Nitrogen dioxide (NO2) is an acidic gas and readily reacts with water (H2O) to produce an acidic solution of nitric acid (HNO3) and nitrogen monoxide (NO). Brown color of nitrogen dioxide gas is disappeared when reaction occurs.
This reaction happens very easily in water. Therefore, produced nitrogen dioxide gas is sent to water to manufacture nitric acid in chemical industry. This reaction is a redox reaction.
Sometimes, we write the chemical equation of nitrogen dioxide and water as below.
In this reaction (consider as reaction 1), we see nitric acid and nitrous acid are formed as products.
When we balance this equation (consider as reaction 2),
By combining reaction 1 and reaction 2 to remove nitrous acid from major equation, we will get our proper equation. ( 3 * reaction 1 + reaction 2 )
Gaseous nitrogen dioxide reacts with liquid water and produce nitric acid which is highly soluble in water (therefore, nitric acid exists as an aqueous solution) and gaseous nitrogen monoxide.
Oxidation number of nitrogen at NO2 is +4 according to the lewis structure of NO2. When reaction occurs, NO2 is oxidized to nitric acid. In nitric acid, oxidation number of nitrogen is +5. Simultaneously to the oxidation, another NO2 molecules are reduced to NO. The oxidation number of nitrogen in NO is +2. Therefore, nitrogen atom in NO2 is reduced and oxidized simultaneously and those reactions are defined as disproportionation reactions.
This reaction is used in nitric acid production; ostwald process. Produced nitrogen dioxide gas is sent to water to produce nitric acid. As a by product, nitric oxide (NO) is produced. It is also used to produce nitric acid by separating and converting to nitrogen dioxide in the manufacturing process.
Questions asked by students
As nitrogen dioxide, dinitrogen trioxide reacts with water and form nitric acid and nitric oxide. Therfeore oxidation number of nitrogen is changed when both nirogen dioxide and dinitrogen trioxide react with water.
pH value will be lover than seven because of nitric acid formation. If lot of nitrogen dioxide is dissolves, pH value will be a value like 1,2 forming a strong acidic solution (Nitric acid is a strong acid).
Both bromine and nitrogen dioxide gases are brown color gases. Therefore we cannot identify them, by observing their colors.
In this tutorial, we have learned that NO2 will readily reacts with water and lost it's brown color during the reaction. But, bromine gas which is a brown color too, does not dissolve like NO2. So brown color of Br2 remains in the water.