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, $\Delta U = 0$.
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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.
Statement-I is correct, but statement-II is not correct
Statement-I is not correct, but statement-II is correct
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The Correct Option isD
Solution and Explanation
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 $\Delta U = 0$.