Oxygen is not released on heating which of the compounds?

The correct option is (NH₄)₂Cr₂O₇.

Let us analyze the chemical reactions that take place when each of the given compounds is heated to understand why oxygen gas is not released from ammonium dichromate.

1. Thermal Decomposition of Ammonium Dichromate:
When ammonium dichromate is heated, it undergoes an intramolecular redox reaction. The ammonium ion (${\mathrm{NH}}_4^{+}$), which contains nitrogen in its -3 oxidation state, is oxidized to nitrogen gas ($N_2$), while the chromium in the dichromate ion (${\mathrm{Cr}}_2{O}_7^{2-}$) is reduced from its +6 oxidation state to +3, forming green chromium(III) oxide (${\mathrm{Cr}}_2{O}_3$). The reaction is written as follows:

$({\mathrm{NH}}_4{)}_2{\mathrm{Cr}}_2{O}_7\to N_2+{\mathrm{Cr}}_2{O}_3+4H_{2}O$

As we can observe from the balanced chemical equation, the products are nitrogen gas, chromium(III) oxide, and water vapor. No oxygen gas ($O_2$) is released in this process.

2. Thermal Decomposition of Potassium Dichromate:
Unlike the ammonium salt, potassium dichromate does not contain a reducing cation. On strong heating, it decomposes to form potassium chromate, chromium(III) oxide, and oxygen gas:

$4K_2{\mathrm{Cr}}_2{O}_7\to 4K_2{\mathrm{CrO}}_4+2{\mathrm{Cr}}_2{O}_3+3O_2$

Here, oxygen gas is released.

3. Thermal Decomposition of Chlorates:
Metal chlorates decompose upon heating to yield the corresponding metal chlorides and oxygen gas:
For zinc chlorate:

$\mathrm{Zn}({\mathrm{ClO}}_3{)}_2\to {\mathrm{ZnCl}}_2+3O_2$

For potassium chlorate:

$2{\mathrm{KClO}}_3\to 2\mathrm{KCl}+3O_2$

Both chlorates release oxygen gas upon thermal decomposition.

Therefore, (NH₄)₂Cr₂O₇ is the only compound among the options that does not release oxygen when heated.