Question

What is the oxidation state of sulfur in Marshall's acid, \(H_2S_2O_8\)?

• A. \(+4\)
• B. \(+5\)
• C. \(+6\)
• D. \(+7\)

Hint:

Always check for peroxide linkage \(( -O-O- )\) in compounds like:
• \(H_2O_2\)
• \(H_2S_2O_8\)
• \(Na_2O_2\) In peroxide linkage, oxygen has oxidation state \(-1\), not \(-2\).

Answer 1

The Correct Option is C

Concept: The oxidation state (oxidation number) of an element represents the apparent charge it would possess if all bonds were completely ionic. While calculating oxidation states in a compound, the following standard rules are commonly used:
• Hydrogen generally has oxidation state \(+1\).
• Oxygen generally has oxidation state \(-2\).
• In peroxides and peroxide linkages \(( -O-O- )\), oxygen has oxidation state \(-1\).
• The algebraic sum of oxidation states of all atoms in a neutral compound is zero. Marshall's acid is also called peroxodisulfuric acid and has the molecular formula: \[ H_2S_2O_8 \] It contains a peroxide linkage \(( -O-O- )\), which is extremely important while calculating oxidation state.

Step 1: Identifying the special peroxide oxygen atoms.
In \(H_2S_2O_8\):
• Two oxygen atoms are present in peroxide linkage \(( -O-O- )\).
• These peroxide oxygen atoms each have oxidation state \(-1\).
• The remaining six oxygen atoms have normal oxidation state \(-2\).

Step 2: Assigning oxidation states to all atoms.
Let oxidation state of each sulfur atom be \(x\). Now calculate contribution from each atom: \[ \text{Hydrogen: } 2(+1)=+2 \] \[ \text{Peroxide oxygen: } 2(-1)=-2 \] \[ \text{Remaining oxygen: } 6(-2)=-12 \] \[ \text{Sulfur: } 2x \]

Step 3: Forming the oxidation number equation.
Since the molecule is neutral: \[ 2x + 2 - 2 - 12 = 0 \] Simplifying: \[ 2x - 12 = 0 \] \[ 2x = 12 \] \[ x = 6 \]

Step 4: Final conclusion.
Therefore, the oxidation state of sulfur in Marshall's acid is: \[ \boxed{+6} \] Hence, the correct option is: \[ \boxed{(3)\ +6} \]