Problem 265 - A Complete Resource Book in Chemistry for JEE Main

Question

When rain is accompanied by a thunderstorm, the collected rain water will have a \(\mathrm{pH}\) value (a) slightly lower than that of rain water without thunderstorm (b) slightly higher than that when the thunderstorm in not there (c) uninfluenced by occurrence of thunderstorm (d) which depends on the amount of dust in air

Step-by-Step Solution

Answer

(a) slightly lower than that of rain water without thunderstorm.

Step 1: Understanding the Chemistry

When rain is accompanied by a thunderstorm, electrical discharges (lightning) occur, which can lead to the formation of nitrogen oxides (NO and NO₂) in the atmosphere.

Step 2: Formation of Acid Rain

The nitrogen oxides react with water vapor in the clouds to form nitric acid (HNO₃), which then dissolves in rainwater, making it more acidic.

Step 3: Comparing pH Values

Since nitric acid is an acid, the presence of this acid lowers the pH of the rainwater when compared to regular rain without a thunderstorm.

Step 4: Conclusion

Based on the formation of nitric acid, the rainwater during a thunderstorm will have a slightly lower pH value than rainwater without a thunderstorm.

Key Concepts

Understanding pH Level

The pH level is an important scale that measures how acidic or basic a substance is. The scale ranges from 0 to 14, with 7 being neutral. Anything below 7 is considered acidic, and above 7 is basic. Rainwater generally has a pH level slightly below 7, making it naturally slightly acidic. This is due to the presence of dissolved carbon dioxide, which forms carbonic acid in the water. When additional acidic substances like nitric acid enter rainwater, the pH drops even more. Understanding this shift in pH is essential in analyzing how natural events, like thunderstorms, influence rainwater composition.

Role of Nitrogen Oxides

Nitrogen oxides, specifically nitrogen monoxide (NO) and nitrogen dioxide (NO₂), play a significant role in atmospheric chemistry. These gases are collectively known as NOx and are produced naturally by lightning or through human activities such as burning fossil fuels. During a thunderstorm, the electrical energy from lightning causes nitrogen (N₂) and oxygen (O₂) in the air to react, forming NO and NO₂. These oxides are not very harmful by themselves but become problematic when they react with other compounds in the atmosphere. This reaction capability significantly affects air quality and water chemistry when they convert into acids.

Formation of Nitric Acid

The process by which nitrogen oxides convert into nitric acid plays a major role in forming acid rain.

• First, nitrogen oxides react with water vapor, a component always present in the atmosphere.
• This reaction results in the formation of nitric acid (HNO₃).
• As the rain falls to the ground, this nitric acid is carried along, dissolving in the water droplets.

This entire process is significantly enhanced during thunderstorms, due to increased NOx emissions from lightning, resulting in more acidic rain.

Impact of Thunderstorms on Rainwater pH

Thunderstorms have a direct impact on the pH level of rainwater due to the presence of electrical discharges. The lightning generated during these storms facilitates the formation of nitrogen oxides. Once formed, these oxides lead to more nitric acid in the atmosphere, which eventually makes its way into the rain. The increased acidity results in a lower pH, meaning the rain is more acidic than that from a clear day. This phenomenon illustrates why rain after a thunderstorm tends to contribute to acid rain. It is important to monitor these changes, as more acidic rain can potentially harm ecosystems, infrastructure, and human health.