Oxides of Nitrogen

Oxides of nitrogen or nitrogen oxides (NOx) are the compounds of nitrogen and oxygen. They come under the category of major air pollutants, and thus, it is important to control their concentration in the atmosphere. Nitrogen is present in different oxidation states, ranging from +1 to +5 in these NOx, and the oxides of nitrogen formula must be known to check the oxidation state of nitrogen. The oxidation state of oxygen is always -2.  

Nitrous Oxide

Also called laughing gas or dinitrogen oxide. It is a colourless and non-flammable gas in which N exists in +1 oxidation state. It is produced by biological sources such as yeasts and plants. Chemically, the following reaction produces nitrous oxide:

NH4NO3 → 2H2O + N2O.

It can deplete the ozone layer in the stratosphere and troposphere by directly reacting with it.

 It is a greenhouse gas. Hence excess of it in the atmosphere can cause global warming.

Nitric Oxide

To a small extent, it is generated by natural processes like lightning, the nitrogen cycle in soil, or forest fires. Majorly, however, it is generated by human activities, especially vehicular emissions. It has a sweet odour. It is not generated at low temperatures (not below 750oC). It is a linear molecule, which is chemically produced by the following reaction:

3H2SO4 + 2NaNO2 + 2FeSO4  →   2NaHSO4 + Fe2(SO4)3 + 2H2O + 2NO

Nitrogen Dioxide

NO2 is a reddish-brown gas with an irritating odour. It is liquid at room temperature and exists in a gaseous state above 70oC. Both nitrogen dioxide and nitric oxide are present near the ozone layer in large amounts. 

Nitrogen dioxide is an acidic gas that is produced by the decomposition of lead nitrate by the following reaction at high temperature (673K):

 2Pb(NO3)2 →  4NO2 + 2PbO  + O2 

Dinitrogen Tetroxide and Dinitrogen Pentoxide

When one talks about NOx, they mainly refer to NO and NO2 as they are present in highest concentrations out of all the given oxides of nitrogen. N2O4 is a dimer of NO2, which is highly oxidising in nature. It is more prevalent at lower temperatures.

N2O5 is a colourless crystalline solid. It is formed when nitric acid reacts with phosphorus pentoxide.

 4HNO3 + P4O10 →    4HPO3  + 2N2O5 

It is an acidic oxide and a planar molecule that decomposes at higher temperatures to give NO2 and O2.

Sources of NOx

Almost half of the NOx comes from vehicular emissions. Manufacturing plants, like steel manufacturing, cement manufacturing, petroleum refineries, also release large amounts of oxides of nitrogen. 

 Natural sources of NOx are lightning, forest fires, yeasts, plants, etc. However, their contribution is minor compared to that from anthropogenic sources. Since they are major environmental pollutants, it is necessary to keep those activities in check that produce NOx.

Problems With Oxides of Nitrogen

Oxides of nitrogen react with water to form acid rain, i.e. rain containing nitric acid. It causes irreversible harm to monuments, buildings, plants, and humans. Other than this, they react with volatile organic compounds (VOCs) to form ozone. It is to be noted that ozone in the atmosphere is different from ozone in the stratosphere. The former is undesirable as it has several side effects like respiration and lung problems. Ozone is also a major component of smog, which leads to problems like asthma, restlessness, headache, nausea, etc.

 NOx can react with other chemicals to form toxic compounds like nitrosamines and others. Prolonged exposure to such compounds can even cause mutations in the DNA, leading to tumours in the worst cases. It is also causing the problem of global warming, which is increasing the average temperature of the earth. 

Problems in Water Bodies: Eutrophication is the phenomenon of excessive plant growth as the water body becomes excessively abundant in minerals and resources. It may seem like a good thing, but, it is a huge problem and a side-effect of oxides of nitrogen. 

 When the water body has too many nitrates or phosphates, then eutrophication can take place. In this, the water body is covered with too many plants. This cuts the supply of atmospheric oxygen to the water body, and the nutrient balance is disturbed. The oxygen content in the bottom of the water body reduces, leading to plant and animal death.

Few uses Of NOx

NOx is indeed a major challenge to environmentalists today, but it has several industrial uses also. Nitric oxide and nitrogen dioxide are used in the production of nitric acid. The latter is then used to produce nitrogen fertilisers and explosives.

 Nitrous oxide is a fairly non-toxic gas that has several everyday applications. It is used in dentistry and as an analgesic as well. It is clinically proven to decrease anxiety.

Conclusion

There are several oxides of nitrogen found on earth, but the major ones are NO and NO2. They pose a significant environmental threat as they can react with VOCs to form ozone. They are greenhouse gases that also increase the earth’s temperature, causing global warming. Acid rain is also one of its consequences. Since vehicular emissions are the major source of NOx pollution, it is crucial to keep them in control. The valency of nitrogen is different in the different oxides of nitrogen.