Molar heat of vaporization The energy

Molar heat of vaporization. The energy (in kilojoules) required to vaporize one mole of a hquid. (11.8)... 

In Section 15.2, we described molar heat of vaporization, the energy required to change one mole of a liquid to a gas. It is expressed in kJ/mol. In this section, we use the more convenient unit kJ/g to express heat of vaporization. Use the molar mass of a substance (g/mol) to convert between kJ/g and kJ/mol, if necessary. 

In Eq. (8-35), Afvap is the molar energy of vaporization, and AH p is the molar heat of vaporization. In effect, -it is a measure of the energy required to break some of the solvent-solvent forces, whereas ced is a measure of the energy required to... 

Suppose we wish to evaporate one mole of water, as expressed in equation (7). One mole contains the Avogadro number of molecules (6.02 X 1023) and has a weight of 18.0 grams. Using a calorimeter, as you did in Experiment 5, you could measure the quantity of heat required to evaporate one mole of water. It is 10 kilocalories per mole. This value is called the molar heat of vaporization of water. This is the energy required to separate 6.02 X 1023 molecules of water from one another, as pictured in Figure 5-1. 

When water vapor condenses to liquid water, the molecules release the energy it took to separate them. A mole of gaseous water, therefore, will release 10 kilocalories of heat when condensed to liquid water at the same temperature. The amount of heat released is numerically equal to the molar heat of vaporization. 

The amount of heat required to vaporize a substance also depends on the size of the sample. Twice as much energy is required to vaporize two moles of water than one mole. The heat needed to vaporize one mole of a substance at its normal boiling point is called the molar heat of vaporization, A /fvap. 

The three kinds of forces described above, collectively known as the cohesive forces that keep the molecules of liquids together, are responsible for various properties of the liquids. In particular, they are responsible for the work that has to be invested to remove molecules from the liquid, that is, to vaporize it. The energy of vaporization of a mole of liquid equals its molar heat of vaporization, Ay//, minus the pressure-volume work involved, which can be approximated well by Rr, where R is the gas constant [8.3143 J K" mol" ] and T is the absolute temperamre. The ratio of this quantity to the molar volume of the liquid is its cohesive energy density. The square root of the cohesive energy density is called the (Hildebrand) solubility parameter of the liquid, 8 ... 

It requires heat energy to melt (fuse) a solid or vaporize a liquid. The amount of heat energy required to melt one mole of a solid is called the molar heat of fusion. The amount of heat energy required to vaporize one mole of a liquid is called the molar heat of vaporization. These phase changes are constant temperature processes, and the amount of heat is that involved in the phase change itself. Notice in the following table that the vaporization values are larger than those for fusion. 

The heat of vaporization of a liquid (A//yap) is the energy required to vaporize 1.00 g of the liquid at its boiling point. In one experiment, 60.0 g of liquid nitrogen (boiling point — 196°C) are poured into a Styrofoam cup containing 2.00 X 10 g of water at 55.3°C. Calculate the molar heat of vaporization of liquid nitrogen if the final temperature of the water is 41.0°C. 

A measure of the strength of intermolecular forces in a liquid is the molar heat of vaporization (AH ap), defined as the energy (usually in kilojoules) required to vaporize one mole of a liquid. The molar heat of vaporization is directly related to the strength of intermolecular forces that exist in the liquid. If the intermolecular attraction is strong, it takes a lot of energy to free the molecules from the liquid phase. Consequently, the liquid has a relatively low vapor pressure and a high molar heat of vaporization. 

At the boiling point, the vapor pressure of a liquid equals the external pressure. The molar heat of vaporization of a liquid is the energy required to vaporize one mole of the liquid. It can be determined by measuring the vapor pressure of the liquid as a function of temperature and using the Clausius-Clapeyron equation [Equation (11.2)]. The molar heat of fusion of a solid is the energy required to melt one mole of the solid. 

When heat is added to a liquid at its boiling point, the temperature of the liquid does not increase as long as any liquid is present. Instead, the added heat energy is used to overcome the attractive forces between the molecules of the liquid so that the molecules can separate into independent entities characteristic of the vapor state. The quantity of heat energy required to convert one mole of liquid to the vapor state is called the molar heat of vaporization and is designated Slyap ... 

Step 2 Vaporization Now we must use the molar heat of vaporization to calculate the energy required to vaporize the 25 g of water at 100. C. The heat of vaporization is given per mole rather than per gram, so we must first convert the 25 g of water to moles. 

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