JEE Challenger
More from p-Block Elements (Group 15-18)
ChemMedAdv 2019 • Paper 2

Properties and Reactions of Aqua Regia with Gold

Regarding aqua regia, choose the correct option(s).

Options

A

Aqua regia is prepared by mixing conc. HCl\text{HCl} and conc. HNO3\text{HNO}_3 in a 3:13:1 (v/v)(v/v) ratio

Correct
B

Reaction of gold with aqua regia produces an anion having Au\text{Au} in +3+3 oxidation state

Correct
C

Reaction of gold with aqua regia produces NO2\text{NO}_2 in the absence of air

D

The yellow colour of aqua regia is due to the presence of NOCl\text{NOCl} and Cl2\text{Cl}_2

Correct

Topics & Concepts

p-Block Elements (Group 15-18)p-Block Elementsd-Block ElementsOxidation State

Step-by-Step Solution

To determine the correct options regarding aqua regia, let us analyze each statement individually:

  1. Option (A): Aqua regia (meaning "royal water") is a freshly prepared, highly corrosive liquid mixture formed by combining concentrated hydrochloric acid (HCl\text{HCl}) and concentrated nitric acid (HNO3\text{HNO}_3) in a 3:13:1 volume ratio (v/vv/v). Conc. HCl:Conc. HNO3=3:1(v/v)\text{Conc. } \text{HCl} : \text{Conc. } \text{HNO}_3 = 3 : 1 \quad (v/v) Thus, Option (A) is correct.

  2. Option (B): Gold (Au\text{Au}) is a noble metal that does not dissolve in concentrated nitric acid or hydrochloric acid individually, but dissolves in aqua regia due to the combined oxidizing action of HNO3\text{HNO}_3 and the complexing ability of Cl−\text{Cl}^- ions. The chemical equation for the reaction is: Au(s)+4H+(aq)+NO3−(aq)+4Cl−(aq)→[AuCl4]−(aq)+NO(g)+2H2O(l)\text{Au}(s) + 4\text{H}^+(aq) + \text{NO}_3^-(aq) + 4\text{Cl}^-(aq) \rightarrow [\text{AuCl}_4]^-(aq) + \text{NO}(g) + 2\text{H}_2\text{O}(l) The complex anion formed is the tetrachloroaurate(III) ion, [AuCl4]−[\text{AuCl}_4]^-.

    To determine the oxidation state of gold (Au\text{Au}) in [AuCl4]−[\text{AuCl}_4]^-: x+4(−1)=−1  ⟹  x=+3x + 4(-1) = -1 \implies x = +3 Therefore, gold is present in the +3+3 oxidation state in the resulting anion. Thus, Option (B) is correct.

  3. Option (C): From the balanced redox reaction between gold and aqua regia: NO3−(aq)+4H+(aq)+3e−→NO(g)+2H2O(l)\text{NO}_3^-(aq) + 4\text{H}^+(aq) + 3e^- \rightarrow \text{NO}(g) + 2\text{H}_2\text{O}(l) The gaseous product formed from the reduction of nitrate in the absence of air is nitric oxide (NO\text{NO}), not nitrogen dioxide (NO2\text{NO}_2). While NO\text{NO} rapidly reacts with oxygen in air to form reddish-brown NO2\text{NO}_2 gas (2NO+O2→2NO22\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2), in the absence of air, only NO\text{NO} is produced. Thus, Option (C) is incorrect.

  4. Option (D): When concentrated HCl\text{HCl} and concentrated HNO3\text{HNO}_3 are mixed, they undergo a chemical reaction to produce nitrosyl chloride (NOCl\text{NOCl}) and chlorine gas (Cl2\text{Cl}_2): 3HCl(aq)+HNO3(aq)→NOCl(g)+Cl2(g)+2H2O(l)3\text{HCl}(aq) + \text{HNO}_3(aq) \rightarrow \text{NOCl}(g) + \text{Cl}_2(g) + 2\text{H}_2\text{O}(l) The yellow to orange-yellow color characteristic of aqua regia is due to the presence of dissolved volatile nitrosyl chloride (NOCl\text{NOCl}) and free chlorine gas (Cl2\text{Cl}_2). Thus, Option (D) is correct.

Conclusion: The correct options are A, B, and D.

PreviousNext