PYQ + Trends
1. TITLE
NEET asked Entropy Change Signs (ΔS>0 vs <0) in 2019, 2020, 2024 — Expect it in 2025
2. THE ACTUAL NEET QUESTION
NEET 2024 — Question
In which of the following processes entropy increases?
A. A liquid evaporates to vapour.
B. Temperature of a crystalline solid lowered from 130K to 0K.
C. $2NaHCO_{3(s)} \rightarrow Na_{2}CO_{3(s)} + CO_{2(g)} + H_{2}O_{(g)}$
D. $Cl_{2(g)} \rightarrow 2Cl_{(g)}$
Choose the correct answer from the options given below:
A) A and C
B) A, B and D
C) A, C and D
D) C and D
3. OFFICIAL ANSWER
NEET Answer Key: (C)
4. COMPLETE SOLUTION
Concept explanation & Step-by-step breakdown:
Entropy ($S$) is a measure of the degree of randomness or disorder in a system. A process where disorder increases will have a positive entropy change ($\Delta S > 0$). Let's evaluate each statement:
- Statement A (Liquid evaporates to vapour): The gaseous state has the highest entropy, while solids have the lowest. Going from liquid to gas increases molecular randomness, so entropy increases ($\Delta S > 0$).
- Statement B (Cooling a solid to 0K): Lowering the temperature decreases molecular motion. At absolute zero (0K), a perfectly crystalline substance has minimum entropy (taken as zero). Thus, entropy decreases ($\Delta S < 0$).
- Statement C (Solid decomposing into solid + gases): Look at the physical states. A solid reactant breaks down to form one solid and two moles of gaseous products ($CO_2$ and $H_2O$). Because the number of gaseous product molecules increases, the randomness—and hence entropy—increases ($\Delta S > 0$).
- Statement D (Dissociation of $Cl_2$ gas): 1 mole of $Cl_{2(g)}$ breaks apart to form 2 moles of $Cl_{(g)}$ atoms. The number of gas particles increases, leading to a more disordered state, so entropy increases ($\Delta S > 0$).
Therefore, processes A, C, and D result in an entropy increase.
5. WHY NEET ASKED THIS
Explain the examiner's intent:
* What concept were they testing? The examiner is testing your ability to predict the sign of $\Delta S$ by looking at physical state changes (vaporization vs. crystallization/cooling) and changes in the number of gaseous moles ($\Delta n_g$) during a chemical reaction.
* What's the common mistake they expected students to make? Students often forget the Third Law of Thermodynamics and might assume any temperature change implies an increase in entropy, missing that cooling reduces randomness.
* Which option was the "trap"? Option B ("A, B and D") was the trap for students who skimmed Statement B and only registered "temperature of a crystalline solid" without noticing it was being lowered to 0K.
6. TREND ANALYSIS
PATTERN ALERT:
| Year | How they asked | Twist |
|---|---|---|
| 2024 | Identify multiple processes where entropy increases ($\Delta S > 0$). | Combined phase changes (evaporation, cooling) with chemical reactions (decomposition, atomization) in one multiple-statement question. |
| 2020 | Asked for the signs of both $\Delta H$ and $\Delta S$ for the reaction $2Cl_{(g)} \rightarrow Cl_{2(g)}$. | Reversed the typical bond-breaking atomization. Combining two gaseous atoms into one molecule decreases entropy ($\Delta S < 0$). |
| 2019 | "In which case change in entropy is negative?" | Provided simple isolated processes. The answer was $2H_{(g)} \rightarrow H_{2(g)}$ because the number of gaseous moles decreased ($\Delta n_g < 0$). |
Prediction for NEET 2025:
Expect them to link the sign of entropy ($\Delta S$) directly to spontaneity ($\Delta G = \Delta H - T\Delta S$). You may be given a reaction with specific signs for $\Delta H$ and $\Delta S$ and asked at what temperature range (high or low) the reaction becomes spontaneous.
7. SIMILAR QUESTIONS TO PRACTICE
"If you understood this, you can solve:"
* NEET 2019 Q19: In which case change in entropy is negative? (Evaporation vs Expansion vs Sublimation vs Bond formation)
* NEET 2020 Q17: For the reaction $2Cl_{(g)} \rightarrow Cl_{2(g)}$, predict the signs of $\Delta_r H$ and $\Delta_r S$.
* NEET 1998 Q87: Identify the correct statement regarding entropy at absolute zero.
8. QUICK REVISION FOR THIS TYPE
For Entropy Change Sign questions, always:
- First check the physical states of the reactants and products. Entropy order is always: Gas > Liquid > Solid. Phase changes like melting, boiling, and sublimation have $\Delta S > 0$.
- Then apply the "gaseous moles" rule. Calculate $\Delta n_g$ (moles of gaseous products – moles of gaseous reactants). If $\Delta n_g > 0$, entropy increases ($\Delta S > 0$). If $\Delta n_g < 0$, entropy decreases ($\Delta S < 0$).
- Watch out for temperature drops. Cooling a system or forming a perfectly ordered crystalline solid at 0K will always result in a decrease in entropy ($\Delta S < 0$).
9. NCERT BASE
This question tests: NCERT Class 11, Unit 5 (Thermodynamics), Sections on "Entropy and Spontaneity" and "Absolute Entropy and Third Law of Thermodynamics".
10. CTA
All PYQs from this chapter → Chemical Thermodynamics Guide
Practice similar questions → Mock exams