Which factor allows nitrogen and oxygen to react in car engines?

• A. High temperature
• B. Low pressure
• C. Presence of a catalyst
• D. Low temperature

Answer: (A)

High temperature supplies the energy needed to overcome the strong bond in nitrogen gas and to drive it to react with oxygen to form nitrogen oxides. In the hot gases inside an engine, collisions occur with enough energy that N2 and O2 can combine to form NOx, and the reaction rate increases rapidly with temperature (higher kinetic energy means more successful collisions). A catalyst could speed up many reactions, but the formation of NOx inside the engine doesn’t rely on a catalyst; catalysts are used later in catalytic converters to reduce NOx. At low temperature, the collisions aren’t energetic enough for this reaction to proceed appreciably, so NOx formation is much less likely.