Problem 6 - A Complete Resource Book in Chemistry for JEE Main

Question

In qualitative analysis \(\mathrm{NH}_{4} \mathrm{Cl}\) is added before \(\mathrm{NH}_{4} \mathrm{OH}\) (a) to increase \(\left[\mathrm{OH}^{-}\right]\)conc (b) for making \(\mathrm{HCl}\) (c) to decrease \(\left[\mathrm{OH}^{-}\right]\)conc (d) statement is wrong

Step-by-Step Solution

Answer

(c) to decrease \( \left[\mathrm{OH}^{-} \right] \) conc.

Step 1: Interpretation of the Question

The question asks us why \( \mathrm{NH}_{4} \mathrm{Cl} \) is added before \( \mathrm{NH}_{4} \mathrm{OH} \) in qualitative analysis. We are given four statements and must determine which is true.

Step 2: Understanding the Role of \( \mathrm{NH}_{4} \mathrm{Cl} \)

\( \mathrm{NH}_{4} \mathrm{Cl} \) dissociates into \( \mathrm{NH}_{4}^{+} \) and \( \mathrm{Cl}^{-} \) ions in solution. The presence of \( \mathrm{NH}_{4}^{+} \) ions can influence the concentration of \( \mathrm{OH}^{-} \) ions.

Step 3: Analyzing the Reaction Environment

Adding \( \mathrm{NH}_{4} \mathrm{Cl} \) increases the \( \mathrm{NH}_{4}^{+} \) ion concentration. Since \( \mathrm{NH}_{4} \mathrm{OH} \) is a weak base, the addition of \( \mathrm{NH}_{4}^{+} \) from \( \mathrm{NH}_{4} \mathrm{Cl} \) will influence the equilibrium position of the \( \mathrm{NH}_{4} \mathrm{OH} \leftrightarrow \mathrm{NH}_{4}^{+} + \mathrm{OH}^{-} \) reaction, reducing the \( \mathrm{OH}^{-} \) concentration (common ion effect).

Step 4: Conclusion

The addition of \( \mathrm{NH}_{4} \mathrm{Cl} \) is used to decrease the \( \left[ \mathrm{OH}^{-} \right] \) concentration in the solution due to the common ion effect of \( \mathrm{NH}_{4}^{+} \). Therefore, option (c) is the correct statement.

Key Concepts

Common Ion Effect

The common ion effect is an important concept in chemistry that helps us understand how substances in a solution can influence each other. When a compound like ammonium chloride \( \mathrm{NH}_{4} \mathrm{Cl} \) is dissolved in water, it dissociates into its constituent ions, \( \mathrm{NH}_{4}^{+} \) and \( \mathrm{Cl}^{-} \). When these ions are present, they can affect the equilibrium of other related ions in the solution.

• The presence of \( \mathrm{NH}_{4}^{+} \) in solution is the key player in the common ion effect in this scenario.
• It works by shifting the equilibrium according to Le Chatelier's Principle.
• This principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change.

In this case, the addition of more \( \mathrm{NH}_{4}^{+} \) ions from \( \mathrm{NH}_{4} \mathrm{Cl} \) can reduce the formation of \( \mathrm{OH}^{-} \) ions by pushing the equilibrium of the weak base \( \mathrm{NH}_{4} \mathrm{OH} \) to favor the formation of more \( \mathrm{NH}_{4}^{+} \) ions. Therefore, the common ion effect plays a crucial role in controlling the concentration of ions in the solution.

Ammonium Chloride

Ammonium chloride (\( \mathrm{NH}_{4} \mathrm{Cl} \)) is a common compound used in laboratories, particularly in qualitative analysis. It is a white crystalline salt that is highly soluble in water, meaning it breaks apart easily to form ammonium ions (\( \mathrm{NH}_{4}^{+} \)) and chloride ions (\( \mathrm{Cl}^{-} \)). This solubility is key to its role in the common ion effect.

• The introduction of ammonium chloride into a solution increases the \( \mathrm{NH}_{4}^{+} \) ions concentration.
• This, in turn, affects reactions involving \( \mathrm{OH}^{-} \) ions, especially when working with weak bases like \( \mathrm{NH}_{4} \mathrm{OH} \).

Ammonium chloride is often added to solutions to create a controlled environment where we can predict and manage reactions better. It essentially helps in the suppression of base ionization due to the presence of common ions, making it a valuable tool for chemists who need precise control over the chemical environment.

Hydroxide Ion Concentration

The hydroxide ion concentration (\( \left[ \mathrm{OH}^{-} \right] \)) in a solution is an important factor in determining its basicity. In many chemical reactions, it's crucial to control this concentration for desired outcomes.

• A common scenario is in the presence of a weak base like \( \mathrm{NH}_{4} \mathrm{OH} \), where the ability to maintain a low \( \mathrm{OH}^{-} \) concentration can be beneficial.
• Adding an abundant amount of \( \mathrm{NH}_{4}^{+} \) ions, through compounds like ammonium chloride, effectively lowers the \( \mathrm{OH}^{-} \) concentration.
• This decrease happens because the system adjusts to maintain equilibrium, favoring the formation of more \( \mathrm{NH}_{4}^{+} \) and less \( \mathrm{OH}^{-} \).

By understanding and manipulating the hydroxide ion concentration, chemists can control the acidity or basicity of the solution they are working with, which is crucial for various chemical processes and analytical methods.