Problem 41 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Give reasons for the following : (i) Sulphur dioxide is a more powerful reducing agent in an alkaline medium than in acidic medium. [1992-1 Mark] (ii) Valency of oxygen is generally two whereas sulphur shows valency of two, four and six. [1988-1 Mark] (iii) Sulphur melts to a clear mobile liquid at \(119^{\circ} \mathrm{C}\), but on further heating above \(160^{\circ} \mathrm{C}\), it becomes viscous.

Step-by-Step Solution

Answer

(i) In alkaline medium, sulfite ions are easily oxidized. (ii) Oxygen lacks d-orbitals; sulfur can expand octet. (iii) Sulfur forms long chains above 160°C, increasing viscosity.

Step 1: Sulphur Dioxide as a Reducing Agent

In the alkaline medium, sulphur dioxide ( ext{SO}_2) reacts with hydroxide ions to form sulfite ions ( ext{SO}_3^{2-}), which have a higher tendency to transform into sulphate ions ( ext{SO}_4^{2-}) by losing electrons; thus acting as a strong reducing agent. In acidic medium, this transformation is less favored, making ext{SO}_2} a weaker reducing agent.

Step 2: Oxygen and Sulphur Valency

Oxygen typically forms two bonds due to its electron configuration ext{(2s}^2 2p^4 ext{)}, needing two more electrons to complete its octet. Sulphur, with its electron configuration ext{(3s}^2 3p^4 3d^0 ext{)}, can expand its valency shell by utilizing the 3d orbitals, allowing it to form more than two bonds, thus showing valencies of two, four, and six.

Step 3: Sulphur's Melting Properties

At ext{119}^{ ext{oC}}, sulphur molecules ext{(S}_8 ext{)} start to move freely, forming a clear liquid. However, above ext{160}^{ ext{oC}}, the ring-shaped ext{S}_8 molecules break open and form long chains, which increase in entanglement and thus make the liquid more viscous.

Key Concepts

Sulphur Dioxide as a Reducing Agent

Sulphur dioxide ( ext{SO}_2) is a fascinating compound when it comes to its reducing capabilities. The influence of the medium, whether alkaline or acidic, plays a crucial role in its effectiveness as a reducing agent. In an alkaline environment, ext{SO}_2} interacts with hydroxide ions (OH^-). This interaction facilitates the transformation of ext{SO}_2 into sulfite ions ( ext{SO}_3^{2-}).

• The sulfite ions have a stronger propensity to oxidize to sulphate ions ( ext{SO}_4^{2-}).
• During this process, electrons are readily given up, enhancing ext{SO}_2’s reducing potential.

In contrast, when in an acidic medium, acid suppresses this transformation.

• ext{SO}_2} does not transition as readily to ext{SO}_3^{2-}.
• As a result, ext{SO}_2} acts as a weaker reducing agent.

Valency of Oxygen and Sulphur

Understanding the valency differences between oxygen and sulphur requires a clear grasp of their electron configurations. Oxygen typically displays a valency of two. Its electron configuration is ext{(2s}^2 2p^4 ext{)}, which means it needs two additional electrons to achieve a stable octet configuration. Thus, it forms two bonds in most of its compounds.

• This is why we often see oxygen bound in a two-bond formation.

Sulphur, on the other hand, is more versatile in its bonding capabilities due to its electron configuration ext{(3s}^2 3p^4 3d^0 ext{)}.

• Sulphur can use its empty 3d orbitals to expand its valency shell.
• This allows it to display valencies of two, four, and six.
• The ability to form more than two bonds makes sulphur's chemistry rich and varied compared to oxygen.

Sulphur's Melting Properties

Sulphur exhibits unique properties when it is heated, reflecting its structural transformation at different temperatures. At around 119°C, sulphur melts into a clear and mobile liquid. This state is comprised principally of ext{S}_8 molecules, which are ring-shaped structures. As the temperature rises above 160°C, these ext{S}_8 rings break open and start assembling into long chains.

• These chains introduce complexity and entanglement to the structure.
• Because of this increased entanglement, the liquid becomes more viscous.

This intriguing transformation highlights sulphur's ability to change its physical state and properties in response to heat. Its capability to form both rings and chains provides further insights into its diverse applications and behaviors.