Heat of Vaporization Definition

Also known as enthalpy of vaporization, the heat of vaporization (∆Hvap) is defined by the amount of enthalpy (heat energy) that is required to transform a liquid substance into a gas or vapor. It is measured in Joules per mole (J/mol), or sometimes in Calories (C).

Heat of Vaporization Explained

The heat of vaporization always has a positive value because enthalpy is always added to a system in order to vaporize a liquid. As the molecules gain more kinetic energy, they become more likely to separate from the liquid and become a gas.

The required increase in internal energy can be described as the energy needed to break the intermolecular interactions in the liquid. The weaker the bond between atoms, the less energy is needed to break those bonds.

The amount of energy required is a function of the pressure at which the transformation takes place, and is temperature dependent. The hotter the liquid already is, the less energy is required. At higher pressures, more energy is required. There is a critical temperature at which the heat of vaporization vanishes (Tr=1). Past this critical temperature, the substance is distinguishable neither as a liquid nor a vapor. Instead, it becomes known as a supercritical fluid.

In a solution containing both the liquid and gaseous states, the kinetic energy of the vapor is higher than that of the liquid because the particles of vapor are able to flow more easily. The increased movement in gas particles compared to liquid particles creates heat and pressure.

Heat of Vaporization Formula

A very basic equation to calculate the heat of vaporization is:

ΔHvap = Hvapor – Hliquid

This calculates the difference in internal energy of the vapor phase compared to the liquid phase.

However, this equation does not take into consideration the additional energy needed for the gas particles to push back against atmospheric pressure to allow for the increase in volume when a liquid boils.

Hence, a more complete equation to calculate the heat of vaporization is:

ΔHvap = ΔUvap + pΔV

Where ΔUvap is the difference in internal energy between the vapor phase and the liquid phase (ΔUvap = Hvapor – Hliquid), and pΔV is the work done against the ambient pressure.

Heat of Vaporization of Water

Water has high specific heat. This measurement describes the amount of energy it takes to raise the temperature of water 1 degree Celsius. As such, water also has a high heat of vaporization. In fact, water takes over 40,000 Joules per mole to vaporize. This is extremely important for life on Earth.

Since the majority of Earth is made of water, large changes in the amount of solar energy the Earth receives are counteracted by water. Water absorbs heat slowly and releases heat when there is less sun. This helps counter drastic shifts in temperature, which would be devastating to life. By comparison, if the world were made of mostly ethanol, the temperature would fluctuate rapidly because ethanol has a much lower heat of vaporization and specific heat.

However, this high heat of vaporization may not be up to the task of regulating the temperature in the face of human actions. Climate change, and global warming specifically, are adding lots of heat to the atmosphere. While the ocean can absorb much of this heat, it has limits. Further, as the ocean absorbs heat, the molecules expand. This expansion will lead to much of the flooding that is currently estimated by climate scientists.

Differences in the Heat of Vaporization

The main influences over the heat of vaporization are the interactions between molecules in a solution. In a liquid, the molecules move past each other but are constantly interacting. Some form hydrogen bonds, while other substances form other types of mild bonds between molecules. These bonds contain energy, and hold the liquid in a lower energy state. The heat of vaporization describes how much energy is needed to separate these bonds.

Water has a high heat of vaporization because hydrogen bonds form readily between the oxygen of one molecule and the hydrogens of other molecules. These bonds hold the molecules together. To get water to vaporize, you must increase the temperature to make the molecules move faster. At a certain point, the molecules will begin to break away from the liquid and vaporize.

Metals have an even higher heat of vaporization. Many metals form complex interactions with other metal atoms. This holds the molecules together even tighter than water molecules. As such, the heat of vaporization of metals is much higher than that of water.

Quiz

1. The heat of vaporization always has what kind of value?

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2. If a liquid uses 40 Joules of heat to vaporize two moles of liquid, what is the heat of vaporization?

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3. Heat of vaporization calculates ______________.

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4. Why is it important that water has a high heat of vaporization?

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5. In a liquid state, a certain substance forms many temporary, but strong bonds between molecules. Will this substance have a high or low heat of vaporization?

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C.