For a reaction, A(g) → A(l); ΔH = -3RT. The correct statement for the reaction is:

For the reaction A(g) → A(l), ΔH = -3RT. We know that ΔH = ΔU + Δ(PV). For an ideal gas, PV = nRT. In this reaction, one mole of gas is converted to liquid. Thus, Δ(PV) = -RT (since the volume of liquid is negligible compared to the volume of gas). Therefore, ΔH = ΔU - RT. Since ΔH = -3RT, we can write -3RT = ΔU - RT, which implies ΔU = -2RT. Now, let's compare the magnitudes of ΔH and ΔU: |ΔH| = |-3RT| = 3RT and |ΔU| = |-2RT| = 2RT. Since 3RT > 2RT, |ΔH| > |ΔU|. Therefore, the correct statement is |ΔH| > |ΔU|.