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Question

Which of the following ammonium salt, release ${SO}_{3}$ gas on thermal decomposition:

I$\left( {NH}_{4} \right)_{2}{SO}_{4}\overset{\phantom{\Delta\ }}{\rightarrow}$
II$\left( {NH}_{4} \right)_{2}{\text{ }S}_{2}O_{8}\overset{\phantom{\Delta\ }}{\rightarrow}$

Options

A.I only
B.II only
C.Both I and II
D.None of these

Solution

Sol. Key concept: Thermal decomposition pathways depend on conditions.

For scoring this paper, the IAPT final key gives (d), “None of these”. The chemistry needs qualification: ammonium sulfate and ammonium peroxydisulfate do not each have a single universally valid thermal decomposition equation under unspecified conditions.

An initial de ammoniation step for ammonium sulfate is:

$$(NH_{4})_{2}SO_{4} \rightarrow \ NH_{4}HSO_{4} + NH_{3}$$

Further heating can involve dehydration, pyrosulfate formation and sulfur-containing gaseous products. Peroxydisulfate additionally contains a peroxide linkage, so its decomposition is different from that of an ordinary sulfate.

Correction / clarification: The previous categorical statement that SO₃ cannot be released, and its blanket explanation based on trapping by NH₃, were not justified. Published thermal studies show that product pathways depend on heating rate, apparatus and other conditions; SO₃ formation has been reported under some conditions. Retain (d) as the official key, but flag the condition-free question as scientifically under-specified. [R5]

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