Give reasons for the following: (a) Au does not dissolve in concentration \(\mathrm{HCl}\) or concentration \(\mathrm{HNO}_{3}\) but dissolves in aqua regia. (b) \(\mathrm{HgCl}_{2}\) is reduced by \(\mathrm{SnCl}_{2}\) but remains unaffected with \(\mathrm{FeSO}_{4}\).

Exploring the unique capabilities of aqua regia reveals a fascinating interaction with one of the most treasured elements on the periodic table: gold. Gold (symbol Au) is known for its gleaming luster and resistance to tarnish, a feature that typically makes it impervious to reaction with individual acids such as hydrochloric acid (HCl) or nitric acid (HNO_3).

However, combine HCl and HNO_3 in a 3:1 ratio to form aqua regia, and you create a potent mixture that even gold cannot withstand. The reaction occurs because aqua regia generates chlorine gas in situ, along with nitrosyl chloride (NOCl) and nitric oxide (NO). These reactive substances efficiently break down gold into its ionic form, resulting in its dissolution.

The mechanism hinges on a redox reaction where the gold is oxidized, and the chlorine from aqua regia acts as the reducing agent. This remarkable dissolution property is why aqua regia is one of the few substances that can claim to 'conquer' gold, used extensively by chemists when gold needs to be dissolved for analysis or recovery.

Redox reactions are the bread and butter of inorganic chemistry, serving as the fundamental processes that facilitate the transfer of electrons between elements and compounds. Standing for 'reduction-oxidation', these reactions are dual-layered events where one reactant gives up electrons - undergoing oxidation, while the other accepts electrons - undergoing reduction.

To understand redox reactions, consider the changing oxidation states of the elements involved. Using the example from the exercise, when mercuric chloride (HgCl2) reacts with stannous chloride (SnCl2), a redox reaction ensues. Stannous chloride is the reducing agent that donates electrons to mercuric chloride, which is then reduced to mercurous chloride (Hg2Cl2). The tin in stannous chloride becomes oxidized in the process. In contrast, iron(II) sulfate (FeSO4) lacks the potential to carry out this reduction, hence, HgCl2 is not affected by it.

The ability to recognize and balance redox reactions is essential for students of chemistry, since it plays a crucial role not only in theoretical equations but also in real-world applications like battery technology, metallurgy, and biogeochemical cycles.

Noble metals, a select group including gold, silver, and the platinum group metals, are characterized by their exceptional resistance to corrosion and oxidation. These metals are well-regarded both for their intrinsic shine and stability, which are attributed to their minimal reactivity.

Electronically, noble metals have filled d sub-shells, which contribute to their low affinity for gaining additional electrons and thus, their pronounced inertness. From an electrochemical standpoint, noble metals exhibit positive electrode potentials, indicating that they are not easily inclined to lose electrons or corrode - they are poor oxidizing agents.

This stability makes noble metals highly valuable in a variety of sectors, from jewelry to electronics to catalysis. They do not readily form compounds with other elements, ensuring that they retain their luster and physical properties over time even when exposed to air or moisture. However, as shown by the aqua regia example, noble metals can be coaxed into reaction under aggressive chemical circumstances.