The molar heat capacity of a diatomic ideal gas at constant pressure is (R = 8.314 J/mol·K):
Answer: B
For diatomic gas: Cv = (25)R, and Cp = Cv + R = (25)R + R = (27)R. This is at room temperature where vibration is not excited.
Q.23Medium
A cyclic process ABCA is shown on a P-V diagram where AB is isothermal expansion, BC is adiabatic compression, and CA is isochoric process. Which statement is correct?
Answer: A
In a complete cycle returning to initial state, ΔU = 0, so Q = W. For expansion-dominated processes in a typical cycle, W > 0 and Q > 0.
Q.24Medium
A heat engine absorbs 1000 J from a hot reservoir and rejects 600 J to a cold reservoir in one cycle. Its efficiency is:
A reversible heat engine operates between two thermal reservoirs. If the temperature of the cold reservoir decreases while hot reservoir temperature remains constant, the maximum efficiency:
Answer: A
Carnot efficiency η = 1 − (T_c/T_h). As T_c decreases while T_h remains constant, the ratio T_c/T_h decreases, so η increases.
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Q.26Medium
Which process results in zero change of entropy for an ideal gas?
Answer: B
For a reversible adiabatic process, dQ = 0, so dS = dQ/T = 0, hence ΔS = 0. This is also called isentropic process.
Q.27Medium
In a throttling process (Joule-Thomson expansion), which of the following remains constant?
Answer: D
Throttling is an irreversible, adiabatic process where enthalpy remains constant (H_initial = H_final). Temperature and pressure both change, and internal energy remains nearly constant only for ideal gases.
Q.28Medium
Five moles of an ideal monatomic gas are heated at constant volume from 300 K to 600 K. The change in internal energy is:
Two kilograms of water at 100°C is converted to steam at 100°C at 1 atm pressure. The change in entropy is (Latent heat of vaporization = 2.26 × 10⁶ J/kg):
A real gas deviates from ideal behavior. Which condition favors ideal behavior?
Answer: B
At low pressure, molecules are far apart (negligible intermolecular forces), and at high temperature, kinetic energy dominates, making gases behave ideally
Q.33Medium
An open system differs from a closed system in that:
Answer: A
An open system allows both mass and energy exchange with surroundings (e.g., a boiling kettle), while a closed system allows only energy exchange
Q.34Medium
The heat capacity at constant pressure Cp is greater than heat capacity at constant volume Cv for ideal gases because:
Answer: A
At constant P: Q = ΔU + PΔV, so more heat is needed to produce same temperature rise. Relation: Cp - Cv = R
Q.35Medium
Two identical containers of ideal gas are connected by a tube with a valve. Initially, container A has pressure 2P and volume V at temperature T, while container B has pressure P and volume V at the same temperature. When the valve is opened and the system reaches equilibrium, what is the final pressure?
Answer: A
Using conservation of moles: n_A = 2PV/RT, n_B = PV/RT. Total moles = 3PV/RT. Final pressure = (3PV/RT)RT/(2V) = 1.5P
Q.36Medium
For a Carnot heat engine operating between reservoirs at temperatures T_h = 500 K and T_c = 300 K, if the engine absorbs 5000 J per cycle from the hot reservoir, how much work is done per cycle?
