In an experiment, $\mathrm{NO}_2$ gas is prepared and taken into 3 test tubes $X, Y$ and $Z, \mathrm

Question

In an experiment, $\mathrm{NO}_2$ gas is prepared and taken into 3 test tubes $X, Y$ and $Z, \mathrm{NO}_2$ gas which is brown in colour dimerises into $\mathrm{N}_2 \mathrm{O}_4$ which is colourless. Test tube $X$ is kept at room temperature, $Y$ is kept in ice and Z is kept in hot water. What colour changes will you observe in the test tubes and why?
$
\underset{	ext { Brown }}{2 \mathrm{NO}_{2(g)}} ightleftharpoons \underset{	ext { Colourless }}{\mathrm{N}_2 \mathrm{O}_{4(g)}} ; \Delta H=-57.2 \mathrm{~kJ} \mathrm{~mol}^{-1}
$

Professor PC Thomas Classes · Answer

(c) : It is an exothermic reaction, hence the forward reaction is favoured at low temperature which means colourless $\mathrm{N}_2 \mathrm{O}_4$ will be formed resulting in decrease in intensity of brown colour. High temperature favours backward reaction resulting in formation of $\mathrm{NO}_2$, thus intensifying brown colour.