An engineer wants to determine an efficient method for condensing large amounts of steam into liquid water. Which constant should she use?

Condensation changes steam into liquid water, the reverse of vaporization. Vaporization has an enthalpy change of ΔH_vap; reversing the process changes the sign, so condensation uses −ΔH_vap. Therefore, the correct choice is negative ΔH_vap.

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