H Vaporization Of Water

[Brandi Wendelberger]

Theenthalpy of vaporization is often quoted for the normal boiling temperature of the substance. Although tabulated values are usually corrected to 298 K, that correction is often smaller than the uncertainty in the measured value.

The enthalpy of condensation (or heat of condensation) is by definition equal to the enthalpy of vaporization with the opposite sign: enthalpy changes of vaporization are always positive (heat is absorbed by the substance), whereas enthalpy changes of condensation are always negative (heat is released by the substance).

It is equal to the increased internal energy of the vapor phase compared with the liquid phase, plus the work done against ambient pressure. The increase in the internal energy can be viewed as the energy required to overcome the intermolecular interactions in the liquid (or solid, in the case of sublimation). Hence helium has a particularly low enthalpy of vaporization, 0.0845 kJ/mol, as the van der Waals forces between helium atoms are particularly weak. On the other hand, the molecules in liquid water are held together by relatively strong hydrogen bonds, and its enthalpy of vaporization, 40.65 kJ/mol, is more than five times the energy required to heat the same quantity of water from 0 C to 100 C (cp = 75.3 J/Kmol). Care must be taken, however, when using enthalpies of vaporization to measure the strength of intermolecular forces, as these forces may persist to an extent in the gas phase (as is the case with hydrogen fluoride), and so the calculated value of the bond strength will be too low. This is particularly true of metals, which often form covalently bonded molecules in the gas phase: in these cases, the enthalpy of atomization must be used to obtain a true value of the bond energy.

An alternative description is to view the enthalpy of condensation as the heat which must be released to the surroundings to compensate for the drop in entropy when a gas condenses to a liquid. As the liquid and gas are in equilibrium at the boiling point (Tb), ΔvG = 0, which leads to:

As neither entropy nor enthalpy vary greatly with temperature, it is normal to use the tabulated standard values without any correction for the difference in temperature from 298 K. A correction must be made if the pressure is different from 100 kPa, as the entropy of a gas is proportional to its pressure (or, more precisely, to its fugacity): the entropies of liquids vary little with pressure, as the compressibility of a liquid is small.

These two definitions are equivalent: the boiling point is the temperature at which the increased entropy of the gas phase overcomes the intermolecular forces. As a given quantity of matter always has a higher entropy in the gas phase than in a condensed phase ( Δ v S \displaystyle \Delta _\textvS is always positive), and from

The enthalpy of vaporization is a function of the pressure at which that transformation takes place. The heat of vaporization diminishes with increasing temperature and it vanishes completely at a certain point called the critical temperature (Critical temperature for water: 373.946 C or 705.103 F, Critical pressure: 220.6 bar = 22.06 MPa = 3200 psi ).

See Water and Heavy Water - for thermodynamic properties. See also Water Boiling points at high pressure , Boiling points at vacuum pressure , Density, specific weight and thermal expansion coefficient , Dynamic and kinematic viscosity , Enthalpy and entropy , Ionization Constant, pK w , of normal and heavy water , Melting points at high pressure , Saturation pressure , Specific gravity , Specific heat (heat capacity) and Specific volume for online calculatores, and similar figures and tables as shown below.

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The astonishing new discovery could have a wide range of significant implications. It could help explain mysterious measurements over the years of how sunlight affects clouds, and therefore affect calculations of the effects of climate change on cloud cover and precipitation. It could also lead to new ways of designing industrial processes such as solar-powered desalination or drying of materials.

The findings, and the many different lines of evidence that demonstrate the reality of the phenomenon and the details of how it works, are described today in the journal PNAS, in a paper by Carl Richard Soderberg Professor of Power Engineering Gang Chen, postdocs Guangxin Lv and Yaodong Tu, and graduate student James Zhang.

Chen and his co-researchers have proposed a physical mechanism that can explain the angle and polarization dependence of the effect, showing that the photons of light can impart a net force on water molecules at the water surface that is sufficient to knock them loose from the body of water. But they cannot yet account for the color dependence, which they say will require further study.

They have named this the photomolecular effect, by analogy with the photoelectric effect that was discovered by Heinrich Hertz in 1887 and finally explained by Albert Einstein in 1905. That effect was one of the first demonstrations that light also has particle characteristics, which had major implications in physics and led to a wide variety of applications, including LEDs. Just as the photoelectric effect liberates electrons from atoms in a material in response to being hit by a photon of light, the photomolecular effect shows that photons can liberate entire molecules from a liquid surface, the researchers say.

The finding may solve an 80-year-old mystery in climate science. Measurements of how clouds absorb sunlight have often shown that they are absorbing more sunlight than conventional physics dictates possible. The additional evaporation caused by this effect could account for the longstanding discrepancy, which has been a subject of dispute since such measurements are difficult to make.

The work was partly supported by an MIT Bose Award. The authors are currently working on ways to make use of this effect for water desalination, in a project funded by the Abdul Latif Jameel Water and Food Systems Lab and the MIT-UMRP program.

The opposite of evaporation is condensation. Condensation is the process of water vapor turning back into liquid water. Condensation occurs when saturated air is cooled, such as on the outside of a glass of ice water.

This does vary geographically, though. Evaporation is more prevalent over the oceans than precipitation, while over the land, precipitation routinely exceeds evaporation. Most of the water that evaporates from the oceans falls back into the oceans as precipitation. Only about 10 percent of the water evaporated from the oceans is transported over land and falls as precipitation. Once evaporated, a water molecule spends about 10 days in the air. The process of evaporation is so great that without precipitation runoff, and groundwater discharge from aquifers, oceans would become nearly empty.

Seawater contains other valuable minerals that are easily obtained due to evaporation. Water from the Dead Sea is ideal for the extraction of not only table salt, but also magnesium, potash, and bromine. The Dead Sea is actually a lake, located in the Middle East within a closed watershed and without any means of outflow. This closed basin system is abnormal for most lakes. Water primarily leaves the lake by evaporating, resulting in upwards of 1,300 - 1,600 millimeters of evaporated water per year in this desert area! The result is that the waters of the Dead Sea have the highest salinity and density of any sea in the world, too high to support life.

In climates where the humidity is low and the temperatures are hot, an evaporative cooler can lower the air temperature by 20 degrees F., while it increases humidity. As this map shows, evaporative coolers work best in the dry areas of the United States (red areas marked A) and can work somewhat in the blue areas marked B. But in section C, in the humid eastern U.S., normal air conditioners must be used.

Evaporative coolers are really quite simple devices, at least compared to air conditioners, because they pull in the dry, hot outdoor air and pass it through an evaporative pad that is kept wet by a supply of water. In a home device, a fan draws the air through the pad causing the water in the pad to evaporate, resulting in cooler air which is then pumped through the house. Much less energy is used as compared to an air conditioner.

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