Question

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

• A. +5 only
• B. +3 and +5 only
• C. 0 and +5 only
  (D) +6 only
• D.

Hint:

Never rely solely on the average oxidation state calculated from a formula if you suspect atoms of the same element are bonded together (e.g., in peroxides, thiosulfates, polythionic acids). Always draw the structure and assign oxidation states based on electronegativity rules for each atom individually. The bond between two identical atoms contributes zero to the oxidation state of each.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We need to find the different oxidation states of sulfur atoms in the general structure of polythionic acids, H\(_2\)S\(_x\)O\(_6\).
Step 2: Key Formula or Approach:
Calculating an average oxidation state from the formula can be misleading when atoms of the same element are bonded to each other. The best method is to determine the oxidation states from the chemical structure of the molecule, assigning electrons based on electronegativity differences.
Step 3: Detailed Explanation:
Let's draw the general structure of a polythionic acid anion, S\(_x\)O\(_6^{2-}\). The structure consists of a chain of sulfur atoms, with two terminal sulfur atoms each bonded to three oxygen atoms. The general structure is: \[ \left[ \text{O}_3\text{S} - (\text{S})_{x-2} - \text{SO}_3 \right]^{2-} \] For example, for tetrathionic acid (x=4), the structure is HO\(_3\)S-S-S-SO\(_3\)H.
Let's analyze the oxidation states based on this structure:
Terminal Sulfur Atoms: Each of the two terminal sulfur atoms is bonded to three more electronegative oxygen atoms and one other sulfur atom. When a bond is between atoms of the same element (S-S), the electrons are shared equally, and this bond does not contribute to the oxidation state. We only consider the bonds to oxygen. Each terminal sulfur is double-bonded to two oxygens and single-bonded to one OH group. Assigning charges based on electronegativity, each double-bonded oxygen contributes +2 and the OH group contributes +1 to the sulfur's oxidation state. Total oxidation state = 2(+2) + 1(+1) = +5. Or, more simply, each terminal S is bonded to 3 O atoms and 1 S atom. The S-S bond contributes 0. For the S-O bonds, S gets +2 for each double bond and +1 for the single bond to the OH group's oxygen. Thus, the oxidation state is +5.
Central Sulfur Atoms: There are (x-2) sulfur atoms in the middle of the chain. Each of these sulfur atoms is bonded only to other sulfur atoms. Since there is no electronegativity difference, the oxidation state of these central sulfur atoms is 0.
So, in any polythionic acid (for x \(\ge\) 3), there are always two sulfur atoms with an oxidation state of +5 and (x-2) sulfur atoms with an oxidation state of 0.
Step 4: Final Answer:
The oxidation states of sulfur present in polythionic acids are +5 and 0. This corresponds to option (C).