Write the chemical equation and
describe the color change that occurs when heating nitrogen dioxide gas in a sealed
glass flask at room temperature.
The first thing we need to do to
answer this question is recall the structure and properties of nitrogen dioxide gas
and recall what happens when it’s mixed with itself. Two molecules of nitrogen dioxide
can react together to form one molecule of dinitrogen tetroxide. Nitrogen dioxide gas is a powerful
reddish brown, while dinitrogen tetroxide is colorless.
In a sealed system, this reaction
is in fact an equilibrium. Molecules of nitrogen dioxide react
to form dinitrogen tetroxide, while simultaneously molecules of dinitrogen tetroxide
can decompose to form molecules of nitrogen dioxide. So a flask containing nitrogen
dioxide gas at room temperature would in fact contain a mixture of nitrogen dioxide
and dinitrogen tetroxide. Since it contains a colorless gas
and a reddish-brown gas, the flask would appear reddish brown.
So the question is, what will
happen to this equilibrium when the flask is heated? To answer that question, we’re
going to need to call on Le Chatelier’s principle. Le Chatelier’s principle is that
the position of an equilibrium will shift to oppose an external change. In this case, the change is heating
of the flask.
Any practical equilibrium will
consist of two reactions: one that is exothermic and the reverse that is
endothermic. In this case, the reaction of two
molecules of nitrogen dioxide gas is exothermic, producing dinitrogen tetroxide and
some heat. The reverse reaction, the
decomposition of dinitrogen tetroxide, absorbs heat. Therefore, the change, which is
heating, favors the decomposition of dinitrogen tetroxide, shifting the equilibrium
to the left. This increases the concentration of
nitrogen dioxide and increases the strength of the reddish-brown color.
We now have all the information we
need to answer the question. So the chemical equation in effect
is 2NO₂ in equilibrium with N₂O₄ plus heat. And the color change is that the
strength of the reddish-brown color increases.