Chapter 6: Thermodynamics
NCERT Solutions for CBSE Class 10 Chemistry — 25 solved questions with detailed explanations.
Solved Questions
Q1. For an exothermic reaction, ΔH is:
- Positive
- Negative
- Zero
- Undefined
Difficulty: Easy · Topic: Enthalpy
Solution
Exothermic: heat is released, enthalpy decreases, ΔH < 0.
Q2. The standard enthalpy of formation of an element in its standard state is:
- 1 kJ/mol
- 0 kJ/mol
- -1 kJ/mol
- Depends on element
Difficulty: Easy · Topic: Standard Enthalpy
Solution
By convention, ΔH°_f of an element in its standard state is zero.
Q3. An endothermic reaction has ΔH:
- negative
- positive
- undefined
- zero
Difficulty: Easy · Topic: Thermodynamics
Q4. Bond energy is always:
- positive
- zero
- negative
- sometimes negative
Difficulty: Easy · Topic: Thermodynamics
Q5. Entropy of a gas is generally:
- less than liquid
- greater than liquid
- zero
- same as solid
Difficulty: Easy · Topic: Thermodynamics
Solution
greater than liquid.
Q6. ΔG = ΔH - TΔS predicts spontaneity at:
- constant V
- any condition
- constant T and P
- constant T only
Difficulty: Easy · Topic: Thermodynamics
Solution
constant T and P.
Q7. The standard state temperature is:
- 300 K
- 298.15 K (25°C)
- 273.15 K (0°C)
- 373.15 K
Difficulty: Easy · Topic: Thermodynamics
Q8. A reaction with ΔH = -100 kJ/mol is:
- adiabatic
- exothermic
- isothermal
- endothermic
Difficulty: Easy · Topic: Enthalpy
Solution
ΔH < 0, so exothermic.
Q9. A reaction with ΔH = 200 kJ/mol is:
- adiabatic
- exothermic
- isothermal
- endothermic
Difficulty: Easy · Topic: Enthalpy
Solution
ΔH > 0, so endothermic.
Q10. A reaction with ΔH = -50 kJ/mol is:
- adiabatic
- exothermic
- isothermal
- endothermic
Difficulty: Easy · Topic: Enthalpy
Solution
ΔH < 0, so exothermic.
Q11. A reaction with ΔH = 150 kJ/mol is:
- exothermic
- endothermic
- isothermal
- adiabatic
Difficulty: Easy · Topic: Enthalpy
Solution
ΔH > 0, so endothermic.
Q12. A reaction with ΔH = -500 kJ/mol is:
- exothermic
- isothermal
- endothermic
- adiabatic
Difficulty: Easy · Topic: Enthalpy
Solution
ΔH < 0, so exothermic.
Q13. Hess's law is a consequence of:
- First law of thermodynamics
- Second law of thermodynamics
- Third law of thermodynamics
- Zeroth law
Difficulty: Easy-Medium · Topic: Hess's Law
Solution
Hess's law (enthalpy is a state function) follows from the first law of thermodynamics (conservation of energy).
Q14. For a spontaneous process at constant T and P:
- ΔG > 0
- ΔG < 0
- ΔG = 0
- ΔH < 0 always
Difficulty: Easy-Medium · Topic: Spontaneity
Solution
Gibbs free energy: a process is spontaneous when ΔG < 0 at constant T and P.
Q15. Bond energy of C-H is approximately:
- 363 kJ/mol
- 463 kJ/mol
- 413 kJ/mol
- 513 kJ/mol
Difficulty: Easy-Medium · Topic: Bond Energy
Solution
Standard bond energy of C-H = 413 kJ/mol.
Q16. Bond energy of C-C is approximately:
- 397 kJ/mol
- 447 kJ/mol
- 347 kJ/mol
- 297 kJ/mol
Difficulty: Easy-Medium · Topic: Bond Energy
Solution
Standard bond energy of C-C = 347 kJ/mol.
Q17. Bond energy of C=C is approximately:
- 564 kJ/mol
- 664 kJ/mol
- 714 kJ/mol
- 614 kJ/mol
Difficulty: Easy-Medium · Topic: Bond Energy
Solution
Standard bond energy of C=C = 614 kJ/mol.
Q18. Bond energy of C≡C is approximately:
- 939 kJ/mol
- 889 kJ/mol
- 839 kJ/mol
- 789 kJ/mol
Difficulty: Easy-Medium · Topic: Bond Energy
Solution
Standard bond energy of C≡C = 839 kJ/mol.
Q19. Bond energy of O-H is approximately:
- 413 kJ/mol
- 513 kJ/mol
- 463 kJ/mol
- 563 kJ/mol
Difficulty: Easy-Medium · Topic: Bond Energy
Solution
Standard bond energy of O-H = 463 kJ/mol.
Q20. Bond energy of C-O is approximately:
- 358 kJ/mol
- 408 kJ/mol
- 308 kJ/mol
- 458 kJ/mol
Difficulty: Easy-Medium · Topic: Bond Energy
Solution
Standard bond energy of C-O = 358 kJ/mol.
Q21. Bond energy of C=O is approximately:
- 899 kJ/mol
- 799 kJ/mol
- 749 kJ/mol
- 849 kJ/mol
Difficulty: Easy-Medium · Topic: Bond Energy
Solution
Standard bond energy of C=O = 799 kJ/mol.
Q22. Bond energy of N-H is approximately:
- 391 kJ/mol
- 441 kJ/mol
- 491 kJ/mol
- 341 kJ/mol
Difficulty: Easy-Medium · Topic: Bond Energy
Solution
Standard bond energy of N-H = 391 kJ/mol.
Q23. Bond energy of H-H is approximately:
- 436 kJ/mol
- 486 kJ/mol
- 386 kJ/mol
- 536 kJ/mol
Difficulty: Easy-Medium · Topic: Bond Energy
Solution
Standard bond energy of H-H = 436 kJ/mol.
Q24. Bond energy of O=O is approximately:
- 548 kJ/mol
- 498 kJ/mol
- 598 kJ/mol
- 448 kJ/mol
Difficulty: Easy-Medium · Topic: Bond Energy
Solution
Standard bond energy of O=O = 498 kJ/mol.
Q25. Bond energy of N≡N is approximately:
- 891 kJ/mol
- 991 kJ/mol
- 941 kJ/mol
- 1041 kJ/mol
Difficulty: Easy-Medium · Topic: Bond Energy
Solution
Standard bond energy of N≡N = 941 kJ/mol.
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