Question

Consider the following
Statement-I: During isothermal expansion of an ideal gas its enthalpy decreases.
Statement-II: When 2.0 L of an ideal gas expands isothermally into vacuum, $\Delta U = 0$.

• A. Both statement-I and statement-II are correct
• B. Both statement-I and statement-II are not correct
• C. Statement-I is correct, but statement-II is not correct
• D. Statement-I is not correct, but statement-II is correct

Hint:

In isothermal processes for ideal gases, $\Delta U = 0$ because internal energy depends only on temperature. Enthalpy also remains constant if pressure-volume product doesn't change.

Answer 1

The Correct Option is D

Statement-I is incorrect because during isothermal expansion of an ideal gas, the internal energy remains constant and hence enthalpy ($H = U + PV$) also remains constant, not decreases.
Statement-II is correct because for free expansion into vacuum (against zero external pressure), the gas does no work and the process is isothermal, so $\Delta U = 0$.