50 MCQs with Answers & Explanations | +4/-1 Pattern
Q1
First law of thermodynamics is based on:
Answer: B. Conservation of energy
Explanation: First law of thermodynamics is the law of conservation of energy for thermodynamic systems: ΔU = Q - W.
Q2
Which thermodynamic process occurs at constant temperature?
Answer: C. Isothermal
Explanation: Isothermal process occurs at constant temperature (ΔT = 0).
Q3
In an isochoric process, work done is:
Answer: D. Zero
Explanation: In isochoric process, volume is constant, so dV = 0, hence work W = ∫PdV = 0.
Q4
The internal energy of an ideal gas depends only on:
Answer: C. Temperature
Explanation: For an ideal gas, internal energy depends only on temperature (U = nCvT).
Q5
The ratio of specific heats Cp/Cv for monatomic gas is:
Answer: A. 5/3
Explanation: For monatomic gas, γ = Cp/Cv = 5/3 ≈ 1.67.
Q6
Second law of thermodynamics states that:
Answer: B. Entropy of the universe increases
Explanation: Second law: For any spontaneous process, the total entropy of the universe always increases.
Q7
In an adiabatic process:
Answer: A. ΔQ = 0
Explanation: Adiabatic process has no heat exchange with surroundings (Q = 0).
Q8
The efficiency of a Carnot engine depends on:
Answer: B. Temperatures of source and sink
Explanation: Carnot efficiency η = 1 - T₂/T₁, depends only on temperatures.
Q9
The zeroth law of thermodynamics leads to the concept of:
Answer: C. Temperature
Explanation: Zeroth law establishes the concept of temperature and thermal equilibrium.
Q10
For an isothermal expansion of an ideal gas:
Answer: A. ΔU = 0
Explanation: For isothermal process, temperature constant → internal energy constant (ΔU = 0).
Q11
The value of Cp - Cv for an ideal gas is:
Answer: A. R
Explanation: For an ideal gas, Cp - Cv = R (molar specific heats).
Q12
In a cyclic process, the net change in internal energy is:
Answer: C. Zero
Explanation: In cyclic process, system returns to initial state, so ΔU = 0.
Q13
The entropy of a system increases in:
Answer: B. Irreversible process
Explanation: Entropy increases in irreversible (spontaneous) processes.
Q14
The efficiency of a heat engine can never be:
Answer: C. 100%
Explanation: According to second law, efficiency of a heat engine cannot be 100% (cannot convert all heat into work).
Q15
In an adiabatic compression, the temperature of the gas:
Answer: A. Increases
Explanation: In adiabatic compression, work is done on the gas, increasing its internal energy and temperature.
Q16
The specific heat of a gas at constant pressure is greater than that at constant volume because:
Answer: A. Work is done by the gas
Explanation: At constant pressure, gas expands doing work, requiring additional heat energy.
Q17
The working of a refrigerator is based on:
Answer: B. Second law of thermodynamics
Explanation: Refrigerator transfers heat from cold to hot body by doing work, following second law (Clausius statement).
Q18
For a diatomic gas, the value of γ = Cp/Cv is:
Answer: B. 7/5
Explanation: For diatomic gas, γ = 7/5 = 1.4.
Q19
The area under P-V diagram represents:
Answer: B. Work
Explanation: Work done = ∫P dV, which is the area under the P-V curve.
Q20
The process that occurs without exchange of heat with surroundings is called:
Answer: C. Adiabatic
Explanation: Adiabatic process has Q = 0, no heat exchange.
Q21
The internal energy of an ideal gas is a function of:
Answer: C. Temperature only
Explanation: For ideal gas, U depends only on temperature (U = nCvT).
Q22
A Carnot engine works between 27°C and 127°C. Its efficiency is:
Answer: B. 25%
Explanation: η = 1 - T₂/T₁ = 1 - (300/400) = 1 - 0.75 = 0.25 = 25%.
Q23
The temperature at which a real gas behaves ideally is:
Answer: B. Boyle temperature
Explanation: At Boyle temperature, a real gas obeys Boyle's law and behaves ideally.
Q24
The change in entropy for a reversible isothermal process is:
Answer: D. Both A and C
Explanation: For isothermal process, ΔS = Q/T = nR ln(V₂/V₁).
Q25
The coefficient of performance of a refrigerator is given by:
Answer: A. Q₂/W
Explanation: COP = Heat extracted from cold body / Work input = Q₂/W.
