Vapor Pressure: Concept, Examples And Solved Exercises

The vapor pressure is one that experiences the surface of a liquid or solid, as a product of a thermodynamic equilibrium of the particles in a closed system. A closed system is understood as a container, container or bottle that is not exposed to air and atmospheric pressure.

Therefore, all liquid or solid in a container exerts on themselves a vapor pressure characteristic and characteristic of their chemical nature. An unopened bottle of water is in equilibrium with water vapor, which “tamps” the surface of the liquid and the inner walls of the bottle.

As long as the temperature remains constant, there will be no variation in the amount of water vapor present in the bottle. But if it increases, there will come a point where pressure will be created such that it can shoot the lid up; as happens when you deliberately try to fill and close a bottle with boiling water.

Carbonated beverages, on the other hand, are a more obvious (and safer) example of what is meant by vapor pressure. When uncovered, the gas-liquid balance inside is interrupted, releasing the vapor to the outside in a sound similar to a hiss. This would not happen if its vapor pressure were lower or negligible.

Vapor pressure concept

Vapor pressure and intermolecular forces

Uncapping several carbonated beverages, under the same conditions, offers a qualitative idea of ​​which ones have higher vapor pressure, depending on the intensity of the sound emitted.

A bottle of ether would also behave the same way; not so one of oil, honey, syrup, or a heaping of ground coffee. They would not make any noticeable noise unless they release gases from decomposition.

This is because their vapor pressures are lower or negligible. What escapes from the bottle are molecules in the gas phase, which must first overcome the forces that keep them “trapped” or cohesive in the liquid or solid; that is, they must overcome the intermolecular forces or interactions exerted by the molecules in their environment.

If there were no such interactions, there would not even be a liquid or solid to enclose inside the bottle. Therefore, the weaker the intermolecular interactions, the more likely the molecules will be to leave the messy liquid, or the orderly or amorphous structures of the solid.

This applies not only to pure substances or compounds, but also to mixtures, where the already mentioned drinks and spirits come in. Thus, it is possible to predict which bottle will have higher vapor pressure knowing the composition of its content.

Evaporation and volatility

The liquid or solid inside the bottle, assuming it is uncapped, will be continuously evaporating; that is, the molecules on its surface escape into the gaseous phase, which are dispersed in the air and its currents. That is why the water ends up evaporating completely if the bottle is not closed or the pot is covered.

But the same does not happen with other liquids, and much less when it comes to solids. The vapor pressure for the latter is usually so ridiculous that it may take millions of years before a decrease in size is perceived; assuming they haven’t rusted, eroded, or decomposed in all that time.

A substance or compound is then said to be volatile if it evaporates rapidly at room temperature. Note that volatility is a qualitative concept: it is not quantified, but is the product of comparing evaporation between various liquids and solids. Those that evaporate faster will be considered more volatile.

On the other hand, the vapor pressure is measurable, gathering by itself what is understood by evaporation, boiling and volatility.

Thermodynamic equilibrium

Molecules in the gas phase collide with the surface of the liquid or solid. In doing so, the intermolecular forces of the other, more condensed molecules can stop and hold them, thus preventing them from escaping again as vapor. However, in the process other molecules on the surface manage to escape, integrating the vapor.

If the bottle is closed, there will come a time when the number of molecules that enter the liquid or solid will be equal to those that leave them. So we have an equilibrium, which depends on the temperature. If the temperature increases or decreases, the vapor pressure will change.

The higher the temperature, the higher the vapor pressure, because the molecules of the liquid or solid will have more energy and can escape more easily. But if the temperature remains constant, equilibrium will be reestablished; that is, the vapor pressure will stop increasing.

Examples of vapor pressure

Suppose you have n -butane, CH 3 CH 2 CH 2 CH 3 , and carbon dioxide, CO 2 , in two separate containers. At 20 ° C, their vapor pressures were measured. The vapor pressure for n- butane is approximately 2.17 atm, while that of carbon dioxide is 56.25 atm.

Vapor pressures can also be measured in units of Pa, bar, torr, mmHg, and others. CO 2 has a vapor pressure almost 30 times higher than that of n- butane, so at first glance its container must be more resistant to be able to store it; and if it has cracks, it will shoot with greater violence around the surroundings.

This CO 2 is found dissolved in carbonated beverages, but in quantities small enough so that the bottles or cans do not explode when they escape, but only a sound is produced.

On the other hand we have diethyl ether, CH 3 CH 2 OCH 2 CH 3 or Et 2 O, whose vapor pressure at 20 ºC is 0.49 atm. A container of this ether when uncovered will sound similar to that of a soda. Its vapor pressure is almost 5 times lower than that of n- butane, so in theory it will be safer to handle a bottle of diethyl ether than a bottle of n- butane.

Solved exercises

Exercise 1

Which of the following two compounds is expected to have a vapor pressure greater than 25 ° C? Diethyl ether or ethyl alcohol?

The structural formula of diethyl ether is CH 3 CH 2 OCH 2 CH 3 , and that of ethyl alcohol, CH 3 CH 2 OH. In principle, diethyl ether has a higher molecular mass, it is larger, so it could be believed that its vapor pressure is lower since its molecules are heavier. However, the opposite is true: diethyl ether is more volatile than ethyl alcohol.

This is because the CH 3 CH 2 OH molecules , like the CH 3 CH 2 OCH 2 CH 3 , interact through dipole-dipole forces. But unlike diethyl ether, ethyl alcohol is capable of forming hydrogen bonds, which are characterized by being especially strong and directional dipoles: CH 3 CH 2 HO— HOCH 2 CH 3 .

Consequently, the vapor pressure of ethyl alcohol (0.098 atm) is lower than that of diethyl ether (0.684 atm) despite its lighter molecules.

Exercise 2

Which of the following two solids is believed to have the highest vapor pressure at 25ºC? Naphthalene or iodine?

The naphthalene molecule is bicyclic, having two aromatic rings, and a boiling point of 218ºC . For its part, iodine is linear and homonuclear, I 2 or II, having a boiling point of 184 ºC. These properties alone rank iodine as possibly the solid with the highest vapor pressure (it boils at the lowest temperature).

Both molecules, that of naphthalene and iodine, are apolar, so they interact through London dispersive forces.

Naphthalene has a higher molecular mass than iodine, and therefore it is understandable to assume that its molecules have a harder time leaving the black, fragrant, tarry solid; while for the iodine it will be easier to escape the dark purple crystals.

According to data taken from Pubchem , the vapor pressures at 25ºC for naphthalene and iodine are: 0.085 mmHg and 0.233 mmHg, respectively. Therefore, iodine has a vapor pressure 3 times higher than naphthalene.

References

  1. Whitten, Davis, Peck & Stanley. (2008). Chemistry . (8th ed.). CENGAGE Learning.
  2. Vapor Pressure. Recovered from: chem.purdue.edu
  3. Wikipedia. (2019). Vapor pressure. Recovered from: en.wikipedia.org
  4. The Editors of Encyclopaedia Britannica. (April 03, 2019). Vapor pressure. Encyclopædia Britannica. Recovered from: britannica.com
  5. Nichole Miller. (2019). Vapor Pressure: Definition, Equation & Examples. Study. Recovered from: study.com

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