Question

In which case change in entropy is negative? (1)(2) (3 (4)

• A. Evaporation of water
• B. Expansion of a gas at constant temperature
• C. Sublimation of solid to gas
• D. \(2H(g) → H_2(g)\)

Answer 1

The Correct Option is D

To determine in which case the change in entropy is negative, we need to understand the concept of entropy. Entropy (\(S\)) is a measure of the randomness or disorder of a system. Generally, the more disordered a system is, the higher its entropy. Here we need to analyze each process:

1. Evaporation of water: Involves the transformation of water from liquid to vapor, which increases the disorder as molecules move from a more ordered liquid state to a less ordered gaseous state. This results in an increase in entropy.
2. Expansion of a gas at constant temperature: When a gas expands, the molecules spread out over a larger volume, increasing the disorder of the system. Thus, entropy increases.
3. Sublimation of a solid to gas: This process changes a solid directly into gas, again increasing the disorder as molecules move from a solid (ordered) to a gas (disordered). As a result, entropy increases.
4. \(2H(g) → H_2(g)\): Here, two moles of gaseous hydrogen atoms combine to form one mole of hydrogen molecule. This reaction decreases the number of gas particles, reducing the disorder in the system. Therefore, the entropy decreases, resulting in a negative change in entropy.

Based on the explanations above, the correct answer is option 4: \(2H(g) → H_2(g)\). This reaction exhibits a negative change in entropy because the system becomes more ordered.

Therefore, the reaction \(2H(g) → H_2(g)\) is characterized by a negative entropy change.