International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What factor allows nitrogen to react with oxygen in car engines?

1. The presence of water

2. Use of catalyst

3. The high temperature in the engine

4. The amount of fuel present

The correct answer is: The high temperature in the engine

Nitrogen can react with oxygen in car engines primarily due to the high temperature present during the combustion process. In car engines, fuel burns at high temperatures, usually exceeding 1000 degrees Celsius. At these elevated temperatures, nitrogen molecules have enough energy to overcome the activation energy barrier for the reaction with oxygen. Under normal conditions, nitrogen is quite stable and does not react readily with oxygen. However, when the temperature rises significantly, as it does in car engines, the energy supplied is sufficient to enable the breaking of nitrogen-nitrogen bonds, allowing nitrogen to react with oxygen, primarily forming nitrogen oxides (NO and NO2). This reaction is a significant contributor to air pollution and is a critical consideration in vehicle emissions control technologies. Other factors listed, such as the presence of water, the use of a catalyst, and the amount of fuel present, may play roles in the overall combustion process and efficiency, but they do not directly enable the reaction between nitrogen and oxygen under the extreme conditions experienced in a car engine. Therefore, the high temperature is the key factor facilitating this reaction.