Concept Explanation: Aqua Regia and its Reactions

Aqua regia is a highly corrosive mixture of acids, primarily used to dissolve noble metals like gold and platinum. Let's analyze each option step by step.

Step 1: Understanding the Composition of Aqua Regia

Aqua regia is prepared by mixing concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO₃) in a specific volume ratio. The standard ratio is 3 parts HCl to 1 part HNO₃ (3:1 v/v). This mixture is crucial because it generates reactive species that can oxidize gold.

The reaction involved in the preparation is:

$\mathrm{HNO}3+3\mathrm{HCl}\to \mathrm{NOCl}+\mathrm{Cl}2+2H_{2}O$

Nitrosyl chloride (NOCl) and chlorine (Cl₂) are formed, which are responsible for the yellow-orange color of fresh aqua regia and its strong oxidizing ability.

Step 2: Reaction of Gold with Aqua Regia

Gold (Au) is dissolved by aqua regia to form chloroauric acid. The oxidation state of gold changes from 0 to +3 in this process.

The overall reaction is:

$\mathrm{Au}+\mathrm{HNO}3+4\mathrm{HCl}\to \mathrm{HAuCl}4+\mathrm{NO}+2H_{2}O$

In this reaction, nitric acid oxidizes gold, and hydrochloric acid provides chloride ions to form the stable complex anion [AuCl₄]^-. Nitrogen monoxide (NO) is produced, not nitrogen dioxide (NO₂). However, NO can react with oxygen from air to form NO₂ (brown fumes), but in the absence of air, NO₂ is not produced.

Step 3: Evaluating the Options

Option 1: "Reaction of gold with aqua regia produces NO₂ in the absence of air" - This is incorrect. The direct product is NO, which requires oxygen (from air) to form NO₂.

Option 2: "Reaction of gold with aqua regia produces an anion having Au in +3 oxidation state" - This is correct. The product is [AuCl₄]^-, where gold has an oxidation state of +3.

Option 3: "Aqua regia is prepared by mixing conc. HCl and conc. HNO₃ in 3:1 (v/v) ratio" - This is correct. The standard preparation uses this ratio.

Option 4: "The yellow colour of aqua regia is due to presence of NOCl and Cl₂" - This is correct. Both nitrosyl chloride and chlorine gas contribute to the characteristic color.

Final Answer

The correct options are:

• Reaction of gold with aqua regia produces an anion having Au in +3 oxidation state.
• Aqua regia is prepared by mixing conc. HCl and conc. HNO₃ in 3:1 (v/v) ratio.
• The yellow colour of aqua regia is due to presence of NOCl and Cl₂.

Related Topics

• Oxidation States and Redox Reactions
• Complex Ion Formation
• Properties of Noble Metals
• Strong Oxidizing Agents

Key Formulae and Theory

Aqua Regia Preparation:

$\mathrm{HNO}3+3\mathrm{HCl}\to \mathrm{NOCl}+\mathrm{Cl}2+2H_{2}O$

Gold Dissolution:

$\mathrm{Au}+\mathrm{HNO}3+4\mathrm{HCl}\to \mathrm{HAuCl}4+\mathrm{NO}+2H_{2}O$

Oxidation State in Complex: In [AuCl₄]^-, oxidation state of Au is +3. (x + 4*(-1) = -1, so x = +3)

Color Theory: The yellow color is due to the formation of NOCl and Cl₂, which are intense colored gases.