When a gas was heated at atmospheric pressure and 25 degree C, its color deepene

Question

When a gas was heated at atmospheric pressure and 25 degree C, its color deepened. Heating above 150 degree C caused the color to fade, and at 500 degree C the color was barely detectable. At 550 degree C, however, the color was partially restores by increasing the pressure of the system. Which of the following scenarios best fits the observations? Justify your selection (and rejections.) [Br_2 is reddish; NO_2 is brown; all other gases colorless.] A mixture of hydrogen and bromine. Pure bromine A mixture of nitrogen dioxide and dinitrogen tetroxide.

Explanation / Answer

It is a mixture of hydrogen and bromine.

Initially we have bromine so the color is deep. As we heat the colr fades progressively as more HBr is formed towards equilibrium.

At 550C, by increasing the presure of the system, the color is partialyy restored due to the disturbance of equilibrium, which leads to formation of bromine again.

H2 + Br2 -----> 2 HBr

Option B is pure bromine which does not change color on heating.

Option C it not correct for the given information, as the equilibrium between brown NO2 and dinitrogen tetroxide is established at lower temperature(less than rooom temperature). At higher temperature the mixture exists completely as brown NO2 gas.

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