HEAT OF VAPORIZATION: FROM WATER, ETHANOL, ACETONE, CYCLOHEXANE - CHEMISTRY - 2024

The heat vaporization or enthalpy of vaporization is the energy that a gram of liquid substance must absorb at its boiling point at constant temperature; that is, to complete the transition from the liquid to the gas phase. It is usually expressed in the units j / g or cal / g; and in kJ / mol, when talking about the molar enthalpy of vaporization.

This concept is more everyday than it seems. For example, many machines, such as steam trains, work on the energy released by water vapor. Large masses of vapor can be seen rising skyward on the Earth's surface, like those in the image below.

Source: Pxhere

Also, the vaporization of sweat on the skin cools or refreshes due to the loss of kinetic energy; which translates into a drop in temperature. The feeling of freshness increases when the breeze blows, as it removes the water vapor from the drops of sweat more quickly.

The heat of vaporization depends not only on the amount of substance, but on its chemical properties; especially, of molecular structure, and the type of intermolecular interactions present.

What does it consist of?

The heat of vaporization (ΔH _vap) is a physical variable that reflects the cohesion forces of the liquid. Cohesion forces are understood to be those that hold molecules (or atoms) together in the liquid phase. Volatile liquids, for example, have weak cohesion forces; while those of water are very strong.

What is the reason that one liquid is more volatile than another and that, as a result, it needs more heat to completely evaporate at its boiling point? The answer lies in the intermolecular interactions or Van der Waals forces.

Depending on the molecular structure and the chemical identity of the substance, its intermolecular interactions vary, as well as the magnitude of its cohesion forces. To understand this, different substances with different ΔH _vap must be analyzed.

Average kinetic energy

The cohesion forces within a liquid cannot be very strong, otherwise its molecules would not vibrate. Here, "vibrate" refers to the free and random movement of each molecule in the liquid. Some go slower, or faster than others; that is, they do not all have the same kinetic energy.

Therefore, we speak of an average kinetic energy for all the molecules of the liquid. Those molecules fast enough will be able to overcome the intermolecular forces that hold it in the liquid, and will escape into the gaseous phase; even more so, if they are on the surface.

Once the first molecule M with high kinetic energy escapes, when the average kinetic energy is estimated again, it decreases.

Why? Because as the faster molecules escape into the gas phase, the slower ones remain in the liquid. Higher molecular slowness equals cooling.

Vapor pressure

As M molecules escape to the gas phase, they can return to the liquid; However, if the liquid is exposed to the environment, inevitably all the molecules will tend to escape and it is said that there was an evaporation.

If the liquid is kept in a hermetically sealed container, a liquid-gas equilibrium can be established; that is, the speed with which the gaseous molecules leave will be the same with which they enter.

The pressure exerted by gas molecules on the surface of the liquid in this equilibrium is known as the vapor pressure. If the container is open, the pressure will be lower compared to that acting on the liquid in the closed container.

The higher the vapor pressure, the more volatile the liquid is. Being more volatile, the weaker are its cohesion forces. And therefore less heat will be required to evaporate it to its normal boiling point; that is, the temperature at which the vapor pressure and the atmospheric pressure equal, 760 torr or 1atm.

Heat of vaporization of water

Water molecules can form the famous hydrogen bonds: H – O – H-OH ₂. This special type of intermolecular interaction, although weak if you consider three or four molecules, is extremely strong when it comes to millions of them.

The heat of vaporization of water at its boiling point is 2260 J / g or 40.7 kJ / mol. What does it mean? That to evaporate a gram of water at 100ºC you need 2260J (or 40.7kJ to evaporate a mole of water, that is, around 18g).

Water at human body temperature, 37ºC, has a higher ΔH _vap. Why? Because, as its definition says, the water must be heated to 37ºC until it reaches its boiling point and evaporates completely; therefore ΔH _vap is higher (and even higher when it comes to cold temperatures).

From ethanol

The ΔH _vap of ethanol at its boiling point is 855 J / g or 39.3 kJ / mol. Note that it is inferior to that of water, because its structure, CH ₃ CH ₂ OH, can hardly form a hydrogen bond. However, it continues to rank among the liquids with the highest boiling points.

From acetone

The ΔH _vap of acetone is 521 J / g or 29.1 kJ / mol. As it reflects its heat of vaporization, it is a much more volatile liquid than water or ethanol, and therefore boils at a lower temperature (56ºC).

Why? Because its CH ₃ OCH ₃ molecules cannot form hydrogen bonds and can only interact through dipole-dipole forces.

Of cyclohexane

For cyclohexane, its ΔH _vap is 358 J / g or 30 kJ / mol. It consists of a hexagonal ring with the formula C ₆ H ₁₂. Its molecules interact through London scattering forces, because they are apolar and lack a dipole moment.

Note that although it is heavier than water (84g / mol vs 18g / mol), its cohesion forces are lower.

Of benzene

The ΔH _vap of benzene, an aromatic hexagonal ring with formula C ₆ H ₆, is 395 J / g or 30.8 kJ / mol. Like cyclohexane, it interacts through dispersion forces; but, it is also capable of forming dipoles and relocating the surface of the rings (where their double bonds are delocalized) on others.

This explains why, being apolar, and not very heavy, it has a relatively high ΔH _vap.

From toluene

The ΔH _vap of toluene is even higher than that of benzene (33.18 kJ / mol). This is due to the fact that, in addition to the aforementioned, its methyl groups, –CH ₃ collaborate at the dipole moment of toluene; as well, they can interact by dispersion forces.

Of hexane

And finally, the ΔH _vap of hexane is 335 J / g or 28.78 kJ / mol. Its structure is CH ₃ CH ₂ CH ₂ CH ₂ CH ₂ CH ₃, that is to say linear, unlike that of cyclohexane, which is hexagonal.

Although their molecular masses differ very little (86g / mol vs 84g / mol), the cyclic structure directly influences the way in which the molecules interact. Being a ring, the dispersion forces are more effective; on the other hand, they are more "errant" in the linear structure of hexane.

The ΔH _vap values for hexane conflict with those for acetone. In principle, hexane, because it has a higher boiling point (81ºC), should have a _higher ΔH _vap than acetone, which boils at 56ºC.

The difference is that acetone has a higher heat capacity than hexane. This means that to heat a gram of acetone from 30 ° C to 56 ° C and evaporate it, it requires more heat than is used to heat a gram of hexane from 30 ° C to its boiling point of 68 ° C.

References

1. TutorVista. (2018). Enthalpy of Vaporization. Recovered from: chemistry.tutorvista.com
2. Chemistry LibreTexts. (April 3, 2018). Heat of Vaporization. Recovered from: chem.libretexts.org
3. Dortmund Data Bank. (sf). Standard Heat of Vaporization of Cyclohexane. Recovered from: ddbst.com
4. Chickos JS & Acree WE (2003). Enthalpies of Vaporization of Organic and Organometallic Compounds, 1880-2002. J. Phys. Chem. Ref. Data, Vol. 32, No. 2.
5. Whitten, Davis, Peck & Stanley. Chemistry. (8th ed.). CENGAGE Learning, p 461-464.
6. Khan Academy. (2018). Heat capacity, heat of vaporization and density of water. Recovered from: es.khanacademy.org