Video: EC17-18-S2-Q05

EC17-18-S2-Q05

02:52

Video Transcript

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.

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