Molar heat of vaporization

The constant-molar-overflow assumption represents several prior assumptions. The most important one is equal molar heats of vaporization for the two components. The other assumptions are adiabatic operation (no heat leaks) and no heat of mixing or sensible heat effects. These assumptions are most closely approximated for close-boiling isomers. The result of these assumptions on the calculation method can be illustrated with Fig. 13-28, vdiich shows two material-balance envelopes cutting through the top section (above the top feed stream or sidestream) of the column. If L + i is assumed to be identical to L 1 in rate, then 9 and the component material balance... 

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... 

The y-coordinale in Figure 9.2b is In P, and the x-coordinate is 1/T. The slope, which is a negative quantity, turns out to be — AHvap/R, where AHnv is the molar heat of vaporization and R is the gas constant, in the proper units. Hence the equation of the straight line in Figure 9.2b is... 

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. 

Table 5-1. the normal boiling points and molar heats of vaporization... 

The heat accompanying the phase change (2) is 1.44 kcal/mole. This is much less than the molar heat of vaporization of water, 10 kcal/mole. Table 5-II contrasts the melting points and the heats of melting per mole (the molar heat of melting, or the molar heat of fusion) of the same pure substances listed in Table 5-1. 

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 equation shows the change in enthalpy when one mole of liquid water vaporizes into water vapor. This is called the molar heat of vaporization. Given this information, which of these is the proper value for the molar heat of condensation ... 

If it is assumed that the molar heat of vaporization is independent of temperature, then the equation integrates to... 

Use the Clausius-Clapeyron equation to solve for the molar heat of vaporization of isopropyl alcohol, A7/vap. 

The molar heat of vaporization, AHvap, at the boiling point is a measure of the heat required to change 1 mole of a liquid into a gas. The stronger the intermolecular forces of attraction between the molecules, the greater will be the value of AHvap. Since vapor pressure at a given temperature decreases with increasing intermolecular force, we expect the AHvap values to be the reverse of the vapor pressure trend. 

Making and Using Graphs The heats required to melt or to vaporize a mole (a specific amount of matter) of matter are known as the molar heat of fusion (Hf) and the molar heat of vaporization (Hy), respectively. These heats are unique properties of each element. You will investigate if the molar heats of fusion and vaporization for the period 2 and 3 elements behave in a periodic fashion. 

Use Table C-6 in Appendix C to look up and record the molar heat of fusion and the molar heat of vaporization for the period 3 elements listed in the table. Then, record the same data for the period 2 elements. 

Graph molar heats of fusion versus atomic p number. Connect the points with straight f = lines and label the curve. Do the same for = = =l molar heats of vaporization. 

What is the molar heat of vaporization of water, given the following thermochemical equations ... 

Trouton s rule phys chem An approximation rule for the derivation of molar heats of vaporization of normal liquids at their boiling points. traCit anz. riil ) true condensing point See critical condensation temperature. trii kan dens ir). point) true electrolyte puys chem A substance in the solid state that consists entirely of ions. trir i lek-tr9,lTt)... 

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 ... 

Trouton s rule says that the ratio of the molar heat of vaporization of a liquid to its normal boiling point (in kelvins) is approximately the same for all liquids ... 

A straight component balance line (i.e., constant molar heat of vaporization, Sec. 2,2.2). 

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