A chemical engineer needs to calculate the amount of coolant required to cool a reaction vessel to a specific temperature. The coolant reduces the temperature of the vessel by absorbing heat from it. During this process, the coolant evaporates. Which constant should the engineer use to determine how much coolant is required?

Because the coolant evaporates, it absorbs heat as it changes from liquid to gas. The heat required per amount of coolant is the enthalpy of vaporization, ΔHvap, which is positive for this endothermic change. Therefore, the engineer should use ΔHvap, not the fusion constant or its negative.

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