Problem 52 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will not give positive chromyl chloride test? (a) zinc chloride (b) copper chloride, \(\mathrm{CuCl}_{2}\) (c) mercuric chloride, \(\mathrm{HgCl}_{2}\) (d) anilinium chloride \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3} \mathrm{Cl}\)

Step-by-Step Solution

Answer

Mercuric chloride (HgCl2) will not give a positive chromyl chloride test.

Step 1: Understanding Chromyl Chloride Test

To solve this problem, we need to understand that the chromyl chloride (CrO2Cl2) test is specific for detecting chloride ions from ionic chlorides. It involves reacting a chloride compound with potassium dichromate (K2Cr2O7) and concentrated sulfuric acid (H2SO4), resulting in a red vapor of chromyl chloride, CrO2Cl2. This test is only positive for ionic chlorides, primarily of alkali and alkaline earth metals.

Step 2: Analyzing Each Option

Analyze each compound to see if it will produce ionic chloride (Cl-) ions that react to form chromyl chloride. (a) Zinc chloride (ZnCl2) dissociates in water to give Zn²⁺ and Cl⁻ ions, hence will give a positive test. (b) Copper(II) chloride (CuCl2) is an ionic compound that dissociates into Cu²⁺ and Cl⁻ ions, so it will give a positive test. (c) Mercuric chloride (HgCl2) exists largely as an associated molecule rather than dissociating into ions completely, hence it will not give a positive chromyl chloride test. (d) Anilinium chloride (C6H5NH3Cl) will dissociate in solution to give C6H5NH3⁺ and Cl⁻ ions; thus, it could give a positive chromyl chloride test.

Step 3: Concluding the Answer

Based on the analysis, the compound that will not give the positive chromyl chloride test is mercuric chloride, HgCl2, because it does not dissociate into free chloride ions due to its molecular nature.

Key Concepts

Ionic Chlorides

Ionic chlorides are compounds made up of positively charged cations and negatively charged chloride ions. These chlorides are usually derived from metals and are particularly prevalent among alkali and alkaline earth metals. When dissolved in water, ionic chlorides readily dissociate into their constituent ions.
This dissociation is key for most chemical reactions involving chlorides. For example, sodium chloride (NaCl) splits into sodium (Na⁺) and chloride (Cl⁻) ions when it dissolves in water. The behavior of ionic chlorides in aqueous solutions is crucial in many industrial and laboratory chemical procedures.

• Common examples of ionic chlorides include sodium chloride (NaCl), potassium chloride (KCl), and calcium chloride (CaCl₂).
• These compounds tend to have high melting and boiling points due to the strong electrostatic forces between the ions.
• The solubility of ionic chlorides in water makes them useful in applications such as de-icing roads and as preservatives in food.

Understanding ionic chlorides is fundamental for predicting the outcome of various chemical tests, including the chromyl chloride test.

Chloride Ions

Chloride ions, denoted as Cl⁻, are the negatively charged particles formed when chlorine gains an electron. They play a vital role as an indicator in many chemical reactions.
Chloride ions are omnipresent in both natural and industrial settings. In the human body, chloride ions help maintain fluid balance and are part of essential biological processes.

• Chloride ions are detected in chemical tests, such as the chromyl chloride test, which is used to identify their presence in a compound.
• In water treatment processes, chloride ions are used to control microbial growth.
• They often act as a counterpart to various metal cations in ionic compounds.

In chemical analysis, the presence of free chloride ions is crucial. They participate in various reactions, highlighting their versatility and importance in scientific studies.

Detection of Chlorides

Detecting chlorides is a fundamental task in chemistry, and one of the methods is using the chromyl chloride test. This test specifically identifies the presence of chloride ions in ionic chlorides by forming chromyl chloride, which is indicated by a red vapor.
The procedure involves reacting the suspected chloride compound with potassium dichromate ( K₂Cr₂O₇ ) and concentrated sulfuric acid ( H₂SO₄ ). If chloride ions are present, chromyl chloride ( CrO₂Cl₂ ) is formed, which is identifiable through its characteristic vapor.

• The test is limited to detecting chlorides from ionic compounds, specifically those of alkali and alkaline earth metals.
• Chlorides from non-ionic compounds or those not forming chloride ions in solution, like mercuric chloride, do not yield a positive result.
• This test is a qualitative analytical technique and is used primarily in educational and research settings.

While effective, understanding the limitations of the chromyl chloride test is crucial for accurately interpreting results in chemical analysis.