Exams › JEE Advanced › Chemistry

An ideal gas expands adiabatically against a constant external pressure. Which of the following statements is INCORRECT?

1. The temperature of the gas decreases.
2. The relation P * V^gamma = constant holds (where P and V are gas state variables).
3. delta_U + P_ext * delta_V = 0
4. The enthalpy of the gas remains constant.

Correct answer: The relation P * V^gamma = constant holds (where P and V are gas state variables).

Solution

For an irreversible adiabatic expansion: q = 0, w = -P_ext * delta_V. So delta_U = -P_ext * delta_V, or delta_U + P_ext * delta_V = 0 (correct). Temperature drops because the gas does work at the expense of internal energy (correct). Enthalpy = U + PV; since both T and V change, H is not constant (option D is incorrect too, but less clearly stated). The PV^gamma = const relation applies ONLY to reversible adiabatic (quasi-static) processes. An irreversible process against constant external pressure does NOT follow this relation. So option B is incorrect.

Related JEE Advanced Chemistry questions

• If the specific heat at constant pressure (Cp) is 10 cal at 1000 K, the initial temperature (T1) is 1000 K, the final temperature (T2) is 100 K, and the mass (m) is 32 g, what is the entropy change (ΔS) under constant pressure?
• For the reaction H2(g) + 1/2 O2(g) → H2O(l), where the enthalpy of formation (ΔHf) is given as −285.9 kJ mol⁻¹, determine the value of ΔHf.
• For the reaction C3H8(g) → 3C(g) + 8H(g), determine the enthalpy change (ΔHf) given that ΔHf for C2H2(g) → 2C(g) + 6H(g) is −620 kJ mol⁻¹.
• The standard enthalpies of formation of CO₂(g), H₂O(l) and glucose(s) at 25°C are -400 kJ/mol, -300 kJ/mol and -1300 kJ/mol, respectively. The standard enthalpy of combustion per gram of glucose at 25°C is -
• Benzene and naphthalene form an ideal solution at room temperature. For this process, the true statement(s) is(are) -
• During the transformation of liquid water to water vapor at 100°C and standard atmospheric pressure, which of the following is accurate?

⚔️ Practice JEE Advanced Chemistry free + battle 1v1 →