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

The pH of rainwater during a thunderstorm is slightly lower than without one.

Step 1: Understanding the Role of Thunderstorms

Thunderstorms are known for producing lightning. This lightning results in a series of chemical reactions in the atmosphere that can affect the composition of rainwater.

Step 2: Exploring the Effect of Lightning on pH

When lightning occurs during a thunderstorm, it causes nitrogen and oxygen in the atmosphere to react, forming nitrogen oxides. These nitrogen oxides dissolve in rainwater to form nitric acid, a strong acid that lowers the pH of the rainwater.

Step 3: Comparing pH Changes in Rain

In contrast, rain without thunderstorms does not generally have as much nitric acid formation because there is no lightning to create significant amounts of nitrogen oxides. Therefore, the pH of rainwater during a thunderstorm would be slightly lower than rainwater without a thunderstorm due to increased nitric acid.

Step 4: Choosing the Correct Option

Based on the understanding that thunderstorms lead to the formation of nitric acid in rain, which lowers its pH, the correct option is (a) slightly lower than that of rain water without thunderstorm.

Key Concepts

Thunderstorm Effects

Thunderstorms are fascinating atmospheric events characterized by thunder, lightning, and often intense rainfall. They play a significant role in influencing atmospheric conditions. During a thunderstorm, there isn't just a display of electrical energy; these natural occurrences also trigger unique chemical processes. Lightning, for instance, is a spectacular discharge of electricity that can affect the molecules in the air. When a lightning bolt flashes across the sky, it can reach temperatures hotter than the surface of the sun, providing the energy needed for certain nitrogen oxides to form. This makes thunderstorms a key player in the atmospheric chemistry that influences our environment, especially through rainwater composition alteration.

Nitrogen Oxides Formation

During a thunderstorm, the intense energy from lightning strikes a reactive catalyst in the atmosphere. This energy facilitates the rapid formation of nitrogen oxides. Air around us is composed largely of nitrogen gas (N₂) and oxygen gas (O₂). When lightning strikes, the intense heat and energy cause these gases to react and form nitrogen oxides (NO and NO₂).

These nitrogen oxides are crucial players in atmospheric chemistry. They contribute to several processes, including the formation of ozone and particulate matter, but more directly, they react with water in the rain to create nitric acid. Thus, thunderstorms function as natural factories for nitrogen oxides, changing the chemical makeup of rain that falls.

Rainwater Acidity

The acidity of rainwater is an important environmental factor that affects both natural ecosystems and human-made structures. In the context of thunderstorms, the acidity of rainwater tends to increase due to the formation of nitric acid. When nitrogen oxides produced by lightning dissolve in rain droplets, they form nitric acid, a potent acidic compound. This reaction can decrease the pH level of rainwater, making it more acidic.

Regular rainwater is slightly acidic, with a typical pH around 5.6, mainly due to dissolved carbon dioxide forming carbonic acid. However, during thunderstorms, the pH can drop lower because nitric acid is a stronger acid than carbonic acid. Consequently, rain collected during thunderstorms is slightly more acidic than usual.

Atmospheric Chemical Reactions

The atmosphere is a dynamic space where numerous chemical reactions occur, many of which are influenced by environmental phenomena like thunderstorms. These reactions can significantly alter atmospheric composition and, subsequently, the chemical nature of rainwater. In the case of thunderstorms, several atmospheric chemical reactions are set in motion by the heat and energy of lightning.

Firstly, the formation of nitrogen oxides is a primary reaction initiated by the energy from lightning. These nitrogen oxides then react with atmospheric moisture, resulting in the creation of nitric acid. This series of transformations exemplifies the complexity and interconnectedness of atmospheric chemistry. Understanding these processes helps in predicting and managing environmental impacts, particularly changes in rainwater pH caused by these reactions.