Chemistry · Redox Equilibrium · NEET
If you took all 5 oxygens as -2, you would get sulphur = +8, which is impossible because sulphur only has 6 valence electrons (it cannot lose more than 6). The catch is that H2SO5 has one peroxide linkage (an O-O bond). The two oxygens in that O-O bond are -1 each, not -2. So the charge balance is: 2(+1 for H) + x(S) + 3(-2 for normal O) + 2(-1 for peroxide O) = 0, giving x = +6.
Rule of thumb for NEET: whenever a sulphur oxoacid has the prefix 'peroxo' or 'peroxy' (Caro's acid H2SO5, Marshall's acid H2S2O8), there is at least one O-O bond, so sulphur stays capped at +6. If you calculate and get more than +6 for sulphur, you have missed a peroxide linkage.
No. A wrong calculation with all 8 oxygens as -2 gives sulphur = +7 (average). The correct structure has one peroxide linkage (two O at -1). Correct balance: 2(+1) + 2x + 6(-2) + 2(-1) = 0, so 2x = +12 and x = +6. Both sulphur atoms are +6.
In ordinary sulphuric acid H2SO4, all four oxygens are normal (-2), so sulphur is +6. In Caro's acid H2SO5 there is one extra oxygen forming a peroxide (O-O) linkage. Even though there is an extra oxygen, sulphur is still +6 because the extra oxygen sits in a peroxide bond at -1, not -2.
Most oxygens are the usual -2. But the two oxygens forming the O-O (peroxide) bond are -1 each. This is the whole trick: NEET wants you to treat the peroxide oxygens as -1 so that sulphur comes out to a sensible +6 instead of an impossible +8 or +7.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
+6. H2SO5 (Caro's acid) has one peroxide linkage, so two oxygens are -1 and the rest are -2, giving sulphur +6.
+6 for each sulphur. Marshall's acid has one peroxide linkage; correcting for the two -1 oxygens gives sulphur +6, not the wrong +7 average.
Sulphur has only 6 electrons in its valence shell (3s2 3p4), so its maximum oxidation state is +6. Any calculation giving +7 or +8 means a peroxide linkage was ignored.
A peroxide linkage is an oxygen-oxygen single bond (O-O). Each oxygen in it has oxidation number -1 instead of the usual -2.
Yes. Oxidation-state calculation with peroxide-linkage traps (H2SO5, H2S2O8, CrO5, H2O2) is a repeatedly favoured pattern in the Redox and p-block chapters.
