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 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 Question

The question asks how the pH level of rainwater is affected by a thunderstorm. We need to know whether this makes the pH higher, lower, or has no effect.

Step 2: Background Knowledge

Thunderstorms produce lightning, which can lead to the formation of nitric acid when atmospheric nitrogen combines with water. This would lower the pH, making it more acidic.

Step 3: Analyzing Options

Option (a) suggests a lower pH, which corresponds to increased acidity. Option (b) suggests a higher pH, meaning less acidic or more basic. Option (c) indicates no change. Option (d) relates pH change to dust, which is not a direct result of thunderstorms.

Step 4: Evaluating Each Option

Considering the science, thunderstorms can cause rain to become more acidic due to the formation of nitric acid. This directly supports option (a). Option (b) is incorrect because the addition of acidic compounds would not raise pH. Option (c) is incorrect based on the known chemical reactions. Option (d) is unrelated to thunderstorms.

Step 5: Conclusion

Since thunderstorms can produce nitric acid, which makes the rain slightly more acidic, the best answer is option (a), as it directly relates to the pH being lower due to increased acidity.

Key Concepts

pH of rainwater

The pH of rainwater is an important indicator of its acidic or basic nature. Normally, pure rainwater has a pH of around 5.6. This is slightly acidic due to the natural presence of carbon dioxide in the atmosphere, which dissolves in rainwater to form carbonic acid. As a result, even without pollutants, rainwater is never neutral.
However, the actual pH of rainwater can vary based on several factors including environmental pollution and natural events. If rainwater contains more acidic components, like sulfuric acid or nitric acid, its pH will drop, indicating increased acidity. Conversely, if it contains basic components, the pH would increase, making it less acidic.

effect of thunderstorms on pH

Thunderstorms have a direct impact on the pH of rainwater. During a thunderstorm, the conditions are right for lightning, which can initiate chemical reactions in the atmosphere.
Lightning provides the energy needed for nitrogen ( _2) and oxygen ( _2) in the air to react and form nitric oxide (NO). Following this, nitrogen dioxide (NO_2) can form and further react with water. This series of reactions creates nitric acid (HNO_3), a strong acid that mixes with rainwater, lowering its pH by making it more acidic.

• Formation of nitric oxide through lightning
• Conversion to nitrogen dioxide
• Formation of nitric acid in the presence of water

Thus, the acidic nature of rain increases following a thunderstorm due to the formation of these acidic compounds, which lowers the pH.

formation of nitric acid in rainwater

The formation of nitric acid in rainwater is a complex process driven by atmospheric reactions. This typically begins when energetic lightning strikes break apart nitrogen molecules ( _2) and oxygen molecules ( _2), leading to the formation of nitrogen oxides.
Nitrogen Oxides and Water:
In the atmosphere, nitrogen oxide (NO) further reacts with oxygen to form nitrogen dioxide (NO_2). Both NO and NO_2 are collectively referred to as nitrogen oxides. In the presence of water, such as in rain, these nitrogen oxides can convert to nitric acid by reacting with oxygen and water molecules.
The chemical response can be represented roughly as:
2NO_2 + H_2O → HNO_3 + HNO_2
The resulting nitric acid is a strong acid that significantly lowers the pH of rain, contributing to the acidity of rainwater. This process demonstrates the key role of natural phenomena, like thunderstorms, in influencing the chemical properties of precipitation.