Question

What is the oxidation state of sulfur in Marshall's acid (H$_2$S$_2$O$_8$)?

• A. +4
• B. +5
• C. +6
• D. +7

Hint:

If oxidation state comes out greater than +6 for sulfur (or +7 for chlorine, etc.), always check for peroxide (-O-O-) or similar abnormal oxygen bonding situations.

Answer 1

The Correct Option is C

Concept: The oxidation state of an atom is the hypothetical charge it would have if all bonds were treated as ionic. In compounds containing peroxide linkages (\(-\mathrm{O{-}O}-\)), oxygen does not always have an oxidation state of \(-2\); instead, each peroxide oxygen is assigned \(-1\). This is crucial in compounds like peroxydisulfuric acid (Marshall's acid), H$_2$S$_2$O$_8$. Sulfur belongs to Group 16 and has a maximum oxidation state of +6.

Step 1: Applying the naive oxidation state method.
Assuming H = +1 and all O = -2: \[ 2(+1) + 2x + 8(-2) = 0 \] \[ 2 + 2x - 16 = 0 \] \[ 2x = 14 \Rightarrow x = +7 \] This result is impossible because sulfur cannot exceed +6, indicating incorrect treatment of oxygen atoms.

Step 2: Recognizing peroxide linkage in structure.
Marshall's acid (H$_2$S$_2$O$_8$) has the following structure: \[ HO-SO_2-O-O-SO_2-OH \] Oxygen atoms are classified as:
• 6 oxygen atoms with oxidation state -2 (double bonded O and OH oxygens)
• 2 peroxide oxygen atoms with oxidation state -1 (-O-O- linkage)

Step 3: Correct oxidation state calculation.
\[ 2(+1) + 2x + 6(-2) + 2(-1) = 0 \] \[ 2 + 2x - 12 - 2 = 0 \] \[ 2x - 12 = 0 \] \[ x = +6 \] Thus, oxidation state of sulfur is +6.

Step 4: Verification using valence rule.
Sulfur is in Group 16 and its maximum oxidation state is +6. The result matches this limit, confirming correctness.