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 gaseous 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. On the earth's surface, large masses of vapor can be seen rising towards the sky, like those in the image below.
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 sweat drops more quickly.
The heat of vaporization depends not only on the amount of the substance, but on its chemical properties; especially, of molecular structure, and the type of intermolecular interactions present.
Article index
The heat of vaporization (ΔHvap) is a physical variable that reflects the cohesive 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 the water, are very strong.
Why is it 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 chemical identity of the substance, its intermolecular interactions vary, as well as the magnitude of its cohesion forces. To understand it, different substances must be analyzed with ΔHvap different.
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, there is talk of a average kinetic energy for all 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; 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. Greater molecular slowness equals cooling.
As M molecules escape into 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 atmospheric pressure are equal, 760 torr or 1atm.
Water molecules can form the famous hydrogen bonds: H-O-H-OHtwo. This special type of intermolecular interaction, although weak when considering 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 the temperature of the human body, 37ºC, has a ΔHvap higher. Why? Because, as its definition says, the water must be heated to 37ºC until it reaches its boiling point and evaporates completely; therefore, ΔHvap is higher (and even more so when it comes to cold temperatures).
The ΔHvap 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, CH3CHtwoOH, it can barely form a hydrogen bond. However, it continues to be among the liquids with the highest boiling points..
The ΔHvap 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 it boils at a lower temperature (56ºC)..
Why? Because its CH molecules3OCH3 cannot form hydrogen bonds and can only interact through dipole-dipole forces.
For cyclohexane, its ΔHvap it is 358 J / g or 30 kJ / mol. It consists of a hexagonal ring with formula C6H12. Its molecules interact through London dispersion forces, due to the fact that 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.
The ΔHvap of benzene, aromatic hexagonal ring with formula C6H6, it is 395 J / g or 30.8 kJ / mol. Like cyclohexane, it interacts through scattering forces; but, it is also capable of forming dipoles and relocating the surface of the rings (where their double bonds are delocalized) on other.
This explains why being apolar, and not very heavy, it has a ΔHvap relatively high.
The ΔHvap toluene is even higher than benzene (33.18 kJ / mol). This is because, in addition to the aforementioned, its methyl groups, -CH3 they collaborate at the dipole moment of toluene; as in turn, they can interact by dispersion forces.
And finally, the ΔHvap of the hexane is 335 J / g or 28.78 kJ / mol. Its structure is CH3CHtwoCHtwoCHtwoCHtwoCH3, 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, in the linear structure of hexane, they are more "errant".
The values of ΔHvap for hexane they conflict with those for acetone. In principle, hexane, because it has a higher boiling point (81ºC), should have a ΔHvap larger than acetone, which boils at 56ºC.
The difference is that acetone has a heat capacity higher than that of hexane. This means that, to heat a gram of acetone from 30ºC to 56ºC and evaporate it, it requires more heat than is used to heat a gram of hexane from 30ºC to its boiling temperature of 68ºC..
Yet No Comments