Nitric Oxide (NO) and Nitrogen Dioxide (NO2) – Structure, Preparation, Physical and Chemical Properties, Uses, Practice Problems and FAQ

If we were to consider oxides of all chemical elements as residents of a city, then the oxides of nitrogen would be unarguably crowned as the King owing to their vivid variations and multiple possibilities. Just like a superstar, capable of acting in a vivid variety of roles, nitrogen is extremely efficient in forming different kinds of oxides upon reaction with oxygen. 

Only a few actors have built a name for themselves in both good and bad roles. For example, only a few performers have had a career as varied as Leonardo DiCaprio's. From playing witty and humbling roles in the movies to playing the baddie in some, he has a lot up his sleeves. Nitrogen is as versatile as him, at least in terms of its capabilities to form oxides. Well, all thanks to nitrogen’s ability to form multiple bonds! 

Nitric oxide (or nitrogen monoxide) and nitrogen dioxide are two such oxides of nitrogen. Though they are both made of nitrogen and oxygen, they differ greatly. Let’s try to decode them separately!

TABLE OF CONTENTS

• Oxides of Nitrogen
• Nitric Oxide - Introduction 
• Nitric Oxide - Oxidation State and Structure
• Nitric Oxide - Preparation
• Nitric Oxide - Physical Properties 
• Nitric Oxide - Chemical Properties
• Nitric Oxide - Significance
• Nitrogen Dioxide - Introduction and Source
• Nitrogen Dioxide - Oxidation State and Structure
• Nitrogen Dioxide - Preparation
• Nitrogen Dioxide - Physical Properties 
• Nitrogen Dioxide - Chemical Properties
• Nitrogen Dioxide - Uses
• Nitrogen Dioxide - Health Hazards
• Practice Problems
• Frequently Asked Questions - FAQ

Oxides of Nitrogen

Nitrogen oxides are produced when oxygen combines with nitrogen. Its oxides have distinct oxidation states ranging from +1 to +5. They form a class of highly reactive gases. Nitrogen oxides with a greater oxidation state are more acidic than those with a lower oxidation state. The major sources of these gases being transportation (e.g. on-road and off-road motor vehicles and engines, rail), electric power stations that use fossil fuels and the oil and gas industry's upstreams (e.g. natural gas plants, oil sands).

Nitric oxide (NO) and nitrogen dioxide (NO₂) are two of the most toxicologically relevant chemicals. 

Nitric Oxide - Introduction 

Nitric oxide (•N=O) is a colourless gas which is also called nitrogen monoxide or nitrogen oxide. It is the simplest of all oxides of nitrogen. It has an unpaired free electron and hence is also considered a free radical. It is an unstable oxide due to the presence of a free electron, but it is of immense biological significance. 

Nitric oxide is a chemical that is created naturally by our body, and it is crucial for many aspects of our health. Its primary function is vasodilation, which involves relaxing the inner muscles of blood arteries, allowing them to expand and enhance circulation. Nitric oxide production is critical for general health because it permits blood, nutrients, and oxygen to flow properly and efficiently to all parts of the body.

Nitric Oxide - Oxidation State and Structure

Let the oxidation state of N in NO be x.

So, x+(-2)= 0

∴ x= +2

The structure of NO is denoted as:

Nitric oxide is linear in shape. It is an odd electron molecule and hence paramagnetic in nature.

Nitric Oxide - Preparation

• In the Ostwald process, nitric oxide is generated in industrial settings by oxidising ammonia with platinum as a catalyst at 750–900 °C (typically 850 °C). This in an intermediate step in the production of nitric acid (Ostwald’s process)

4 NH₃+ 5O₂ → 4NO + 6 H₂O

• Another method is reacting sodium nitrite with ferrous sulphate and sulphuric acid.

2NaNO2(aq) + 2FeSO₄(aq) + 3H₂SO₄(aq)

→ Fe₂(SO₄)₃ (aq)+ 2 NaHSO₄(aq) + 2H₂O(l) + 2NO(g)

• Nitric oxide is easily produced in the laboratory by reducing dilute nitric acid with copper.

3Cu + 8HNO₃(dil.) 3Cu(NO3)₂+2NO+ 4H₂O

Nitric Oxide - Physical Properties 

• Nitric oxide is a neutral oxide.

• The molar mass of nitric oxide is 30.006 g mol^-1.

• Its density is 1.3402 g L^-1

• The melting point of nitric oxide is -164 °C (−263 °F; 109 K) and the boiling point is −152 °C (−242 °F; 121 K)

• The solubility of NO in water is 0.0098 g/100 mL at 0 °C and 0.0056 g / 100 ml at 20 °C.

Nitric Oxide - Chemical Properties

• Metal nitrosyls are formed when nitric oxide combines with transition metals. 

