Oxides of Nitrogen

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Oxides of Nitrogen Oxides of Nitrogen

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    NO? That's the right answer!

    Oxides of nitrogen are a group of gases, made of just nitrogen and oxygen atoms. These gases are pollutants, contributing to global warming and affecting human health. If you want to know more about where these gases from, and the effect that they have on the planet, read on!


    Oxides of Nitrogen: Meaning

    Let's begin with a definition.

    Oxides of nitrogen are a group of highly reactive gases made of nitrogen and oxygen.

    Oxides of nitrogen include:

    • Nitrogen dioxide (NO2)

    • Nitrogen monoxide (NO)

    • Nitrous oxide (N2O)

    • Dinitrogen pentoxide (N2O5)

    Oxides of Nitrogen: Structure

    Nitrogen dioxide and nitrogen monoxide are the most toxicologically significant gases of the group, so we're going to focus on their structure.

    The formula NOx refers to both nitrogen dioxide and nitrogen monoxide.

    Nitrogen Dioxide (NO2)

    NO2 is a covalent compound. It's composed of a central nitrogen atom and two oxygen atoms.

    One oxygen atom is single bonded to the nitrogen; the other oxygen atom is double bonded.

    O=N−O

    Nitrogen Monoxide (NO)

    Nitrogen monoxide is composed of one nitrogen atom and two oxygen atoms. The molecule is a free radical – the atoms share a double bond, but the nitrogen has an unpaired electron (indicated by a dot).

    O=N•

    Free radicals are atoms or molecules with at least one unpaired electrons.

    Due to their unpaired electron, free radicals are highly unstable and reactive. Free radicals affect human health – they break down cells over time. As the body ages, it loses its ability to fight free radicals, resulting in oxidative stress.

    Oxidative stress is the imbalance of free radicals and antioxidants (molecules that fight free radicals).

    Oxidative stress leads to cell damage, degenerative processes, and ageing.

    Examples of Oxides of Nitrogen

    Earlier, we heard about four different gases that are classed as oxides of nitrogen. Let's go into more detail about each one.

    PropertiesNitrogen DioxideNitrogen MonoxideNitrous OxideDinitrogen Pentoxide
    Chemical FormulaNO2NON2ON2O5
    CharacteristicsReddish-brown gas with unpleasant odourColourless, sweet-smelling gasColourless, sweet-tasting gasColourless gas
    Boiling Point21.15°C-151.8°C-88.5°C47.0°C
    UsesNitric acid synthesis, rocket fuel, producing explosivesIndustrial chemistry intermediate, mammal signalling moleculeAnaesthetic (often abused recreationally), propellant, foaming agentChloroform, explosives production
    HazardsInhalation leads to respiratory conditions and airway inflammationInhalation leads to respiratory conditions and oxidative stressInhalation causes dizziness, unconsciousness, and even deathHighly explosive

    Causes of Oxides of Nitrogen

    Where do oxides of nitrogen come from?

    Natural Sources

    The majority of NOx are produced naturally. Sources include:

    • Volcanic eruptions

    • Biological decay

    • Microbial activity in the ocean

    • Lightning strikes

    How do lightning strikes produce NOx?

    The core of lightning bolts can reach a temperature close to 30,000ºC – so hot that it can break apart molecules of oxygen and nitrogen. The rapid heating and cooling of oxygen and nitrogen produces molecules of nitrogen monoxide (NO). Later, NO combines with oxygen to create NO2.

    Oxides of Nitrogen lightning natural source causes StudySmarterFigure 1: Scientists have estimated that lightning strikes produce 10% of NOx in the atmosphere. unsplash

    Combustion Processes

    Hydrocarbon fuels are burnt in hot engines. In these hot conditions, typically between 75 and 105ºC, nitrogen and oxygen molecules in the air combine to form nitrogen monoxide.

    N2 + O2 → 2NO

    When nitrogen monoxide is released from vehicle exhausts or flue chimneys, it reacts with oxygen in the air to form nitrogen dioxide.

    2NO + O2 → 2NO2

    Oxides of Nitrogen Power Plant at sunset  StudySmarterFigure 2: Fossil fuel power plants burn hydrocarbons, leading to NOx pollution. unsplash

    Fertiliser Use

    Farmers add chemical fertilisers to their crops. Fertilisers contain important plant nutrients such as nitrogen, phosphorus, and potassium. Plants need these nutrients to grow and function effectively.

    Fertilisers contain nitrogen because it is used by plants to produce chlorophyll.

    Chlorophyll is a green pigment that absorbs light energy for photosynthesis.

    Without nitrogen and chlorophyll, plants cannot grow.

    Using too much fertiliser results in an excess of nitrogen-rich organic matter in the soil. Microbes feeding on excess nitrate matter produce large amounts of NOx, polluting the air.

    Fertiliser use doesn't only lead to NOx pollution. Excess fertiliser runs into waterways, causing eutrophication.

    Eutrophication is the excessive richness of nutrients in a body of water.

    Eutrophication leads to algal overgrowth. Too much algae limits photosynthesis elsewhere in the water, leaving anaerobic 'dead zones' below.

    The Effects of Oxides of Nitrogen

    Oxides of nitrogen are pollutants, affecting the environment and human health.

    Global Climate Change

    Nitrogen dioxide is the third most potent greenhouse gas, trapping heat in the atmosphere and contributing to global climate change.

    Furthermore, NOx destroys stratospheric ozone. The ozone layer plays an essential role in protecting our planet from harmful UV radiation. Without it, UV radiation would kill plants and damage the DNA of living organisms.

