Chemical Thermodynamics NEET Previous Year Questions (PYQs)
Practice NEET previous year questions from Chemical Thermodynamics. These MCQs cover enthalpy, entropy, Gibbs free energy, spontaneity of reactions, work done and thermodynamic criteria with answers and clear explanations.
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Chemical Thermodynamics Chapter Notes
Q1. The bond dissociation energies of X2, Y2 and XY are in the ratio 1 : 0.5 : 1. If ΔH for formation of XY is −200 kJ mol−1, the bond dissociation energy of X2 is:
(NEET 2018)
- (A) 100 kJ mol−1
- (B) 200 kJ mol−1
- (C) 400 kJ mol−1
- (D) 800 kJ mol−1
Show Answer
(D)
Show Explanation
Using bond energy relation ΔH = ΣB.E.(reactants) − ΣB.E.(products), bond energy of X2 comes out to be 800 kJ mol−1.
Q2. For a reaction, ΔH = 35.5 kJ mol−1 and ΔS = 83.6 J K−1 mol−1. The reaction is spontaneous at:
(NEET 2017)
- (A) T > 425 K
- (B) All temperatures
- (C) T > 298 K
- (D) T < 425 K
Show Answer
(A)
Show Explanation
For spontaneity, ΔG = ΔH − TΔS < 0. This is possible only when T > 425 K.
Q3. A gas expands in a well-insulated container against a constant external pressure of 2.5 atm from 2.5 L to 4.5 L. The change in internal energy (ΔU) is:
(NEET 2017)
- (A) −500 J
- (B) −505 J
- (C) +505 J
- (D) +1136 J
Show Answer
(B)
Show Explanation
For an adiabatic process, q = 0. Hence ΔU = w = −PΔV = −2.5 × 2 L-atm ≈ −505 J.
Q4. The correct thermodynamic condition for a reaction to be spontaneous at all temperatures is:
(NEET 2016)
- (A) ΔH > 0, ΔS < 0
- (B) ΔH > 0, ΔS > 0
- (C) ΔH < 0, ΔS > 0
- (D) ΔH < 0, ΔS < 0
Show Answer
(C)
Show Explanation
ΔG = ΔH − TΔS is negative at all temperatures only when ΔH < 0 and ΔS > 0.
Q5. The heat of combustion of carbon to CO2 is −393.5 kJ mol−1. The heat released on formation of 35.2 g of CO2 is:
(NEET 2015)
- (A) −315 kJ
- (B) +315 kJ
- (C) −630 kJ
- (D) −3.15 kJ
Show Answer
(A)
Show Explanation
35.2 g CO2 = 0.8 mol. Heat released = 0.8 × 393.5 ≈ 315 kJ.
Q6. For a reaction, ΔU = 2.1 kcal and ΔS = 20 cal K−1 at 300 K. The value of ΔG is:
(NEET 2014)
- (A) +2.7 kcal
- (B) −2.7 kcal
- (C) +9.3 kcal
- (D) −9.3 kcal
Show Answer
(B)
Show Explanation
First calculate ΔH, then ΔG = ΔH − TΔS. The value comes out to be −2.7 kcal.
Q7. Under isothermal condition, a gas expands from 0.1 L to 0.25 L against a constant pressure of 2 bar. Work done is:
(NEET 2019)
- (A) −30 J
- (B) 5 kJ
- (C) 25 J
- (D) 30 J
Show Answer
(A)
Show Explanation
Work = −PΔV = −2 × (0.25 − 0.1) bar L = −0.30 bar L = −30 J.
Q8. In which case is entropy change negative?
(NEET 2019)
- (A) Evaporation of water
- (B) Expansion of gas
- (C) Sublimation of solid
- (D) 2H(g) → H2(g)
Show Answer
(D)
Show Explanation
Entropy decreases when gas molecules combine to form fewer moles.
Q9. For free expansion of an ideal gas under adiabatic condition, which is correct?
(NEET 2020)
- (A) q ≠ 0, w ≠ 0
- (B) q = 0, w ≠ 0
- (C) q ≠ 0, w = 0
- (D) q = 0, w = 0
Show Answer
(D)
Show Explanation
In free expansion, q = 0 (adiabatic) and w = 0 (no external pressure).
Q10. Which relation is correct for one mole of an ideal gas?
(NEET 2021)
- (A) Cp − Cv = R
- (B) Cp + Cv = R
- (C) Cp/Cv = R
- (D) Cp = Cv
Show Answer
(A)
Show Explanation
For one mole of an ideal gas, Cp − Cv = R.
Q11. For a spontaneous reaction, which condition must be satisfied?
(NEET 2005)
- (A) ΔH < 0
- (B) ΔS < 0
- (C) ΔG < 0
- (D) ΔG > 0
Show Answer
(C)
Show Explanation
A reaction is spontaneous when Gibbs free energy change (ΔG) is negative.
Q12. During isothermal expansion of an ideal gas, its:
(NEET 1998)
- (A) Internal energy increases
- (B) Internal energy decreases
- (C) Internal energy remains constant
- (D) Enthalpy becomes zero
Show Answer
(C)
Show Explanation
For an ideal gas, internal energy depends only on temperature.
Q13. Unit of entropy is:
(NEET 2002)
- (A) J
- (B) J K−1
- (C) J mol−1
- (D) J mol−1 K−1
Show Answer
(D)
Show Explanation
Entropy is expressed as energy per mole per kelvin.
Q14. Which pair of reactions is always spontaneous?
(NEET 2005)
- (A) Endothermic & decreasing entropy
- (B) Exothermic & decreasing entropy
- (C) Endothermic & increasing entropy
- (D) Exothermic & increasing entropy
Show Answer
(D)
Show Explanation
Exothermic reaction with increase in entropy is spontaneous at all temperatures.
Q15. Which of the following is a state function?
(NEET 2008)
- (A) Work
- (B) Heat
- (C) Entropy
- (D) Path
Show Answer
(C)
Show Explanation
Entropy depends only on the state of the system.
Q16. Three moles of an ideal gas expand freely into vacuum. The work done is:
(NEET 2010)
- (A) Infinite
- (B) 3 J
- (C) 9 J
- (D) Zero
Show Answer
(D)
Show Explanation
Free expansion occurs against zero external pressure, so work done is zero.
Q17. At equilibrium for a reaction:
(NEET 2006)
- (A) ΔG < 0
- (B) ΔG > 0
- (C) ΔG = 0
- (D) ΔH = 0
Show Answer
(C)
Show Explanation
At equilibrium, Gibbs free energy change is zero.
Q18. For an endothermic reaction, the value of ΔH is:
(NEET 1999)
- (A) Negative
- (B) Positive
- (C) Zero
- (D) Constant
Show Answer
(B)
Show Explanation
Endothermic reactions absorb heat, hence ΔH is positive.
Q19. Entropy of a perfectly crystalline substance at absolute zero is:
(NEET 1998)
- (A) Maximum
- (B) Minimum
- (C) Zero
- (D) Infinite
Show Answer
(C)
Show Explanation
This is the third law of thermodynamics.
Q20. Which thermodynamic quantity is the criterion for spontaneity at constant temperature and pressure?
(NEET 2006)
- (A) Enthalpy
- (B) Entropy
- (C) Internal energy
- (D) Gibbs free energy
Show Answer
(D)
Show Explanation
At constant temperature and pressure, spontaneity is determined by ΔG.
👉 More Chemical Thermodynamics NEET PYQs will be added gradually.