Question

In which case change in entropy is negative ?

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

Answer 1

The Correct Option is B

To determine in which scenario the change in entropy is negative, we need to understand the concept of entropy and how it varies with different processes:

1. Entropy (S) is a measure of disorder or randomness in a system. A higher entropy means more disorder.
2. Entropy changes can occur in various processes:
   • Sublimation of a solid to gas: This involves changing from a more ordered state (solid) to a less ordered state (gas), hence, entropy increases.
   • Evaporation of water: This process also involves the transition from a liquid (less chaotic) to a vapor state (more chaotic), increasing entropy.
   • Expansion of a gas at constant temperature: When a gas expands, it occupies a larger volume, hence the disorder increases, leading to an increase in entropy.
   • The reaction 2H(g) \rightarrow H_2(g): Here, two moles of hydrogen atoms (in gaseous state) combine to form one mole of a hydrogen molecule. This reduction in the number of gas molecules results in a decrease in entropy, as the system becomes more ordered.

Thus, the change in entropy is negative for the process where two moles of hydrogen atoms react to form one mole of hydrogen molecules (2H(g) \rightarrow H_2(g)), as the system becomes more ordered.