Outline the Contact process for the manufacture of sulphuric acid, giving the key equations and reaction conditions.

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Accepted Answer

The Contact process manufactures sulphuric acid in three main stages. Stage 1: sulphur (or sulphide ores) is burnt in air to give sulphur dioxide: $\text{S} + \text{O}_2 \rightarrow \text{SO}_2$ Stage 2: the $\text{SO}_2$ is catalytically oxidised with more oxygen over a vanadium(V) oxide catalyst, at about 720 K and 2 bar pressure: $2\text{SO}_2 + \text{O}_2 \underset{720\text{ K},\ 2\text{ bar}}{\overset{\text{V}_2\text{O}_5}{\rightleftharpoons}} 2\text{SO}_3$ Stage 3: rather than adding the $\text{SO}_3$ directly to water (which would form a hard-to-condense acid mist), it is absorbed into concentrated sulphuric acid to give oleum (pyrosulphuric/disulphuric acid): $\text{SO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{S}_2\text{O}_7$ The oleum is then carefully diluted with the correct amount …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set A1 markMCQ
$Q. The peroxy acid of sulphur is$
(a) H2S2O7
(b) H2S2O8
(c) H2SO4
(d) H2SO3
›Reveal solutionSolution

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 …

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CBSE 2026Set SEM31 markMCQ
$Q. Which of the following shows temporary bleaching action through reduction?$
(a) H2O2
(b) O3
(c) SO2
(d) Cl2
›Reveal solutionSolution
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. …
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CBSE 2026Set SEM31 markMCQ
$Q. Which of the following compounds does not contain per-oxo-linkage?$
(a) H2O2
(b) H2SO5
(c) H2S2O8
(d) H2S2O7
›Reveal solutionSolution
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. …
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CBSE 2021Set A1 markMCQ
$Q. Which of the following has pπ - dπ bond?$
(a) NO3^-
(b) CO3^2-
(c) BO3^2-
(d) SO3^2-
›Reveal solutionSolution

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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CBSE 2016Set ANNUAL1 markMCQ
$H_2S_2O_2$
(a) Hydrosulphurous acid
(b) Thiosulphurous acid
(c) Sulphuric acid
(d) Pyrosulphurous acid
›Reveal solutionSolution

$H_2S_2O_2$ = 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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Q.Outline the Contact process for the manufacture of sulphuric acid, giving the key equations and reaction conditions.

Concept understanding — Oxoacids of Sulphur

Q.Outline the Contact process for the manufacture of sulphuric acid, giving the key equations and reaction conditions.

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