Question

Entropy decreases in which of the following reactions? $ \text{(A) } \text{Br}_2 (l) \to \text{Br}_2 (g) $ $ \text{(B) } \text{Br}_2 (g) \to \text{Br}_2 (l) $ $ \text{(C) } \text{Br}_2 (l) \to \text{Br}_2 (g) \text{ and } \text{Br}_2 (g) \to \text{Br}_2 (l) $

• A. \( \text{Br}_2 (l) \to \text{Br}_2 (g) \)
• B. \( \text{Br}_2 (g) \to \text{Br}_2 (l) \)
• C. Both A and B
• D. None of the above

Hint:

For phase transitions, the general rule is: gas to liquid or solid results in a decrease in entropy, while liquid to gas or solid to liquid results in an increase in entropy.

Answer 1

The Correct Option is B

Entropy is a measure of the disorder or randomness of a system. For a phase transition, such as from liquid to gas, entropy increases due to the increase in the freedom of motion of molecules in the gas phase. In the reaction: \[ \text{Br}_2 (l) \to \text{Br}_2 (g) \] there is a phase transition from liquid to gas, which increases entropy. However, when the phase transition is from gas to liquid: \[ \text{Br}_2 (g) \to \text{Br}_2 (l) \] the molecules in the gas phase become more ordered when they condense into the liquid phase, which results in a decrease in entropy.
Thus, entropy decreases in the reaction \( B \).