Reactivity with Water & Hydrolysis
Elemental inertness, the $SF_6$ vs $SF_4$ anomaly, and Acid Rain.
When studying the interaction of Group 16 elements (Chalcogens) with water, we must distinguish between the free elements and their compounds. The elements themselves are famously unreactive towards water under normal conditions, but their halides and oxides undergo highly specific and frequently tested hydrolysis reactions.
1. Reaction of Free Elements with Water
As a general rule, the free elements of Group 16 do not chemically react with cold or hot water.
- Oxygen ($O_2$): Oxygen gas dissolves sparingly in water physically ($\sim 30 \text{ cm}^3$ per liter at room temperature). This physical dissolution is absolutely vital for the survival of aquatic life, but it does not involve a chemical reaction (redox) with the water molecules.
- Sulfur ($S_8$), Selenium, Tellurium: Completely insoluble and inert to cold and boiling water.
If Sulfur is heated to a very high temperature (red heat) and steam is passed over it, a chemical reaction finally occurs. The sulfur reduces the water, producing Hydrogen Sulfide ($H_2S$) and Sulfur Dioxide ($SO_2$).
2. Hydrolysis of Halides (A High-Yield Topic)
Because the elements themselves are mostly inert, competitive exams focus almost entirely on the hydrolysis of Chalcogen Halides. The most famous comparison in all of inorganic chemistry is between $SF_4$ and $SF_6$.
The Classic Anomaly: $SF_4$ vs. $SF_6$
When added to water, Sulfur tetrafluoride ($SF_4$) hydrolyzes instantly and violently. However, Sulfur hexafluoride ($SF_6$) bubbles through water completely unaffected. Why does the addition of two fluorine atoms completely stop the reaction?
$SF_4$ has a See-Saw geometry. The central sulfur atom has one lone pair, leaving a large, open physical space (steric accessibility) and available vacant 3d-orbitals. The water molecule easily attacks the exposed Sulfur atom.
(Sulfur dioxide & Hydrogen fluoride)
$SF_6$ has a perfect Octahedral geometry. Six fluorine atoms tightly pack around the central sulfur atom. The sulfur is physically blocked from all sides. Water cannot reach the sulfur to initiate an attack. This is known as Severe Steric Hindrance.
Figure 1: The six fluorine atoms act as a physical shield, preventing the water molecule's oxygen lone pair from interacting with the central sulfur atom.
While $SF_6$ is inert, Tellurium hexafluoride ($TeF_6$) does undergo hydrolysis! Because Tellurium is a much larger atom than Sulfur, six fluorine atoms are not enough to completely cover its surface. Water can still squeeze in and attack the exposed Tellurium atom.
3. Hydrolysis of Oxides (Acid Rain)
Group 16 elements are non-metals/metalloids, meaning their oxides are acidic. When dissolved in water, these oxides form their corresponding oxoacids. This is the exact chemical mechanism behind Acid Rain.
Dissolves in water to form weak Sulfurous Acid.
Reacts violently with water to form strong Sulfuric Acid.
Mastery Check: Hydrolysis & Water Reactivity
15 High-Yield Questions to test your JEE/NEET Preparation
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