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Nitric Oxide and Oxygen Reaction

Nitric oxide and oxygen gas can react to form nitrogen dioxide. This reaction is very important for industry because it's used in the production of nitric acid. It's also important for studying pollution in the environment because nitric oxide and nitrogen dioxide are pollutants. While nitric oxide is clear and colorless, nitrogen dioxide is a dull brown color.

Mechanism
- When nitric oxide and oxygen react, one molecule of oxygen (O2) and two molecules of nitric oxide (NO) combine to form two molecules of nitrogen dioxide (NO2). According to a 2011 study in the Journal of Chemical Theory and Computation, several different reaction mechanisms have been proposed. For example, it could be that NO initially combines with O2 to form a weakly bound intermediate, OO.NO, which then reacts with another molecule of NO to give ONOONO. This intermediate then breaks up into two molecules of NO2. Other mechanisms have been proposed, and the precise mechanism by which the reaction takes place is still a subject of controversy.
Products
- The reaction between nitric oxide and oxygen in air is rapid, and it starts as soon as the gas is exposed to air. The product, nitrogen dioxide, has an odd number of electrons, and two molecules of nitrogen dioxide can combine to form N2O4. Ordinarily nitrogen dioxide gas exists in equilibrium, with some N2O4 molecules breaking down to form NO2 even as other NO2 molecules combine to form N2O4.
Reverse Reaction
- Nitric oxide is a more unstable and hence very reactive molecule, which is why it is oxidized by air so quickly. When nitrogen dioxide dissolves in water, however, it reacts with the water to form nitric acid and nitric oxide gas. The nitric acid is a problem because in the environment nitrogen dioxide from smog can of course dissolve in water to create acid rainwater. In the lab, however, the formation of nitric acid actually offers a way to watch the reaction take place.
Indicator
- Suppose you have a flask containing a little water with a pH indicator chemical dissolved in it. If you introduce nitric oxide into the flask followed by air, the nitric oxide and the air will start to react, and the gas inside the flask will turn a leaden brown. As the nitrogen dioxide dissolves in the water, however, it will form nitric acid, acidifying the water and causing the pH indicator chemical to change color. The change in the color of the pH indicator and the change in the color of the gas give you two different ways to watch the reaction in action.

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