Entropy — ΔS, spontaneity
Thermodynamics & Thermochemistry
9
JEE Qs
8%
Hard
60
min
Always remember that spontaneity is governed by the total entropy change of the universe (ΔS_universe), which is the sum of system and surroundings entropy changes.
🧮 Key Formulas
✅ Key Points for JEE
- 1Entropy (S) is a measure of the degree of randomness or disorder in a system. It is a state function.
- 2The Second Law of Thermodynamics states that for any spontaneous process, the total entropy of the universe (system + surroundings) must increase: ΔS_universe > 0.
- 3For an equilibrium process (e.g., phase transition at its equilibrium temperature), ΔS_universe = 0.
- 4Factors that generally increase entropy include phase transitions (solid to liquid to gas), increase in the number of gaseous moles in a reaction, increase in temperature, and dissolution of a solid in a liquid.
- 5The Third Law of Thermodynamics states that the entropy of a perfect crystalline substance is zero at absolute zero (0 K).
⚠️ Common Mistakes
- ✕Confusing ΔS_system > 0 with spontaneity; spontaneity is determined by ΔS_universe > 0, not just ΔS_system.
- ✕Incorrectly calculating ΔS_surroundings by forgetting the negative sign or not using the correct ΔH_system.
- ✕Not using absolute temperature (Kelvin) in entropy calculations, especially for phase transitions and ΔS_surroundings.
- ✕Failing to convert units (e.g., J to kJ) when combining ΔH and ΔS values, leading to incorrect magnitudes.
📝 Practice Questions
See allQ32.Given are statements for certain thermodynamic variables, (A) Internal energy, volume (V) and mass (M) are extensive variables. (B) Pressure (P), temperature (T) and density ( ρ ) are intensive variables. (C) Volume (V), temperature (T) and density ( ρ ) are intensive variables. (D) Mass (M), temperature (T) and internal energy are extensive variables. Choose the correct answer from the options given below : (1) (B) and (C) Only (2) (C) and (D) Only (3) (D) and (A) Only (4) (A) and (B) Only
Q64.Ice at −5∘C is heated to become vapor with temperature of 110∘C at atmospheric pressure. The entropy change associated with this process can be obtained from 2025 (23 Jan Shift 1) JEE Main Previous Year Paper (1) (2) (3) (4)
Q53.A liquid when kept inside a thermally insulated closed vessel at 25∘C was mechanically stirred from outside. What will be the correct option for the following thermodynamic parameters ? (1) ΔU < 0, q = 0, w > 0 (2) ΔU = 0, q = 0, w = 0 (3) ΔU > 0, q = 0, w > 0 (4) ΔU = 0, q < 0, w > 0
Q31.Water of mass m gram is slowly heated to increase the temperature from T1 to Tz The change in entropy of the water, given specific heat of water is 1Jkg−1 K−1 , is : (1) m ln ( T2T1 ) (2) zero (3) m ln ( T1T2 ) (4) m (T2 −T1)
Q74.Niobium ( Nb ) and ruthenium (Ru) have " x " and " y " number of electrons in their respective 4 d orbitals. The value of x + y is ______ -.
Q70.The correct stability order of the following species/molecules is: (1) q > r > p (2) r > q > p (3) q > p > r (4) p > q > r Q71. 1 The standard enthalpy and standard entropy of decomposition of N2O4 to NO2 are 55.0 kJ mol−1 and 175.0 J/K/mol respectively. The standard free energy change for this reaction at 25∘C in J mol−1 is ______ (Nearest integer)
NCERT Chapters
- Class 11 Chemistry Ch 6: Thermodynamics