Problem 23 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{NH}_{4} \mathrm{Cl}\) solution is (a) neutral (b) acidic (c) basic (d) amphoteric

Step-by-Step Solution

Answer

(b) acidic

Step 1: Analyze the Compound

Identify the components of the compound \( \mathrm{NH}_{4} \mathrm{Cl} \). It is composed of the ammonium ion \( \mathrm{NH}_4^+ \) and the chloride ion \( \mathrm{Cl}^- \).

Step 2: Consider the Nature of the Ions

The ammonium ion \( \mathrm{NH}_4^+ \) is the conjugate acid of ammonia \( \mathrm{NH}_3 \), which makes it act as a weak acid in solution. The chloride ion \( \mathrm{Cl}^- \) is the conjugate base of hydrochloric acid \( \mathrm{HCl} \), a strong acid, meaning it doesn't affect the acidity.

Step 3: Assess the Overall Solution

Since the ammonium ion adds acidic properties and the chloride ion does not contribute to basicity, the overall solution of \( \mathrm{NH}_{4} \mathrm{Cl} \) is acidic.

Key Concepts

Ammonium Ion

Ammonium ion, denoted as \( \mathrm{NH}_4^+ \), is a positively charged polyatomic ion. It is derived from ammonia \( \mathrm{NH}_3 \), which is a common compound containing nitrogen and hydrogen. When ammonia accepts a proton, it becomes the ammonium ion. This addition of a proton gives the \( \mathrm{NH}_4^+ \) ion its positive charge.

In aqueous solutions, the ammonium ion interacts with water to donate protons, which is typical behavior for acids. This is because \(\mathrm{NH}_4^+\) is the "conjugate acid" of ammonia. Thus, it has the tendency to release a proton and revert back to ammonia in the solution, slightly lowering the pH.

The significance of knowing whether a solution contains ammonium ions is that it allows us to predict its acidic behavior. In practical applications and experiments, understanding this can help in determining the nature of chemical reactions and their outcomes.

Conjugate Acid-Base Pairs

In acid-base chemistry, the concept of conjugate acid-base pairs helps us understand how substances interact in a solution to either donate or accept protons. A conjugate pair consists of a base and its corresponding acid, or an acid and its corresponding base.

Using the ammonium ion example: \( \mathrm{NH}_4^+ \) is the conjugate acid of the base \( \mathrm{NH}_3 \). This indicates that \( \mathrm{NH}_4^+ \) can donate a proton to revert to its base form, ammonia. Conversely, when \( \mathrm{NH}_3 \) accepts a proton, it becomes \( \mathrm{NH}_4^+ \), completing the cycle of conjugate acid-base interaction.

Understanding these pairs is crucial as they allow chemists to predict and explain the outcomes of acid-base reactions. For example, in an \( \mathrm{NH}_4\mathrm{Cl} \) solution, even though \( \mathrm{Cl}^- \) is present as the conjugate base of \( \mathrm{HCl} \), it doesn't participate in making the solution basic because \( \mathrm{HCl} \) is a strong acid and completely dissociates.

Solution Acidity

Acidity of a solution is mainly determined by the presence and concentration of hydrogen ions \( \mathrm{H}^+ \). When a substance like ammonium chloride \( \mathrm{NH}_4 \mathrm{Cl} \) dissolves in water, the ammonium ion \( \mathrm{NH}_4^+ \) contributes to the acidity by donating protons to the water, forming hydronium ions \( \mathrm{H}_3\mathrm{O}^+ \). This process occurs because \( \mathrm{NH}_4^+ \) is a weak acid.

Chloride ions \( \mathrm{Cl}^- \) are the conjugate base of strong hydrochloric acid \( \mathrm{HCl} \), and they have negligible effect on altering the pH of the solution. This is because strong acids like \( \mathrm{HCl} \) dissociate completely, leaving their conjugate base unable to reverse the process for any basic effect.

Therefore, in a \( \mathrm{NH}_4 \mathrm{Cl} \) solution, the overall effect is an acidic solution due to the presence of \( \mathrm{NH}_4^+ \), despite the existence of \( \mathrm{Cl}^- \). Understanding this is particularly important for fields focusing on chemical reactions, buffer solutions, and titrations, where control over pH is essential.