11.7
Phase Transitions: Vaporization and Condensation
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Q1: What is the difference between surface evaporation and boiling?
Surface evaporation occurs only at the liquid's surface below the boiling point without forming vapor bubbles. Boiling happens at the boiling point and involves vapor bubble formation throughout the bulk liquid at all interior points. Both are vaporization processes, but boiling is more vigorous and occurs at a specific temperature.
Q2: Why does vaporization require energy input?
Vaporization is an endothermic process because thermal energy must overcome intermolecular forces holding molecules together in the liquid state. The greater the heat supplied, the higher the vaporization rate. This energy requirement is quantified as the molar heat of vaporization, which varies depending on the strength of intermolecular forces between molecules.
Q3: How do intermolecular forces affect the molar enthalpy of vaporization?
Stronger intermolecular forces require more energy to overcome during vaporization, resulting in higher molar enthalpy values. Water, with strong hydrogen bonds, requires approximately 40.65 kilojoules per mole to vaporize, while acetone with weaker dipole-dipole forces needs only 31.3 kilojoules per mole. Weak intermolecular attractions present less of a barrier to vaporization.
Q4: What is condensation and how does it relate to vaporization?
Condensation is the reverse of vaporization, occurring when gas molecules collide with cooler surfaces and lose heat. It is an exothermic process with negative enthalpy. The magnitude of condensation enthalpy equals vaporization enthalpy but with opposite sign. When both processes occur in a closed system, they reach vapor-liquid equilibrium.
Q5: Why does your skin feel cool after swimming or showering?
Water evaporating from your skin removes heat through the endothermic vaporization process. As water molecules escape the liquid phase, they absorb thermal energy from your skin, causing the cooling sensation. This demonstrates the practical effect of vaporization as an energy-absorbing phase transition.
Q6: What happens when vaporization and condensation occur simultaneously in a closed container?
When opposing transitions occur in a closed system, the system reaches dynamic equilibrium called vapor-liquid equilibrium. At this state, the rate of vaporization equals the rate of condensation, and gas molecules cannot escape. The system maintains constant pressure and composition despite ongoing molecular exchanges.