Animated Solution for Chemistry - Environmental Chemistry: The statement that is not true about ozone is

This question takes us high above the Earth's surface into the stratosphere, where a silent, invisible battle is constantly being fought. It tests our fundamental understanding of environmental chemistry, specifically the mechanism of ozone depletion.

First, let's establish the baseline. The ozone layer, situated in the stratosphere, is absolutely vital for life on Earth. It acts as a planetary sunscreen, absorbing the majority of the sun's harmful ultraviolet (UV) radiation. Without it, life as we know it would be severely compromised by radiation damage. Therefore, the statement that ozone forms a protective shield against UV radiation is a well-established scientific fact.

CFCs

The trouble begins when human-made chemicals called chlorofluorocarbons (CFCs) are released into the atmosphere. These compounds are incredibly stable in the lower atmosphere (troposphere), which allows them to slowly drift upward over many years until they reach the stratosphere.

Once in the stratosphere, they are exposed to the very UV radiation that the ozone layer is trying to block. This high-energy radiation is strong enough to break the carbon-chlorine bonds in the CFC molecules. This process, known as photodissociation, releases highly reactive chlorine free radicals ($C{l}^{\bullet }$).

These chlorine radicals are the true culprits of ozone depletion. Because they possess an unpaired electron, they are chemically aggressive and immediately seek out stable molecules to react with. They attack the abundant ozone ($O_3$) molecules in their vicinity.

The reaction proceeds as follows:

Notice carefully what is produced here. The chlorine radical strips an oxygen atom from the ozone molecule, forming oxygen gas ($O_2$) and a chlorine monoxide radical ($Cl{O}^{\bullet }$).

This is the critical catch in the question! The first option claims that this reaction produces chlorine dioxide radicals ($Cl{O}_2^{\bullet }$). This is factually incorrect. The chemistry dictates the formation of chlorine monoxide, making statement (a) the false statement we are looking for.

To fully appreciate the danger of CFCs, we must look at what happens next. The chlorine monoxide radical ($Cl{O}^{\bullet }$) can further react with a free atomic oxygen atom (which is naturally present in the stratosphere due to the continuous breakdown and formation of ozone):

This reaction regenerates the original chlorine radical ($C{l}^{\bullet }$)! This means a single chlorine atom can act as a catalyst, destroying up to $100,000$ ozone molecules in a continuous chain reaction before it is eventually removed from the stratosphere.

Finally, regarding the other statements: Ozone is indeed a toxic gas when present at ground level (troposphere), acting as a severe respiratory irritant and a component of photochemical smog. It also readily reacts with nitric oxide ($NO$) to produce nitrogen dioxide ($N{O}_2$). Thus, all statements except (a) are perfectly true.