Oxides of Nitrogen
A concise revision guide to nitrogen oxides, NOx, how they form in high-temperature combustion, why engines and power stations produce them, and why they matter in air pollution.
What Are Oxides of Nitrogen?
Oxides of nitrogen are commonly written as NOx. This is a general term for nitrogen oxide gases formed when nitrogen and oxygen react together during high-temperature combustion.
The two main examples at this level are nitric oxide, NO, and nitrogen dioxide, NO2.
Although nitrogen gas, N2, makes up a large proportion of the air, it is usually quite unreactive because it contains a very strong N≡N triple bond.
Key idea: Nitrogen oxides form from nitrogen and oxygen in air when combustion temperatures are high enough to overcome the stability of nitrogen molecules.
Why High Temperature Is Needed
Inside a car engine, the temperature during combustion is extremely high. The spark in the engine also supplies energy to help start the reaction.
At these high temperatures, enough energy is available to break bonds in nitrogen and oxygen molecules. Once nitrogen atoms are available to react, they can combine with oxygen to form nitrogen oxides.
High-temperature formation: Nitrogen oxides are mainly produced when the high temperatures in engines or power stations allow nitrogen and oxygen from the air to react.
High-temperature combustion in engines and power stations provides the conditions needed for nitrogen and oxygen in air to react.
Equations for Nitrogen Oxide Formation
The first important product is often nitric oxide, NO. This can form when nitrogen and oxygen react at high temperature:
N2 + O2 → 2NO
Nitric oxide can then react further with oxygen in the air to form nitrogen dioxide, NO2:
2NO + O2 → 2NO2
The overall idea can also be summarised as nitrogen and oxygen reacting to form nitrogen dioxide under high-temperature conditions:
N2 + 2O2 → 2NO2
Where Oxides of Nitrogen Are Produced
Nitrogen oxides are produced mainly where combustion temperatures are very high. This explains why they are associated with car engines and power stations.
In a car engine, fuel burns in air. Air contains both nitrogen and oxygen, so the engine provides both the reactants and the high-temperature conditions needed for NOx formation.
| Source | Why NOx forms | Key exam point |
|---|---|---|
| Car engines | Fuel burns at very high temperature in air. | Nitrogen and oxygen from air react together. |
| Power stations | Combustion occurs at high temperature on a large scale. | High temperature makes normally unreactive nitrogen able to react. |
Exam focus: Do not say nitrogen oxides come directly from the fuel. The nitrogen and oxygen are mainly from the air, and the high temperature allows them to react.
Why Nitrogen Oxides Matter
Nitrogen oxides are air pollutants. They are important because they contribute to poor air quality and are linked with environmental problems.
At this stage, the key chemical point is to understand how they form: nitrogen and oxygen from the air react during very high-temperature combustion.
Reducing NOx emissions is one reason vehicles use catalytic converters, which are covered in the next connected part of this topic.
Check Your Understanding
Use these short activities to check the key ideas about nitrogen oxide formation during high-temperature combustion.
Master Topic 6A/6B Organic Chemistry and Alkanes for Edexcel A Level Chemistry
Continue from these free revision notes into the full Topic 6A/6B Organic Chemistry and Alkanes course for Edexcel A Level Chemistry Paper 2 (9CH0/02), with guided video teaching, diagnostic MCQ practice, teacher-marked short-answer questions and a specification assignment with a personalised progress report.
Guided video teaching
Learn the chemistry and exam technique through structured video lessons with worked examples and walkthroughs.
Instant MCQ feedback
Auto-marked MCQ quizzes provide immediate diagnostic feedback for every answer choice.
Teacher-marked SAQs
Submit written exam responses and receive chemistry specialist feedback with improvement guidance.
Progress tracking
Identify strengths and weaknesses across the full Topic 6A/6B specification with targeted reporting.
See how the course works
Click play to start the course preview animation.
Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Oxides of Nitrogen FAQs
These questions cover the main exam points on nitrogen oxides formed during combustion.
What are oxides of nitrogen?
Oxides of nitrogen, commonly written as NOx, are nitrogen oxide gases such as NO and NO2.
How are nitrogen oxides formed in car engines?
Nitrogen oxides form when nitrogen and oxygen from the air react together at the very high temperatures produced inside a car engine.
Why is nitrogen usually unreactive?
Nitrogen is usually unreactive because N2 contains a strong N≡N triple bond, which requires a large amount of energy to break.
What equation shows nitric oxide formation?
Nitric oxide can form from nitrogen and oxygen at high temperature: N2 + O2 → 2NO.
Why do power stations produce nitrogen oxides?
Power stations involve high-temperature combustion, so nitrogen and oxygen from the air can react to form nitrogen oxides.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.