Given below are two statements.
One is labelled as Assertion (A) and the other is labelled as Reason

Q: Why does rubber crack specifically, and not other materials like plastic?

Rubber (polyisoprene) contains many carbon-carbon double bonds ($C=C$) in its main polymer backbone. These double bonds are highly susceptible to oxidative cleavage by ozone (ozonolysis). Many common plastics (like polyethylene or PVC) have saturated single bonds, which are much more resistant to ozone attack.

Q: Is classical smog also responsible for cracking rubber?

No. Classical smog is primarily composed of smoke, fog, and sulfur dioxide ($SO_2$), making it a reducing mixture. It lacks the strong oxidizing agents like ozone required to cleave the double bonds in rubber.

Q: What is PAN and what does it do?

PAN stands for Peroxyacetyl Nitrate. It is a secondary pollutant present in photochemical smog. While it is a powerful eye irritant and toxic to plants, ozone is the primary culprit for the cracking of rubber.

Q: How can we prevent rubber from cracking due to smog?

Manufacturers often add 'antiozonants' to rubber products like tires. These chemicals migrate to the surface of the rubber and react with ozone before the ozone can attack the polymer's double bonds, thereby protecting the material.

Q: Why is photochemical smog called 'oxidizing' smog?

It is called oxidizing smog because it contains high concentrations of oxidizing agents like ozone ($O_3$), nitrogen dioxide ($NO_2$), and PAN, which readily oxidize other substances they come into contact with.

Question

Given below are two statements.
One is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) Photochemical smog causes cracking of rubber.
Reason (R) Presence of ozone, nitric oxide, acrolein, formaldehyde and peroxyacetyl nitrate in photochemical smog makes it oxidising.
Choose the most appropriate answer from the options given below.

Options:

(A) Both (A) and (R) are true but (R) is not the correct explanation of (A).

(B) Both (A) and (R) are true and (R) is the correct explanation of (A).

(C) (A) is false but (R) is true.

(D) (A) is true but (R) is false.

Sigma Percentile · Accepted Answer

### Analyzing the Setup

When we look at the problem, we are presented with an Assertion and a Reason regarding the effects of photochemical smog on rubber. To truly understand what is happening, we need to dive into the molecular structure of rubber and the chemical composition of photochemical smog.

Rubber, in its natural form, is a polymer known as **polyisoprene**. If you visualize its long polymer chain, you will notice that it is packed with carbon-carbon double bonds ($C=C$). These double bonds are electron-rich areas, making them prime targets for any electron-hungry chemical species.

### The Master Equation: Photochemical Smog

Now, let's shift our focus to photochemical smog. Unlike classical smog, which is reducing in nature, photochemical smog is highly **oxidizing**. It is a toxic soup of chemicals formed when sunlight reacts with nitrogen oxides ($NO_x$) and volatile organic compounds (VOCs) in the atmosphere.

The key players in photochemical smog are strong oxidizing agents such as **ozone ($O_3$)**, nitric oxide ($NO$), acrolein, formaldehyde, and peroxyacetyl nitrate (PAN). Among these, ozone is exceptionally reactive.

### The Attack: Ozonolysis

Imagine you are a highly reactive ozone molecule floating in the air, and you bump into a rubber tire. What happens next? The ozone molecule immediately seeks out the electron-rich double bonds in the polyisoprene chain.

This chemical attack leads to a reaction known as **ozonolysis**. The ozone molecule cleaves the carbon-carbon double bond, effectively snapping the long polymer chain into smaller, weaker fragments.

### Final Conclusion

Macroscopically, this microscopic chain-breaking manifests as physical degradation. The rubber loses its elasticity and structural integrity, leading to visible **cracks** on its surface. This phenomenon is widely known as the cracking of rubber.

Therefore, the Assertion that photochemical smog causes cracking of rubber is absolutely **True**. The Reason, which states that the presence of ozone and other chemicals makes the smog oxidizing, is also **True**. Furthermore, it is precisely this oxidizing nature (specifically the action of ozone) that causes the cracking. Thus, the Reason is the correct explanation for the Assertion.

Select Answer:

Visualized Solution

$Structure of Rubber$

$Photochemical Smog Components$

$Ozonolysis of Rubber$

$Cracking Mechanism$

$Conclusion$

$The Way Forward$

The Sigma Insight: Atmospheric Pollution

Analyzing the Setup

The Master Equation

The Attack

Final Conclusion

Similar Questions

Assertion (A) Ozone is destroyed by CFCs in the upper stratosphere. Reason (R) Ozone holes increase the amount of UV radiation reaching the earth.

The correct set of species responsible for the photochemical smog is

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.

Identify the wrong statements in the following

The smog is essentially caused by the presence of

The primary pollutant that leads to photochemical smog is

The molecule that has minimum/no role in the formation of photochemical smog, is