascinating chemical species. It is derived from ammonia \( \mathrm{NH_3} \), a common and well-known compound. When ammonia accepts an extra proton (\( \mathrm{H}^+ \)), it transforms into the ammonium ion. This ability to accept a proton makes ammonium a conjugate acid. In an aqueous solution, \( \mathrm{NH}_4^+ \) can donate this proton back to the water, forming \( \mathrm{NH_3} \) again and releasing a hydrogen ion (\( \mathrm{H}^+ \)) into the solution.
This release of hydrogen ions is what characterizes the solution as being acidic. When a solution is acidic, it means that there's an increase in the concentration of \( \mathrm{H}^+ \) ions, which subsequently decreases the pH value. The presence of more hydrogen ions makes the environment more acidic, making \( \mathrm{NH}_4Cl \) a resourceful example to illustrate this behavior.

Chloride Ion

Chloride ion, symbolized as \( \mathrm{Cl}^- \), is the negative charge counterpart that forms when hydrochloric acid (\( \mathrm{HCl} \)) dissociates. While \( \mathrm{HCl} \) is a strong acid, \( \mathrm{Cl}^- \) is a very weak conjugate base, which means it does not readily accept protons back. In water, chloride ions do not significantly alter the acidity or basicity of a solution.
Therefore, even though \( \mathrm{Cl}^- \) is present in the \( \mathrm{NH}_4Cl \) solution, it does not contribute to making the solution more or less acidic. Its primary role in the context of \( \mathrm{NH}_4Cl \) is providing electrical neutrality in the solution. It balances the positive charge of the \( \mathrm{NH}_4^+ \) ions without influencing the overall pH.

pH and Acidity

The pH scale is a fundamental concept in chemistry used to gauge the acidity or alkalinity of a solution. A pH range from 0 to 14 is typically used, where lower pH values indicate acidic conditions, and higher pH values denote basic conditions. A pH of 7 is considered neutral, representing the pH of pure water.
In the case of \( \mathrm{NH}_4Cl \), the pH of the solution is influenced by the ammonium ion. As \( \mathrm{NH}_4^+ \) ionizes in water, it releases protons, lowering the pH below 7, indicating an acidic solution. This effect is because the additional protons increase the \( \mathrm{H}^+ \) ion concentration in the solution. It's important to note that the pH is not significantly affected by the chloride ion since it is inert in terms of influencing acidity.

Dissolution of Salts

The dissolution of salts, such as \( \mathrm{NH}_4Cl \), in water involves a straightforward process. A salt is composed of cations (positively charged ions) and anions (negatively charged ions) held together by ionic bonds. When salt dissolves in water, these bonds are broken, and the ions become surrounded by water molecules and disperse across the solution.
For \( \mathrm{NH}_4Cl \), the dissolution process separates it into \( \mathrm{NH}_4^+ \) and \( \mathrm{Cl}^- \) ions. This dissociation is what allows each ion to interact independently within the solution. While the ammonium ion can affect the pH (making the solution acidic), the chloride ion simply exists to balance charge without affecting pH levels. Understanding how salts dissolve highlights why solutions may exhibit certain properties, such as acidity or neutrality, based on the behavior of their constituent ions.