Animated Solution for Chemistry - Environmental Chemistry: Assertion (A) Nitrogen and oxygen are the main components in the atmosphere but these do not react to form oxides of nitrogen. Reason (R) The reaction between nitrogen and oxygen requires high temperature.

Have you ever stopped to think about the air you are breathing right now? Every time you take a breath, your lungs fill with a mixture that is roughly $78\%$ nitrogen ($N_2$) and $21\%$ oxygen ($O_2$). These two gases are constantly bumping into each other, billions of times a second. Yet, nothing happens. They don't react.

If they did react under normal conditions, they would form nitrogen oxides like $NO$ and $N{O}_2$, which are highly toxic gases responsible for acid rain and photochemical smog. We would literally be breathing poison! So, nature has a built-in safety mechanism. Let's dive into the chemistry of why the atmosphere is so stable.

The secret to nitrogen's peaceful coexistence with oxygen lies in its molecular structure. A nitrogen molecule ($N_2$) consists of two nitrogen atoms locked together by a triple covalent bond ($N\equiv N$).

This triple bond is one of the strongest chemical bonds in all of nature. It has a staggering bond dissociation enthalpy of approximately $941.4\text{ kJ/mol}$. To put that into perspective, it takes an immense amount of energy just to pull those two nitrogen atoms apart so they can react with something else. Because of this, nitrogen gas is practically inert (unreactive) at room temperature.

Activation Energy

In chemistry, for a reaction to occur, the reactant molecules must collide with enough energy to break their existing bonds and form new ones. This minimum energy requirement is called the activation energy ($E_a$).

Imagine rolling a boulder up a steep hill. If you don't push it hard enough, it just rolls back down. Similarly, when $N_2$ and $O_2$ molecules collide at room temperature, they simply bounce off each other because they don't have enough kinetic energy to overcome the massive activation energy barrier created by the nitrogen triple bond.

High Temperature

So, how do we force nitrogen and oxygen to react? We need to give them a massive energetic push!

This is where the Reason (R) in our question comes into play. The reaction between nitrogen and oxygen requires extremely high temperatures—around $2000\text{ K}$.

Where do we find such extreme temperatures in nature? Lightning strikes! When lightning tears through the sky, it superheats the surrounding air, providing the massive activation energy needed to shatter the $N\equiv N$ triple bonds. The freed nitrogen atoms immediately react with oxygen to form nitric oxide ($NO$). This same high-temperature reaction also occurs inside the combustion engines of our cars, which is why vehicle exhaust is a major source of nitrogen oxide pollution.

Bringing it all together, the Assertion (A) is absolutely correct: nitrogen and oxygen are the main components of the atmosphere but do not react under normal conditions.

The Reason (R) is also perfectly correct: the reaction requires high temperatures.

Most importantly, the reason is the exact explanation for the assertion. The requirement for high temperature (to overcome the massive activation energy of the triple bond) is precisely why they don't react under normal, everyday conditions. Therefore, both statements are true, and the reason correctly explains the assertion!