Oxoacids of Sulfur
Master the Structures, Linkages, and Oxidation State Traps.
Sulfur forms a numerous and complex series of oxoacids. While some are highly stable and industrially vital (like Sulfuric acid), others exist only in aqueous solutions or as their corresponding salts. In competitive exams, you are rarely asked for deep physical properties; instead, examiners will aggressively test you on the chemical linkages ($S-S, S-O-S, S-O-O-S$) and the resulting oxidation states of sulfur.
1. General Structural Rules
Before memorizing the specific acids, you must know the fundamental rules governing the structure of all sulfur oxoacids:
- Hybridization & Geometry: The central sulfur atom is $sp^3$ hybridized. It is surrounded tetrahedrally by oxygen atoms or hydroxyl groups (and sometimes lone pairs).
- Core Bonds: Every single oxoacid of sulfur contains at least one $S=O$ (double bond) and at least one $S-OH$ (hydroxyl group).
- Basicity: Only the hydrogen atoms attached to oxygen (in the $-OH$ groups) are ionizable. The number of $S-OH$ bonds directly determines the basicity of the acid.
- Complex Linkages: In acids with more than one sulfur atom, the S atoms can be linked by an $S-S$ bond, an $S-O-S$ bridge, or an $S-O-O-S$ peroxide bridge.
2. The "ic" and "ous" Series
The suffix tells you the general oxidation state: "-ous" acids typically have sulfur in the +4 oxidation state, while "-ic" acids have sulfur in the +6 oxidation state.
- Oxidation State of S: +4
- Basicity: Dibasic (2 $S-OH$ bonds)
- Structure: The S atom has 1 lone pair, 1 $S=O$ bond, and 2 $S-OH$ bonds. (Pyramidal shape).
- Nature: Exists only in aqueous solution. Acts as a strong reducing agent.
- Oxidation State of S: +6
- Basicity: Dibasic (2 $S-OH$ bonds)
- Structure: The S atom has 0 lone pairs, 2 $S=O$ bonds, and 2 $S-OH$ bonds. (Tetrahedral shape).
- Nature: The "King of Chemicals." Powerful dehydrating agent.
3. Polymeric & Bridged Oxoacids
| Name | Formula | Key Structural Feature | Notes / O.S. |
|---|---|---|---|
| Pyrosulfuric Acid (Oleum) |
$H_2S_2O_7$ | $S-O-S$ linkage | Formed by absorbing $SO_3$ into $H_2SO_4$. Oxidation state of both S atoms is +6. |
| Dithionic Acid | $H_2S_2O_6$ | $S-S$ direct bond | The two S atoms are bonded directly to each other. O.S. is +5. |
| Thiosulfuric Acid | $H_2S_2O_3$ | $S=S$ double bond | One O atom of sulfuric acid is replaced by an S atom. |
4. The Peroxo Acids: The Ultimate Exam Traps
In standard nomenclature, oxygen is assigned an oxidation state of -2. However, these two acids contain a peroxide linkage ($-O-O-$). In a peroxide linkage, each oxygen atom has an oxidation state of -1. If you ignore this and calculate the oxidation state of Sulfur algebraically, you will get an impossible answer!
WRONG! Sulfur belongs to Group 16; its maximum possible oxidation state is +6.
The Reality: It contains one peroxide linkage. Therefore, 3 oxygens are normal (-2), and 2 oxygens are peroxide (-1).
$2(+1) + x + 3(-2) + 2(-1) = 0 \Rightarrow \mathbf{x = +6}$
WRONG!
The Reality: It contains one $S-O-O-S$ peroxide bridge. Therefore, 6 oxygens are normal (-2), and 2 oxygens are peroxide (-1).
$2(+1) + 2x + 6(-2) + 2(-1) = 0 \Rightarrow 2x = +12 \Rightarrow \mathbf{x = +6}$
Figure 1: Comparison of S-O-S linkage in Oleum and the S-O-O-S peroxide linkage in Marshall's acid.
Mastery Check: Oxoacids of Sulfur
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