Why is the heat of vaporization of water greater at room temperature than at its boiling point?

Verified step by step guidance

Understand that the heat of vaporization is the energy required to convert a liquid into a gas at constant temperature and pressure.

Recognize that at the boiling point, the liquid and gas phases are in equilibrium, meaning the molecules have enough energy to overcome intermolecular forces.

At room temperature, water molecules have less kinetic energy compared to the boiling point, so more energy is needed to overcome the intermolecular forces to vaporize.

Consider that the heat of vaporization is higher at room temperature because the energy difference between the liquid and gas phases is greater than at the boiling point.

Conclude that the additional energy required at room temperature is due to the need to increase the kinetic energy of the molecules to reach the energy level at which they can vaporize.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Heat of Vaporization

The heat of vaporization is the amount of energy required to convert a unit mass of a liquid into vapor without a change in temperature. For water, this process involves breaking intermolecular hydrogen bonds, which requires significant energy. The heat of vaporization is typically higher at lower temperatures because the molecules are more tightly bound, necessitating more energy to overcome these interactions.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring particles (atoms, molecules, or ions). In water, hydrogen bonds are the primary intermolecular forces, and they are stronger at lower temperatures. As water approaches its boiling point, these forces weaken due to increased molecular motion, making it easier for molecules to escape into the vapor phase, thus reducing the heat of vaporization.

Temperature and Molecular Motion

Temperature is a measure of the average kinetic energy of particles in a substance. At room temperature, water molecules have lower kinetic energy compared to those at boiling point, where they move more vigorously. This increased motion at boiling point allows more molecules to escape into the vapor phase with less energy input, resulting in a lower heat of vaporization at that temperature compared to room temperature.

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When a thin glass tube is put into water, the water rises 1.4 cm. When the same tube is put into hexane, the hexane rises only 0.4 cm. Explain.

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Which evaporates more quickly: 55 mL of water (H₂O) in a beaker or 100 mL of acetone [(CH₃)₂CO] in an identical beaker under identical conditions? Is the vapor pressure of the two substances different? Explain.

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Suppose that 0.95 g of water condenses on a 75.0-g block of iron that is initially at 22 °C. If the heat released during condensation goes only to warming the iron block, what is the final temperature (in °C) of the iron block? (Assume a constant enthalpy of vaporization for water of 44.0 kJ/mol.)

Textbook Question

This table displays the vapor pressure of ammonia at several different temperatures. Use the data to determine the heat of vaporization and normal boiling point of ammonia.

Temperature (K) Pressure (torr)

200 65.3

210 134.3

220 255.7

230 456.0

235 597.0

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