The standard enthalpy of formation for liquid water is -285.8 kJ / mole.
Which of the following could be the standard enthalpy of formation for water vapor?
A) -480.7 kJ / mole
B) -285.8 kJ
C) -241.8 kJ / mole
D) +224.5 kJ / mole
The Attempt at a Solution
The correct answer in the solutions manual was C. The logic behind the answer is that the formation of water vapor must be exothermic, and it must be less exothermic than the formation of liquid water.
Could anyone help me see the reasons behind those last 2 statements? Why is the formation of a gas less exothermic than formation of a liquid? Why must the process be exothermic? The formation of water vapor from liquid water seems like an endothermic process from my perspective, requiring the input of energy.