Problem 137 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following of compound will not turn black on adding \(\mathrm{NH}_{4} \mathrm{OH}\) to it (a) \(\mathrm{Hg}_{2} \mathrm{Cl}_{2}\) (b) \(\mathrm{PbCl}_{2}\) (c) \(\mathrm{BaCl}_{2}\) (d) \(\mathrm{AgCl}\)

Step-by-Step Solution

Answer

The compound \(\mathrm{BaCl}_2\) will not turn black with \(\mathrm{NH}_4\mathrm{OH}\).

Step 1: Understand the Reaction with Ammonium Hydroxide

When ammonium hydroxide (\(\mathrm{NH}_4\mathrm{OH}\)) is added to certain metal chlorides, chemical reactions occur which may produce a black precipitate. Specifically, this is common with mercury(I) chloride (\(\mathrm{Hg}_2\mathrm{Cl}_2\)) and silver chloride (\(\mathrm{AgCl}\)), leading to the formation of mercury and silver, respectively, both of which are black in appearance.

Step 2: Identify Non-Reactive Compound

We need to identify the compound that does not form a black precipitate when ammonium hydroxide is added. \(\mathrm{PbCl}_2\) can react and lead to the formation of lead hydroxide, not a typical black precipitate. However, \(\mathrm{BaCl}_2\) does not react in such a way to produce a black substance, as barium does not form a black precipitate with ammonium hydroxide.

Step 3: Choose the Correct Option Based on Chemical Knowledge

Since \(\mathrm{Hg}_2\mathrm{Cl}_2\) and \(\mathrm{AgCl}\) are known to produce black precipitates, and \(\mathrm{PbCl}_2\) reacts but doesn't typically form a black precipitate with \(\mathrm{NH}_4\mathrm{OH}\), the compound that will certainly not turn black is \(\mathrm{BaCl}_2\).

Key Concepts

Ammonium Hydroxide Reactions

Ammonium hydroxide (\(\text{NH}_4\text{OH}\)) is a weak base that plays an essential role in many chemical reactions. When it encounters certain metal chlorides, it causes a series of interactions that can lead to various products, sometimes resulting in color changes. The reaction with \(\text{NH}_4\text{OH}\) is particularly notable for forming hydroxides from certain metal ions. This behavior stems from its ability to dissociate slightly in water to form hydroxide ions (\(\text{OH}^-\)), which are active in these reactions.

• This dissociation makes ammonium hydroxide a versatile reagent in chemical reactions and a common ingredient in many laboratory experiments.
• Its role in precipitate formation and affecting the color of solutions is significant and can help in the identification of metal ions.

Metal Chlorides

Metal chlorides are compounds that include a metal ion and chloride ions. The interactions of these compounds with other substances can lead to interesting chemical phenomena. Ammonium hydroxide can alter the state of metal chlorides by converting them into their corresponding hydroxides.

• \(\text{Hg}_2\text{Cl}_2\) (mercury(I) chloride) and \(\text{AgCl}\) (silver chloride) are both examples of metal chlorides that react with \(\text{NH}_4\text{OH}\) to produce characteristic products.
• These reactions lead to the formation of metallic mercury and silver, both dark substances that can provide visual evidence of the reaction.

Understanding how different metal chlorides interact with bases can be crucial for predicting the results of chemical reactions and for analytical techniques in chemistry.

Precipitate Formation

Precipitate formation is a common outcome in reactions involving ammonium hydroxide and metal chlorides. When a substance is formed in a reaction that is insoluble in water, it appears as a solid, known as a precipitate. The color and nature of this precipitate can often give insights into the reagents involved.

• For instance, when \(\text{NH}_4\text{OH}\) reacts with \(\text{Hg}_2\text{Cl}_2\), a black precipitate of mercury forms.
• Similarly, with \(\text{AgCl}\), the result is the formation of black metallic silver.

Barium chloride (\(\text{BaCl}_2\)), meanwhile, does not form a black precipitate when reacted with \(\text{NH}_4\text{OH}\). It remains soluble, which is an important detail in determining reaction outcomes.

Color Change in Reactions

Color change is a visual indicator of a chemical reaction occurring. In the reactions with ammonium hydroxide, specific metal chlorides undergo a change in color due to the formation of new compounds, such as elemental metals.

• This is particularly noticeable with metallic compounds like \(\text{AgCl}\) and \(\text{Hg}_2\text{Cl}_2\), where the introduction of \(\text{NH}_4\text{OH}\) results in a noticeable shift to black.
• These changes are not just fascinating to observe but also extremely useful in laboratory analyses for identifying unknown compounds or verifying the presence of certain ions.

Understanding the role of these visual cues can greatly aid in chemical experimentation and interpretation of results.