Q26
For an ideal gas undergoing isothermal expansion, the internal energy:
Answer: C. Remains constant
Explanation: Isothermal means constant temperature, and U depends only on T for ideal gas.
Q27
The equation PV^γ = constant holds for:
Answer: C. Adiabatic process
Explanation: For reversible adiabatic process, PV^γ = constant, where γ = Cp/Cv.
Q28
The total entropy of the universe:
Answer: B. Always increases
Explanation: According to second law, total entropy of the universe always increases for spontaneous processes.
Q29
The value of universal gas constant R is:
Answer: A. 8.314 J/mol·K
Explanation: Universal gas constant R = 8.314 J/(mol·K) = 2 cal/(mol·K).
Q30
Which of the following is NOT a state function?
Answer: C. Heat
Explanation: Heat and work are path functions, not state functions.
Q31
The triple point of water occurs at:
Answer: B. 0.01°C, 4.58 mm Hg
Explanation: Triple point of water is 273.16 K (0.01°C) at 4.58 mm Hg pressure.
Q32
In a free expansion of an ideal gas:
Answer: C. Temperature remains constant
Explanation: For free expansion of ideal gas, no work is done and no heat exchange, so ΔU = 0 → ΔT = 0.
Q33
The latent heat of vaporization of water is approximately:
Answer: B. 540 cal/g
Explanation: Latent heat of vaporization of water is 540 cal/g (2260 kJ/kg).
Q34
The efficiency of a Carnot engine operating between 100°C and 25°C is:
Answer: A. 20%
Explanation: η = 1 - (298/373) = 1 - 0.799 = 0.201 ≈ 20%.
Q35
The specific heat of water is:
Answer: C. Both A and B
Explanation: Specific heat of water is 1 cal/g°C = 4.18 J/g°C.
Q36
The process in which no heat enters or leaves the system is called:
Answer: B. Adiabatic
Explanation: Adiabatic process has Q = 0.
Q37
The root mean square speed of gas molecules is proportional to:
Answer: A. √T
Explanation: vrms = √(3RT/M) → vrms ∝ √T.
Q38
The ideal gas equation is:
Answer: A. PV = nRT
Explanation: Ideal gas equation: PV = nRT, where n is number of moles.
Q39
The average kinetic energy of a gas molecule is:
Answer: B. (3/2)kT
Explanation: Average KE per molecule = (3/2)kT, where k is Boltzmann constant.
Q40
The temperature at which the volume of a gas becomes zero is:
Answer: C. -273°C
Explanation: Absolute zero (-273.15°C or 0 K) is the theoretical temperature where gas volume becomes zero.
Q41
The degree of freedom for a monatomic gas is:
Answer: B. 3
Explanation: Monatomic gas has 3 translational degrees of freedom.
Q42
Which law states that entropy of a pure crystalline substance at absolute zero is zero?
Answer: C. Third law
Explanation: Third law of thermodynamics states entropy of a perfect crystal at absolute zero is zero.
Q43
The work done in an isochoric process is:
Answer: C. Zero
Explanation: W = ∫P dV, and dV = 0 in isochoric process, so work = 0.
Q44
In an isobaric process, the heat supplied is used for:
Answer: C. Both increasing internal energy and doing work
Explanation: Q = ΔU + W, so heat increases internal energy and does work.
Q45
The value of Boltzmann constant k is:
Answer: A. 1.38 × 10⁻²³ J/K
Explanation: Boltzmann constant k = R/NA = 1.38 × 10⁻²³ J/K.
Q46
For an adiabatic process, which of the following is constant?
Answer: B. PV^γ
Explanation: For reversible adiabatic process, PV^γ = constant.
Q47
The molar specific heat at constant pressure Cp for monatomic gas is:
Answer: B. (5/2)R
Explanation: For monatomic gas, Cv = (3/2)R, Cp = Cv + R = (5/2)R.
Q48
Which of the following is an example of irreversible process?
Answer: B. Free expansion of gas
Explanation: Free expansion is irreversible as it cannot be reversed without external work.
Q49
The enthalpy change for a process is given by:
Answer: A. ΔH = ΔU + PΔV
Explanation: Enthalpy H = U + PV, so ΔH = ΔU + PΔV + VΔP. At constant pressure, ΔH = ΔU + PΔV.
Q50
The maximum efficiency of a heat engine working between 300 K and 600 K is:
Answer: C. 50%
Explanation: η_max = 1 - T₂/T₁ = 1 - 300/600 = 1 - 0.5 = 0.5 = 50%.
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