Read Peroxo and S-S Linkages Before Naming a Sulfur Oxoacid
Sulfur's oxoacid family varies along two independent structural features — whether a peroxo (O-O) linkage is present, and whether a direct S-S bond is present — and most naming/identification questions reduce to checking for one or both.
Sulfur Dioxide and Other Oxygen-Family Compounds is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Peroxoacids of sulfur contain an O-O linkage
Caro's acid (H2SO5, peroxomonosulfuric acid) and peroxodisulfuric acid (H2S2O8) both contain a peroxo (-O-O-) linkage, distinguishing them from the non-peroxo acids like pyrosulphuric acid (H2S2O7, oleum's acid) which has an S-O-S bridging oxygen instead of a peroxo bond — same-looking formula pattern, genuinely different bonding.
S-S bonds and differing sulfur oxidation states
In H2S4O6 (tetrathionic acid), the two central sulfur atoms are joined by an S-S bond and sit at a DIFFERENT oxidation state than the two terminal (SO3-type) sulfurs — this is why a species can have 'sulfur in different oxidation states' within one formula. Polythionic acids (H2SnO6) generally contain (n-2) S-S bonds in the central chain, one less than a naive atom count might suggest.
SO2's bleaching action is a reduction, not oxidation or hydrolysis
SO2 bleaches by REDUCING the coloured compound (unlike chlorine's oxidative bleaching) — the bleaching is often reversible with time as atmospheric oxygen slowly re-oxidises the reduced (colourless) product back to its coloured form, a classic distinguishing test between chlorine-bleached and sulfur-dioxide-bleached material.
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- ✓Peroxoacids (Caro's acid H2SO5, H2S2O8) contain an O-O linkage; pyrosulphuric acid (H2S2O7) does not — it has a bridging S-O-S oxygen instead.
- ✓H2S4O6 has two sulfurs joined by an S-S bond, at a different oxidation state than the two terminal sulfurs.
- ✓Polythionic acid H2SnO6 contains (n-2) S-S bonds in its chain.
- ✓SO2 bleaches by reduction (often reversible); this is the opposite mechanism to chlorine's oxidative (generally permanent) bleaching.
Don't assume every sulfur oxoacid with a similar formula pattern (H2SxOy) has the same bonding — check specifically for a peroxo (O-O) linkage versus a bridging S-O-S oxygen versus a direct S-S bond, since these are three structurally distinct features that formulas alone don't distinguish.
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