The changing of the colour of $\ce{NO2}$ is because of a change in temperature, resp. heating and cooling?
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The changing color of $\ce{NO_2}$ is because it exits in equilibrium with $\ce{N_2O_4}$ as shown in the following equation $$\ce{2 NO_2 <=> N_2O_4} + \mathrm{58~ kJ/m}$$ $\ce{NO_2}$ is a reddish-brown colored gas, while the $\ce{N_2O_4}$ is colorless. Considering Le Chatelier's principle,
- Taking heat out of the equilibrium will push it to the right
- Putting heat in, will push it to the left
- Increasing pressure will push it to the right
As we shift the equilibrium we change the concentration of the components, and consequently the color of the gaseous mixture.
Back to your question, in the solid state (which is where it would be at 100°K), the equilibrium is shifted completely to the right. So if we had a tube of gaseous $\ce{NO_2}$ and $\ce{N_2O_4}$ at room temperature it would have a distinct red-brown color. If we then plunged it into a bath at 100°K, it would solidify to $\ce{N_2O_4}$ and appear colorless.
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$\begingroup$ Does the reaction shift again to restore the concentrations of NO2 and N2O4 to the left while constant temperature? $\endgroup$ Apr 25, 2014 at 21:58
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1$\begingroup$ Not sure I follow. If the "constant temperature" is high enough that the substances are in the gas phase, then changing pressure will shift the equilibrium at constant temperature. Changing the temperature from 100K to room temp will restore the equilibrium that exists in the gas phase at room temp. $\endgroup$– ronApr 25, 2014 at 22:04
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$\begingroup$ no Chatelier's principle says if concentration changes the equilibrium shifts ,, if the change in temperature changes concentration? does that mean the equilibrium is going to shift again because the concentration changed .. $\endgroup$ Apr 25, 2014 at 22:14
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1$\begingroup$ The concentrations will change with temperature. If a temperature is held constant long enough, then an equilibrium will be reached. At each temperature there will be a distinct equilibrium constant - which means distinct concentrations of reactants and products. At a given temperature, once equilibrium is achieved, the concentrations will not change again unless the equilibrium is disturbed (change in temperature, pressure, adding more reagent, etc.). $\endgroup$– ronApr 25, 2014 at 22:25