E.g. [FeCN5NO]^2-. It forms sodium nitroprusside Na₂[FeCN₅NO], an important analytical reagent.

• Nitric oxide is used in organic reactions for nitrosylation. The addition of NO moiety to an organic molecule is termed nitrosylation.

• On exposure to oxygen, nitric oxide readily converts to nitrogen dioxide.

2NO+O₂ →2NO₂

• Nitric oxide combines with oxygen in water to generate nitrous acid (HNO₂).

4NO + O₂+ 2H₂O → 4HNO₂

• Nitrosyl halides such as nitrosyl chloride, are formed when nitric oxide interacts with fluorine, chlorine, and bromine.

2NO + Cl₂ → 2NOCl

• Nitric oxide combines with nitrogen dioxide (also a radical) to form dinitrogen trioxide.

NO +NO₂ ⇌ N₂O₃

Nitric Oxide - Significance

• It is an important reagent in various organic and inorganic reactions, mainly for nitrosylation.

• It has immense biological significance. 

• Nitric oxide serves as a key signalling molecule in mammals, despite the fact that it is harmful at large quantities. It transmits signals to cells in the cardiovascular, neurological, and immunological systems as a messenger molecule. Because it contains a free radical, the nitric oxide molecule is significantly more reactive than other signalling molecules, and its tiny size allows it to pass through cell membranes and walls to perform a variety of signalling tasks in different organ systems. The enzyme named ‘NO-synthase’ produces NO from the amino acid called ‘L-arginine’ present inside the body.

Nitrogen Dioxide - Introduction and Source

Nitrogen dioxide (NO₂) is an extremely toxic gas. It is known as nitrogen (IV) oxide. It is one of the most common air contaminants in the atmosphere that absorbs UV radiation and prevents it from reaching the earth's surface.

Nitrogen (IV) oxide is a compressed yellowish-brown liquid or a reddish-brown gas. Vapours of NO₂ are thicker than that of air.

Source: 

• NO₂ enters the environment by natural processes such as stratospheric entry, bacterial respiration, volcanoes, and lightning. NO₂ is a trace gas in the Earth's atmosphere, where it absorbs sunlight and regulates the chemistry of the troposphere, particularly in determining ozone concentrations, according to these sources.

• Combustion of fuels from transportation or heavy scale industries, accounts for almost 98 percent of man-made NO₂ emissions, with stationary sources accounting for the bulk. Combustion-generated nitrogen oxides are mostly exhaled as nitric oxide, NO, a comparatively innocuous gas that is quickly transformed to the lethal nitrogen dioxide in the atmosphere. Nitrogen dioxide has a negative impact on human respiratory functioning, and prolonged exposure can lead to a rise in respiratory illnesses.

• Nitrogen dioxide also serves as a precursor to the creation of nitrate aerosols and nitrosamines, both of which have health implications. Nitrogen oxides are one of the air pollutants for which regulations and frequent restrictions have been created due to the volume produced and the potential for widespread negative impacts on public health and welfare.

Nitrogen Dioxide - Oxidation State and Structure

Let the oxidation state of N in NO₂ be ‘x'.

So, x+2(-2)= 0

∴ x= +4

The structure of NO₂ is bent and is also known to undergo resonance in the following way. 

Nitrogen Dioxide - Preparation

• Some metal nitrates undergo heat degradation (thermal decomposition) to produce nitrogen dioxide.
  
• Nitric oxide in the presence of oxygen quickly forms nitrogen dioxide. 

2NO(g) + O₂(g) ⇌ 2NO₂ (g)

• Oxidation of metals by concentrated nitric acid produces nitrogen dioxide.

Cu + 4HNO₃ (conc.) Cu(NO_3⁾2+ 2NO₂+ 2H₂O

• In the laboratory, NO₂ may be made via a two-step process in which nitric acid is dehydrated to generate dinitrogen pentoxide, which is then thermally decomposed.

2HNO₃ → N₂O₅ + H₂O

2N₂O₅ → 4NO₂+ O₂

• Standing nitric acid decomposes into brown nitrogen dioxide. This is why, despite the fact that fresh nitric acid is colourless, it becomes dark with time.

4HNO₃ → 4NO₂ + O₂ + 2H₂O

Nitrogen Dioxide - Physical Properties

• It is a reddish-brown poisonous gas and is very reactive.

• It is an acidic oxide.

• The density of NO2 is 1.880 g dm^-3

• Its melting point is −9.3 °C and boiling point is 21.15 °C.

• Its molar mass is 46.006 g mol^-1

Nitrogen Dioxide - Chemical Properties

• Thermal property: At low temperatures, NO₂ changes to colourless dinitrogen tetroxide (N₂O₄), which then reverts back to NO₂ at higher temperatures. Hence, NO₂ and N₂O₄ exists in a dynamic equilibrium.

