In polythionic acid, H2SxO6 (x = 3 to 5) the oxidation state(s) of sulphur is/are
(a) only + 5
(b) o

Q: Why can't we just use the average oxidation state formula here?

Using the average formula (like $2(+1) + x(\text{S}) + 6(-2) = 0$) gives a fractional oxidation state, which doesn't represent the actual physical state of individual atoms. Drawing the structure reveals the true, distinct oxidation states of different sulphur atoms.

Q: Why is the oxidation state of the central sulphur atoms exactly zero?

Oxidation state is determined by the electronegativity difference between bonded atoms. Since the central sulphur atoms are only bonded to other sulphur atoms, there is no electronegativity difference, meaning no electrons are 'pulled' away or 'gained'. Hence, their oxidation state is $0$.

Q: What happens if $x=2$ in this formula?

If $x=2$, the compound is dithionic acid ($\text{H}_2\text{S}_2\text{O}_6$). In this case, there are no central sulphur atoms—just two terminal sulphur atoms bonded directly to each other. Both would have an oxidation state of $+5$.

Q: Are all polythionic acids stable?

Polythionic acids with a small number of sulphur atoms (typically $x=3$ to $5$) are the most stable. As the sulphur chain gets longer, the compounds become increasingly unstable and prone to decomposition.

Q: How does oxygen's electronegativity affect the terminal sulphur?

Oxygen is highly electronegative. Each double-bonded oxygen pulls two electrons' worth of density away from sulphur ($+2$ each), and the single-bonded $\text{OH}$ group pulls one ($+1$). This totals an oxidation state of $+5$ for the terminal sulphur.

Question

In polythionic acid, $	ext{H}_2	ext{S}_x	ext{O}_6$ ($x = 3$ to $5$) the oxidation state(s) of sulphur is/are

Options:

(A) only $+ 5$

(B) only $+ 6$

(C) $+ 3$ and $+ 5$

(D) $0$ and $+ 5$

Sigma Percentile · Accepted Answer

### Decoding Polythionic Acid

When we encounter a chemical formula like $	ext{H}_2	ext{S}_x	ext{O}_6$, it can initially look intimidating. This is the general formula for polythionic acids, a fascinating class of compounds where sulphur atoms link together to form a straight chain. The variable $x$ typically ranges from 3 to 5, representing the total number of sulphur atoms in the molecule.

To truly understand the properties of this molecule, we cannot rely on simple mathematical averages. We must draw its structure.

### The Power of Structure

Imagine the molecule as a bridge. At both ends of this bridge, we have identical anchor points: the sulphonic acid groups ($	ext{SO}_3	ext{H}$). These terminal groups consist of a sulphur atom double-bonded to two oxygen atoms and single-bonded to a hydroxyl ($	ext{OH}$) group.

Connecting these two anchors is a straight chain of central sulphur atoms. If the total number of sulphur atoms is $x$, and two are used at the terminals, the central chain contains $x-2$ sulphur atoms.

### Calculating Oxidation States

Now, let's determine the oxidation states by looking at the electronegativity of the bonded atoms.

**The Terminal Sulphur Atoms:**
Oxygen is significantly more electronegative than sulphur. Each double-bonded oxygen pulls two electrons' worth of density away from the sulphur, contributing $+2$ to its oxidation state. The single-bonded $	ext{OH}$ group pulls one electron's worth of density, contributing $+1$. 
Therefore, the oxidation state of each terminal sulphur atom is:
$$+2 + 2 + 1 = +5$$

**The Central Sulphur Atoms:**
The central sulphur atoms are in a unique position. They are bonded *only* to other sulphur atoms. Because there is absolutely no difference in electronegativity between identical atoms, no electron density is pulled away or gained. 
Therefore, the oxidation state of every central sulphur atom is exactly:
$$0$$

### The Final Verdict

By breaking down the structure, we've uncovered a beautiful chemical reality. The sulphur atoms in polythionic acid do not share a single, fractional oxidation state. Instead, they exist in two distinct states depending on their position in the molecule. The terminal atoms sit at **$+5$**, while the central atoms sit at **$0$**.

This perfectly matches our option, proving that structure is the ultimate key to unlocking chemical mysteries!

Animated Solution for Chemistry - s and p-Block Elements: In polythionic acid, $H_{2} S_{x} O_{6}$ ( $x = 3$ to $5$ ) the oxidation state(s) of sulphur is/are

Select Answer:

Visualized Solution

$P o l y t hi o ni c A c i d F or m u l a$

$S t r u c t u r e A na l y s i s$

$O x i d a t i o n S t a t eo f T er mina l S u l p h u r$

$O x i d a t i o n S t a t eo f C e n t r a l S u l p h u r$

$F ina l C o n c l u s i o n$

The Sigma Insight: Group 16 Elements

Decoding Polythionic Acid

The Power of Structure

Calculating Oxidation States

The Final Verdict

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Animated Solution for Chemistry - s and p-Block Elements: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } In polythionic acid, H2​Sx​O6​ (x=3 to 5) the oxidation state(s) of sulphur is/are