Q.What are peroxoacids? Describe the structures of Caro's acid ($\text{H}_2\text{SO}_5$) and Marshall's acid ($\text{H}_2\text{S}_2\text{O}_8$).

H2S2O8 (peroxydisulphuric acid) contains a peroxide (-O-O-) bond and is the peroxy acid of sulphur.

A peroxy acid contains a peroxide linkage (-O-O-). Among the given acids, H2S2O8 (peroxydisulphuric acid, Marshall's acid, HO3S-O-O-SO3H) has this -O-O- bridge and is a peroxy acid. (H2SO5, Caro's acid, is the peroxymonosulphuric acid, but it is not one of the options.) H2S2O7 is pyrosulphuric (oleum) a …

SO2 bleaches by reduction (it removes oxygen / adds hydrogen to the dye); the bleached material re-oxidises in air, so the effect is temporary. Correct option (c).

Mechanisms of bleaching:

• SO2 (with moisture) acts as a reducing agent: dye + reduction -> colourless product. On standing in air, air oxidises it back, so the colour reappears - a temporary bleach (used for wool, silk, straw).
• Cl2, O3 and H2O2 bleach by oxidation, destroying the coloured substance permanently. …

H2O2, Caro's acid (H2SO5) and Marshall's acid (H2S2O8) all contain an -O-O- peroxo linkage; H2S2O7 (pyrosulphuric acid) does not. Correct option (d).

Checking the structures:

• H2O2: H-O-O-H, contains a peroxo (-O-O-) linkage.
• H2SO5 (Caro's acid, peroxomonosulphuric): has one -O-O-H group -> peroxo linkage present.
• H2S2O8 (Marshall's acid, peroxodisulphuric): has an -O-O- bridge between the two sulphur atoms -> peroxo linkage present. …

Only sulphur (period 3) has vacant 3d orbitals, so SO3^2- shows pπ-dπ bonding; NO3-, CO3^2-, BO3^2- (period-2 central atoms) cannot.

pπ-dπ bonding requires the central atom to have empty d-orbitals that can overlap with filled p-orbitals of oxygen.

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$H_2S_2O_2$ = thiosulphurous acid.

The 'thio' prefix in oxoacid nomenclature indicates that one oxygen atom of the parent oxoacid has been replaced by a sulphur atom. Sulphurous acid is $H_2SO_3$; replacing one O with S gives $H_2SO_2S$, whose molecular formula is $H_2S_2O_2$ — this is thiosulphurous acid.

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