Entropy Decrease Processes Explained

Entropy ($S$) measures the randomness or disorder of a system. A process involves a decrease in entropy when the system becomes more ordered.

Analyzing Chemical Reactions for Entropy Change

Let's examine each process:

• Burning of coal: This combustion increases the number of gas molecules and randomness, hence entropy increases.
• Formation of NH$_3$ from H$_2$ and N$_2$: The reaction is $N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$. Here, 4 moles of gas react to form 2 moles of gas. This decrease in the number of molecules leads to greater order, meaning entropy decreases.
• Evaporation of water: Liquid water turning into water vapor involves a transition from a less random state to a more random state. Entropy increases.
• Melting of ice: Solid ice transforming into liquid water represents a change from an ordered structure to a less ordered state. Entropy increases.

Identifying Entropy Decrease: Formation of NH$_3$

The formation of ammonia ($NH_3$) from nitrogen ($N_2$) and hydrogen ($H_2$) is the only process listed where the number of moles of gas decreases (4 moles $\rightarrow$ 2 moles). This results in a more ordered system and therefore, a decrease in entropy.