2NO₂ ⇌ N₂O₄

• NO2 is an excellent oxidant due to the fragility of the N–O bond. As a result, many molecules, such as hydrocarbons, may combust, sometimes explosively.

• NO₂(N₂O₄) is a strong oxidising system.

• Upon hydrolysis, nitrogen dioxide produces nitrous acid and nitric acid.

2NO₂(N2O₄) + H₂O → HNO₂ + HNO₃

• To make anhydrous metal nitrates from oxides, NO₂ is employed.

MO + 3NO₂ → M(NO₃)₂ + NO

• Alkyl and metal iodides produce equivalent nitrites in presence of nitrogen dioxide.

2CH₃I + 2NO₂ → 2CH₃NO₂ + I₂

TiI₄ + 4NO₂ → Ti(NO₂)₄ + 2I₂

Nitrogen Dioxide - Uses

• Nitrogen dioxide is employed as an intermediary In the manufacture of nitric acid.

• It is used to make oxidised cellulose molecules.

• As a catalyst, it is used in several reactions.

• It is an intermediate used in the process of making sulphuric acid.

• Rocket fuels use it as an oxidiser.

• As a nitrating agent, it is used in various organic reactions.

• It is used as a sterilising agent for use at room temperature.

Nitrogen Dioxide - Health Hazards

Nitrogen dioxide (NO₂) exposure can be lethal at high doses. When it comes into contact with the eyes and skin, it generates a burning feeling. When in liquid form it causes frostbite. It is said to generate methemoglobin when it reacts with blood. When heated to decompose, it releases toxic fumes of nitrogen oxides.

Nitrogen dioxide is an irritant gas that causes inflammation of the airways at high concentrations. NO₂ is mostly harmful to those who have respiratory problems that create a lot of inflammation in their airways. Long-term exposure reduces lung capacity, raises the risk of respiratory problems, and intensifies allergic reactions. NO₂ also leads to the creation of fine particles (particulate matter) and ozone at ground level, both of which have negative environmental consequences.

Practice Problems

Q1. The colour of nitrogen dioxide standing at warm temperatures, deepens from light yellowish to dark brown, why?
Answer: NO₂ and N₂O₄ exists in a dynamic equilibrium. 

2NO₂ ⇌ N₂O₄ 

Where nitrogen tetroxide is colourless. At low temperatures, NO₂ changes to the colourless dinitrogen tetroxide (N₂O₄), which then reverts back to NO₂ at higher temperatures and hence, the colour deepens.

Q2. NO2 is paramagnetic in nature because

A. It has one electron pair
B. It has a positive charge
C. It has one odd-electron
D. It exists as a dimer 
Answer: NO₂ is paramagnetic due to the presence of one odd electron. 

So, option C) is the correct answer.

Q3. Which of the following properties is not exhibited by nitric oxide (NO)?

A. It is acidic oxide
B. It is neutral oxide
C. It is colourless
D. It combines with oxygen to form NO₂
Answer: 

• Nitric oxide exists in +2 oxidation state of nitrogen and hence, it is not acidic. Rather it is a neutral oxide. 
• Nitric oxide is a colourless gas.
• Nitric oxide combines with oxygen to form NO₂.

2NO+O₂ → 2NO₂

So, option A) is the correct answer.

Q4. What is the product obtained when nitric oxide reacts with halides ?
Answer: Nitrosyl halides such as nitrosyl chloride, are formed when nitric oxide interacts with fluorine, chlorine, and bromine.

2NO + Cl₂ → 2NOCl

Frequently Asked Questions - FAQ

Question 1. How is nitrogen dioxide responsible for acid rain?
Answer: Emissions from industrial zones contain maximum gaseous oxides of nitrogen and sulphur, nitrogen dioxide being one of the major oxides. It mixes with water and undergoes hydrolysis, forming nitrous acid and nitric acid. This is how it generates acid rain.

Question 2. What happens if we inhale high levels of NO₂?
Answer: Breathing air with a high NO₂ content might irritate the human respiratory system's airways. Short-term exposures can worsen respiratory disorders, notably asthma, resulting in respiratory symptoms (coughing, wheezing, or trouble breathing), hospitalizations, and emergency department visits.

Question 3. Do nitric oxide and nitrogen dioxide cause global warming?
Answer: The two most deadly and harmful nitrogen oxides are nitric oxide and nitrogen dioxide. But, nitrous oxide is a greenhouse gas and majorly the one that contributes to global warming.

Question 4. What happens if there is a low nitric oxide level in our body?
Answer: High blood pressure, cognitive problems, limited stamina, and erectile dysfunction are all symptoms of insufficient nitric oxide levels. Cigarette smoking, a high-fat, high-cholesterol diet, and little to no exercise are all potential contributors to these symptoms.

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