Problem 7 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The process with negative entropy change is: [Main Jan. 10, 2019 (II)] (a) Dissociation of \(\mathrm{CaSO}_{4}(\mathrm{~s})\) to \(\mathrm{CaO}(\mathrm{s})\) and \(\mathrm{SO}_{3}(\mathrm{~g})\) (b) Sublimation of dry ice (c) Dissolution of iodine in water (d) Synthesis of ammonia from \(\mathrm{N}_{2}\) and \(\mathrm{H}_{2}\)

Step-by-Step Solution

Answer

Option (d) is the process with negative entropy change.

Step 1: Understand Entropy Change

Entropy change is the measure of disorder or randomness in a system. Processes that increase the number of gas molecules generally have positive entropy changes, while processes that result in fewer or unchanged gas molecules tend to have negative or lesser positive entropy changes.

Step 2: Analyze Option (a)

Dissociation of \( \text{CaSO}_4(\text{s}) \) to \( \text{CaO} (\text{s}) \) and \( \text{SO}_3(\text{g}) \) involves breaking a solid compound into a solid and gaseous component. This results in an increase in the number of gas molecules, indicating a positive entropy change.

Step 3: Analyze Option (b)

Sublimation of dry ice \( \text{(solid CO}_2 \rightarrow \,\text{gaseous CO}_2) \) involves a solid turning directly into a gas, significantly increasing the disorder and the number of gas molecules. This reflects a positive entropy change.

Step 4: Analyze Option (c)

Dissolution of iodine in water involves solid iodine dispersing in water. Typically, dissolving solids in liquids increases entropy because it increases disorder. This process usually leads to a positive entropy change.

Step 5: Analyze Option (d)

Synthesis of ammonia (\( \text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3 \)) involves four gas molecules converting into two gas molecules. This reduces the total number of gas molecules, leading to a decrease in entropy, therefore a negative entropy change.

Key Concepts

Negative Entropy

Entropy is all about the level of disorder in a system. When we talk about a negative entropy change, it means the system is becoming less disordered, moving towards a more ordered state.
This typically happens when there are fewer gas molecules in a chemical reaction, as gases contribute greatly to disorder. Consider the synthesis of ammonia from nitrogen and hydrogen:

• In this reaction, four gas molecules (one nitrogen and three hydrogen) combine to form two ammonia molecules.
• This reduction in the number of gas molecules indicates a more ordered system.
• Hence, the reaction results in a negative entropy change.

Understanding how molecules behave and change during reactions can help predict whether entropy will increase or decrease.

Disorder in Chemical Reactions

Chemical reactions involve the making and breaking of bonds, leading to changes in the structure and energy of the system. Entropy is a key factor here.
Disorder, or an increase in entropy, generally occurs when:

• Solids turn into liquids or gases, which adds to disorder.
• Reactions produce gases from non-gaseous reactants.
• Multiple products are formed from single reactants, sharing energy and space unevenly.

However, not all reactions lead to increased disorder. Some, like the ammonia synthesis, decrease disorder:

• In ammonia synthesis, fewer gas molecules result in more ordered dynamics.
• This minimized disorder is crucial for reactions where product stability is desired.

Recognizing which reactions increase or decrease disorder helps understand the direction and feasibility of chemical processes.

Gas Molecules and Entropy

Gas molecules play a significant role in determining entropy. Since gases have the most freedom of movement compared to solids and liquids, their presence increases system disorder.
Here’s how gas molecules affect entropy:

• When a solid or liquid turns into a gas, the entropy of the system usually increases substantially.
• Sublimation, like that of dry ice turning into carbon dioxide gas, dramatically boosts entropy due to the increase in gas molecules.
• Conversely, reactions like ammonia synthesis result in fewer gas molecules, thus reducing entropy.

Entropy changes involving gases are a hallmark of many processes because the more you have free-flying molecules, the higher the entropy.