Heat of Vaporization Calculator

When heat is added to a liquid substance, the kinetic energy of its particles increases, resulting in a rise in temperature. As more heat is added, the temperature of the liquid continues to rise until it reaches a specific point called the boiling point, at which the liquid starts to transform into a gas or vapor.

heat of vaporization calculator

This article focuses on the energy required to facilitate this transition from the liquid phase to the gaseous phase at the boiling point temperature— referred to as the heat of vaporization.

Heat Of Vaporization Explained

Substances undergo phase transformations when heat is applied or removed. In the case of liquids, the application of heat increases the average kinetic energy of the particles. As a result, the particles move more vigorously, leading to an increase in temperature.

At a specific temperature called the boiling point, the particles’ kinetic energy becomes sufficient to overcome the intermolecular forces holding the liquid together. Consequently, the liquid undergoes vaporization, transforming into a gas phase where the particles are free to move independently and occupy a significantly larger volume. The energy required for this transformation is referred to as the heat of vaporization.

The heat of vaporization, also known as the enthalpy of vaporization or latent heat of vaporization, is a fundamental concept in thermodynamics that quantifies the amount of energy needed to convert a substance from its liquid phase to its gaseous phase at constant temperature and pressure.

Consider the temperature-volume diagram illustrating the heating process of water at a constant pressure of 1 atm, shown below:

Temperature-volume diagram for water undergoing heating process at constant pressure

As heat is added, the temperature of liquid water increases until it reaches the boiling point at 100°C. At point 2, the water is considered a saturated liquid, meaning the liquid has absorbed the maximum amount of energy it can handle without undergoing phase change.

Further addition of heat causes the water to start transforming into vapor at a constant temperature. At point 3, the water becomes a saturated mixture, containing both liquid and gaseous states. Eventually, at point 4, the water becomes saturated vapor, where all of the liquid has been converted into vapor.

Beyond this point, the vapor enters the superheated region, where its temperature continues to rise as more heat is added.

The flat horizontal line on the diagram represents the heat of vaporization of water. This value indicates the amount of energy required to transform the water from point 2 to point 4, which corresponds to the transition from saturated liquid to saturated vapor.

It is important to note that if the process is reversed and the vapor is condensed, the same amount of energy would need to be released to transform the saturated vapor back into saturated liquid. Therefore, the heat of vaporization is also equal to the energy released during condensation.Top of Form

Molar Heat Of Vaporization

In published tables, the heat of vaporization is commonly expressed on a per molar basis, known as the molar heat of vaporization. The table below shows the molar heat of vaporization of various substances, measured in kJ/mol at standard pressure.

Specific Heat Of Vaporization

In some texts, the heat of vaporization is also expressed on a per mass basis, known as the specific heat of vaporization. The table below shows the specific heat of vaporization of various substances, measured in J/g at standard pressure.

Factors Affecting Heat Of Vaporization

The magnitude of heat of vaporization is primarily determined by the strength of the intermolecular forces present within a substance. In order for the liquid phase to transform into the gaseous phase, these intermolecular forces must be overcome.

The heat of vaporization represents the energy needed to break these intermolecular forces and separate the particles, allowing them to escape into the gas phase. Hence, higher heat of vaporization indicates stronger intermolecular forces.

For instance, as shown in the table above, nitrogen exhibits a particularly low heat of vaporization due to the weak van der Waals forces between nitrogen atoms. Conversely, liquid water has relatively strong hydrogen bonds, resulting in a significantly larger heat of vaporization.

Apart from intermolecular forces, the heat of vaporization is also influenced by the temperature and pressure at which the phase change occurs. Remember that the boiling point increases with pressure, and vice versa. This is why water boils at a lower temperature at higher elevations compared to sea level.

In general, as the temperature and pressure increases, the heat of vaporization decreases. To illustrate this relationship, consider the temperature-enthalpy diagram of water provided below.

Temperature-enthalpy diagram of water

The diagram features a red curve representing the saturated liquid line and a blue curve representing the saturated vapor line. The horizontal distance between these two lines represents the heat of vaporization at a specific temperature. As depicted in the diagram, as the temperature rises, the horizontal distance decreases, indicating a decrease in the heat of vaporization.

At the critical point, the heat of vaporization diminishes completely. The critical point is the state at which the liquid and vapor phases can no longer be distinguished.

Heat Of Vaporization Calculation

The heat of vaporization has two components: one is the change in internal energy between the liquid and vapor phases, and the other is the work done caused by the change in volume against ambient pressure. Therefore, the heat of vaporization can be mathematically expressed as:

heat of vaporization formula

Where:

  • ΔHvap = heat or enthalpy of vaporization [J]
  • ΔUvap = change in internal energy between the liquid and vapor phases [J]
  • P = ambient pressure [Pa]
  • ΔV = change in volume of the substance [m3]

However, the relationships among thermodynamic properties for most substances are too complex to be expressed by simple equations. Therefore, properties are often presented in the form of property tables. Property tables that list saturation temperature, pressure, specific volume, internal energy, enthalpy, and entropy are available for practically all pure substances.

Below is an example of a property table for saturated water or steam.

The quantity hfg on the property table is the specific heat of vaporization, expressed in kJ/kg, at the specified temperature. Alternatively, it can also be calculated from the enthalpies of the saturated liquid and vapor states using the formula:

enthalpies of the saturated liquid and vapor states formula

Where:

  • hfg = specific heat of vaporization [kJ/kg]
  • hg = enthalpy at the saturated vapor state [kJ/kg]
  • hf = enthalpy at the saturated liquid state [kJ]/g

Notice that the temperature values on the property table are listed in increments of 5. In order to obtain enthalpy values for temperatures in between, interpolation is required.

Furthermore, the property table shown above uses temperature as a basis. This is useful if the given parameter is temperature. However, there are also property tables that use pressure as a basis if the given parameter is pressure.

Importance And Applications Of Heat Of Vaporization

Knowing the energy required to transform a liquid into its gaseous phase is valuable in various applications like heating and cooling systems, power generation, metallurgy, food and chemical processing, and more.

In heating and cooling systems, the refrigerant undergoes continuous evaporation and condensation to absorb and release heat from the surroundings. The heat of vaporization is useful in estimating the amount of heat that the refrigerant can absorb in the evaporator and the amount it can expel in the condenser.

In power generation, such as in steam generators, the heat of vaporization is used to determine the energy needed to convert liquid water into steam. This steam is then pressurized to power steam turbines that generate electricity.

In metallurgy, the vaporization of metals plays a crucial role in metal vapor synthesis. This method involves combining freshly produced metal atoms or small particles with ligands to prepare metal complexes. The process involves a reactor that evaporates the metal, allowing the vapor to impinge on a cold reactor wall coated with the organic ligand.

In food and chemical processing, vaporization is employed for separation, purification, and concentration purposes. Distillation, for example, utilizes the differences in vapor pressures and heat of vaporization of different components to separate mixtures. Vaporization is also significant in processes like drying and solvent recovery.

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