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JEE Main levelOne or more correct optionsDifficulty 2 of 3td142

Select the correct statement(s) about entropy.

A $Δ S > 0$ for $N X_{2} O X_{4} (g) 2 NO X_{2} (g)$ .
B $Δ S_{sys} < 0$ when an ideal gas is compressed isothermally.
C $Δ S_{sys} = 0$ for a reversible adiabatic process.
DThe entropy of a pure substance decreases when it is heated at constant pressure.

Answer: A, B, C

Statement 1: the number of gas moles increases (

1 \to 2

), so disorder rises. Statement 2:

Δ S = n R ln (V_{2} / V_{1}) < 0

when

V_{2} < V_{1}

. Statement 3: for a reversible adiabatic process

q_{re v} = 0

, so

Δ S = 0

. Statement 4 is false because

Δ S = \int C_{P} d T / T > 0

on heating.

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