Molar enthalpy of vaporisation

ASvapjn is equal to ratio between the molar enthalpy of vaporisation and the boiling temperature, which leads to Eq. (18.10). The molar enthalpy and entropy of vaporisation and the boiling temperature of some simple liquids are presented in Table 18.2 Some liquids deviate sharply from the rule. This is often because these liquids have structure and so a greater amount of disorder is introduced when they evaporate. Examples are water and methanol due to hydrogen bonds between the molecules. (Atkins and De Paula, 2006)... 

The slope of a plot of logP against 1/T for metamfetamine is -2.727 xfO K. Calculate the molar enthalpy of vaporisation of this compound over the given temperature range. 

If the liquid enthalpy change is very small from plate to plate, and if the molar enthalpy of vaporisation of components is close, then we may safely assume that ... 

For the liquid phase is very approximately the molar enthalpy of vaporisation, while for solids it is approximately the molar enthalpy of sublimation. Corrections must be applied for the energy of the molecules in the condensed phase and for the work done in expanding the vapour against the atmosphere, so that the best simple approximation to Uq is obtained from the value of extrapolated to absolute zero. For example, the extrapolated value of for liquid argon is about 6400 J mol , and taking 7 as 10 gives a value for Uq of about 21 X 10" J. 

The difference between the two heats of formation is 44.01 kJ/mol and gives the molar enthalpy of vaporisation of water (latent heat). The efficiency of an ideal fuel cell operation at standard conditions is (H ) = 0.83 or 83% for the free energy of 237.3 kJ/mol. 

Thus the elevation of the boiling point, AZb, of a liquid (molar mass, Mm, boiling point, Tb and enthalpy of vaporisation, AvapH° at 1 bar pressure) caused by addition of zwSoiute can be calculated using equation (52.12). 

The vapour enthalpies are calculated from the molar heat capacity functions for the vapour components and the latent heats of vaporisation at standard temperature. The vapour overflow, V, is then obtained from the energy balance as... 

TABLE 18.2 Molar enthalpy and entropy of vaporisation and boiling temperature of some simple liquids... 

Sublimation, for solids, and vaporisation, for liquids, correspond to the breaking of all the bonds between a molecule and its neighbours. Therefore, the molar enthalpy for sublimation or vaporisation (as is appropriate) is equal to qeNp jl where is the Avogadro constant. Hence ... 

The stream report is an image of the material and energy flows in the plant and the basic document in flowsheeting. The format of results depends on the type of process and on the level of details. This should contain state variables, as temperature, and pressure, vaporised fraction, total and partial flow rates, molar fractions, composition, on molar and mass basis, enthalpy, specific volume, etc. Normally the information can be exported to a spreadsheet. 

In the above relations and Cp y are the molar heat capacity of liquid and vapour, AHy is the molar vaporisation enthalpy, while Tp, T, are the actual feed temperature, feed bubble point and feed dew point. 

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