Question

A yellow metallic powder is burnt in a stream of fluorine to obtain a colourless gas A which is thermally stable and chemically inert. Its molecule has octahedral geometry. Another colourless gas B with same constituent atoms as that of \(\mathrm{A}\) is obtained when sulphur dichloride is heated with sodium fluoride. Its molecule has trigonal bipyramidal geometry. Gases \(\mathrm{A}\) and \(\mathrm{B}\) are respectively (a) \(\mathrm{SF}_{6}\) and \(\mathrm{SF}_{4}\) (b) \(\mathrm{SF}_{4}\) and \(\mathrm{SF}_{6}\) (c) \(\mathrm{SF}_{4}\) and \(\mathrm{S}_{2} \vec{F}_{2}\) (d) \(\mathrm{KCl}\) and \(\mathrm{KBr}\)

Short answer

Gases A and B are \\(\mathrm{SF}_{6}\text{ and }\mathrm{SF}_{4}\), respectively (option a).

Step-by-step solution

Identify the Metal in the Yellow Powder

The yellow metallic powder mentioned is likely elemental sulfur (S), given its common color and reactivity with fluorine to form sulfur fluorides. We've established sulfur is likely a component in the compound solutions.

Understanding the Formation of Gas A

Gas A is formed when sulfur reacts with fluorine. It is described as having an octahedral geometry and is known to be chemically inert and thermally stable, fitting the characteristics of sulfur hexafluoride, \(\mathrm{SF}_6\). \(\mathrm{SF}_6\) is inert due to its strong S-F bonds and 3D symmetric arrangement.

Identify the Formation of Gas B

Gas B is produced when sulfur dichloride is reacted with sodium fluoride. Given it has the same constituent atoms as gas A, it would contain sulfur and fluorine, and it has a trigonal bipyramidal geometry, which is characteristic of sulfur tetrafluoride, \(\mathrm{SF}_4\). \(\mathrm{SF}_4\) has one sulfur with four fluorines, forming a see-saw shape which can be described as trigonal bipyramidal.

Matching the Geometries and Characteristics

Gas A, being \(\mathrm{SF}_6\), matches the description of octahedral geometry and inert nature. Gas B is likely \(\mathrm{SF}_4\), with trigonal bipyramidal geometry. Thus, option (a) aligns with the chemical and structural properties identified: \(\mathrm{SF}_{6}\) is gas A, and \(\mathrm{SF}_{4}\) is gas B.

Key concepts

Sulfur Hexafluoride

Sulfur Hexafluoride, known chemically as \(\mathrm{SF}_6\), is a colorless gas that exhibits unique properties. It is formed from sulfur and fluorine and is renowned for its exceptional stability and lack of reactivity under normal conditions due to strong sulfur-fluorine bonds.

This compound adopts an octahedral geometry, where six fluorine atoms are symmetrically arranged around a central sulfur atom. This geometry provides high stability and renders \(\mathrm{SF}_6\) chemically inert, meaning it doesn't react easily with other substances. Its inertness makes it particularly useful in applications requiring non-reactive environments, such as in electrical circuits as an insulating gas.

In summary, \(\mathrm{SF}_6\)'s key traits include:

• Octahedral geometry for a stable, balanced structure.
• Strong, stable sulfur-fluorine bonds.
• Chemical inertness, useful in specialized industrial applications.

Octahedral Geometry

Octahedral Geometry is a term used to describe a molecular structure where a central atom is surrounded symmetrically by six other atoms. In the case of \(\mathrm{SF}_6\), the central atom is sulfur, and the surrounding atoms are fluorine.

This structure, resembling two pyramids placed base to base, allows for optimal spatial distribution, minimizing repulsion among the electron pairs that surround the central atom.

Here are some defining features of octahedral geometry:

• Six-fold symmetry around the central atom.
• Equal bond lengths and angles, specifically angles of 90 degrees.
• Typically found in molecules that are highly symmetrical and stable.

Octahedral geometry is significant in stabilizing molecules, making it invaluable where stability is desired, as seen in \(\mathrm{SF}_6\).

Chemical Reactivity

Chemical Reactivity refers to the tendency of a substance to engage in chemical reactions. In the realm of sulfur fluorides, the reactivity can be limited or extensive based on their molecular structure and bond strength.

\(\mathrm{SF}_6\), with its strong sulfur-fluoride bonds and symmetrical octahedral geometry, is virtually chemically inert. This implies it resists reactions even under aggressive conditions, contributing to its use in applications requiring stability and non-reactive environments.

In contrast, sulfur tetrafluoride (\(\mathrm{SF}_4\)) displays a different reactivity profile. Its less symmetrical, trigonal bipyramidal structure and weaker bonds make it more reactive. This can lead to reactions with other substances under suitable conditions, unlike \(\mathrm{SF}_6\).

Key aspects of chemical reactivity to remember:

• Structural symmetry often influences reactivity; more symmetry generally means less reactivity.
• Bond strength is crucial—stronger bonds imply less reactivity.
• The nature and environment of the chemical species can change its reactivity profile.

Thus, understanding the structural influences on chemical reactivity helps anticipate how different sulfur fluorides might behave chemically.