Question

Why the oxidation state of s is -2and +4 in h2s2o2?

Answer 1

2(1) + x + 2(-2) = 2
2 + x + -4 = 2
x = 4

Answer 2

The two sulfur atoms in thiosulphurous acid acquire the oxidation states -2 and +4 so that the molecule becomes neutral, which can be explained based on the structure of the compound.

• Oxidation state of an atom is an imaginary charge that indicates the number of electrons gained or lost during a bond formation.
• The oxidation state of a neutral compound as a whole is zero.
• Thiosulphurous acid has two sulfur, two oxygen, and two hydrogen atoms.
• One of the sulfur is bonded to two hydroxyl groups(-OH) through two single bonds, and by a double bond to the other sulfur atom.
• The other sulfur atom has no other bonds except the double bond with the first sulfur atom.
• The structure of the thiosulphurous acid is depicted in the diagram attached. (Check for the attachment)
• As per the structure, one of the sulfur atoms has the normal charge, -2.
• The other sulfur atom acquires the oxidation state, +4, to balance the charges surrounding it, which include:

1.  -1 of one of the hydroxyl groups.
2.  -1 of the other hydroxyl group.
3.  -2 of the doubly bonded sulfur atom.

• Thus the oxidation state of the other sulfur atom is opposite to that of $[(-1)+(-1)+(-2)] = -4$ , that is +4.
• Hence, the oxidation states of the two sulfur atoms are -2 and +4.