Which of the following reactions would result in decreased entropy?

Entropy is a measure of degree of randomness. The higher the degree of randomness, the greater the entropy is.

A. False
The degree of randomness of the system increases when 1 mole of gaseous molecules is converted to 2 moles of gaseous molecules. Hence, entropy increases.

B. False
In solid state, Mg²⁺ and Cl⁻ ions are well arranged in pattern. When MgCl₂ dissolves, the ions become mobile and thus the degree of randomness of the system increases. Hence, entropy increases.

C. False
The degree of randomness of the system increases when 5 moles of gaseous molecules are converted to 6 moles of gaseous molecules. Hence, entropy increases.

D. True
The degree of randomness of the system decreases when 4 moles of gaseous molecules are converted to 2 moles of gaseous molecules. Hence, entropy decreases.

The answer: D. N₂(g) + 3H₂(g) → 2NH₃(g)

D. results in decreased entropy.