    Acid Rain

    Oxides of nitrogen contribute to acid rain. When they react with water and air, they form nitric acid (HNO3). Nitric acid mixes with water and falls to the ground as acid rain. Consequences include damage to forests and erosion of physical structures.

    Photochemical Smogs

    NOx pollution contributes to the formation of photochemical smogs.

    Photochemical smogs are a type of smog produced when UV light reacts with nitrogen oxides and other chemicals in the atmosphere.

    Photochemical smogs are visible as a brown haze. They're most prominent in warm, densely populated areas.

    Cities prone to photochemical smogs include Los Angeles, Sydney, and Mexico City.

    Smogs worsen respiratory illnesses and affect visibility.

    Oxides of Nitrogen Photograph of a city in fog  StudySmarterFigure 3: A photochemical smog in Hong Kong. unsplash

    Health Problems

    Short-term exposure to oxides of nitrogen results in symptoms such as headaches, irritation of the eyes and noise, difficulty breathing, and abdominal pain. Long-term exposure can cause asthma and other respiratory conditions.

    Exposure to very high concentrations of NOx can impact fertility, harm developing fetuses, and even cause death.

    Controlling Oxides of Nitrogen

    How can we prevent NOx from impacting the environment and living organisms? There are three primary methods: catalytic converters, urea sprays, and managing fertiliser use.

    Catalytic Converters

    Catalytic converters are installed in car engines to reduce harmful emissions such as NOx, carbon monoxide, and particulate matter.

    The converters use redox reactions to reduce harmful emissions.

    Redox reactions are reactions where both oxidation (losing electrons) and reduction (gaining electrons) are taking place.

    Catalytic converters are made of rare metals platinum and rhodium. They are positioned in a honeycomb shape to create a large surface area for the oxidation and reduction of harmful emissions.

    The converters reduce nitrogen atoms from NOx molecules.

    Free, highly reactive oxygen atoms collide and form O2 molecules.

    Nitrogen atoms attached to the catalyst react with each other, forming N2 molecules.

    2NO → N2 + O2

    2NO2 → N2 + 2O2

    Oxidation reactions use the newly formed O2 molecules to convert carbon monoxide and methane into less harmful products.

    CO + O2 → CO2

    CH4 + 2O2 → CO2 + 2H2O

    Urea Sprays

    Urea is commonly used in controlling NOx emissions from burning fossil fuels.

    It's synthesised by the body for use in nitrogen excretion.

    Flue chimneys containing oxides of nitrogen are sprayed with an aqueous urea solution.

    Flue chimneys are vertical passageways transporting waste gas outdoors. They're the smoking towers that you see next to power stations!

    Any oxides of nitrogen dissolve in the solution, forming nitrous acid (HNO2).

    Then, the nitrous acid reacts with the urea to form nitrogen, carbon dioxide, and water.

    2HNO2 + NH2CONH2 2N2 + CO2 + 3H2O

    Managing Fertiliser Use

    To use fertiliser as effectively as possible, and limit NOx emissions, farmers follow the “four R's”:

    • Right application rate

    • Right formulation (fertiliser type)

    • Right timing of application

    • Right placement of application

    Using urea-based fertilisers as an alternative to ammonia-based fertilisers may reduce emissions of NOx.


    I hope that this article has explained oxides of nitrogens to you. Remember that they're a group of polluting gases, made of oxygen and nitrogen atoms. They cause global climate change, photochemical smogs, and health problems.

    Oxides of Nitrogen - Key takeaways

    • Oxides of nitrogen are a group of highly reactive gases made of nitrogen and oxygen.
    • Nitrogen dioxide and nitrogen monoxide are collectively referred to as NOx. Nitrogen dioxide (NO2) is a covalent compound, while nitrogen monoxide (NO) is a free radical. Other oxides of nitrogen include nitrous oxide and dinitrogen pentoxide.
    • Causes of NOx pollution include: natural sources (such as volcanoes and lightning strikes), hydrocarbon combustion processes, and excess fertiliser use.
    • Effects of NOx pollution include climate change, ozone depletion, acid rain, photochemical smogs, and health problems.
    • Oxides of nitrogen can be controlled by installing catalytic converters, spraying urea solution into flue chimneys, and managing fertiliser use.

    1. Adam Voiland, NASA Researchers Explore Lightning's NOx-ious Impact on Pollution, Climate, NASA, 2009

    2. Australian Government, Oxides of Nitrogen, Department of Climate Change, Energy, the Environment and Water, 2022

    3. Autodoc, Causes of Overheating, 2022

    4. H. J. Emeléus, Advances in Inorganic Chemistry and Radiochemistry, 1964

    5. PubChem, Nitrous Oxide, National Library of Medicine, 2022

    Frequently Asked Questions about Oxides of Nitrogen

    What are oxides of nitrogen?

    Oxides of nitrogen are a group of highly reactive gases made of nitrogen and oxygen.

    How are oxides of nitrogen formed?

    Oxides of nitrogen can be formed naturally, through combustion processes, or by excessive use of fertiliser.

    What are some examples of oxides of nitrogen?

    Oxides of nitrogen include nitrogen dioxide, nitrogen monoxide, nitrous oxide, and dinitrogen pentoxide.

    What are some effects of oxides of nitrogen?

    Oxides of nitrogen lead to global climate change, ozone depletion, acid rain, photochemical smogs, and health problems.

    What is the formula for oxides of nitrogen?

    The formula for oxides of nitrogen is NOx.

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