Boiling point variation with molecule

Variation in boiling points of nonpolar molecules with van der Waals a parameter. 

The attractions get stronger as the molecules get bigger in size and have more electrons. The pattern of variation of boiUng points of different halides is depicted in Fig. 10.1. For the same allqrl group, the boiling points of allqrl halides deerease in the order Rl> RBr> RC1> RF. This is beeause with the inerease in size and mass of halogen atom, the magnitude of van der Waal forees Inereases. 

In summary, it may be concluded that the uncertainty in calculating absolute cross-sectional areas, the variation in cross-sectional areas with the BET C value and the fact that on porous surfaces less area is available for larger adsorbate molecules all point to the need for a universal, although possibly arbitrary, standard adsorbate. The unique properties of nitrogen have led to its acceptance in this role with an assigned cross-sectional area of 16.2 usually at its boiling point of —195.6 °C. 

As with alkanes, the boiling points and melting points of alkenes decrease with increasing molecular weight, but show some variations that depend on the shape of the molecule. Alkenes with the same molecular formula are isomers of one another if the position and the stereochemistry of the double bond differ. For example, there are four different acyclic structures that can be drawn for butene (C4H8). They have different b.p. and m.p. as follows. 

Uranium hexafluoride yflelds jglistening, colourless or pale yellow, monoclinie crystals, which fume in the air and sublime under reduced pressure at ordinary temperature. It boils at 56-2° C., and the calculated mean latent heat of evaporation between 42° and 57° C. is 29 4 calories per gram ( = 10360 calories per gram molecule). The variation of the boiling-point with the vapour pressure is as follows ... 

The linear correlation between the refractive indices and the boiling points was found in 27 series of organic compounds with a wide range of structural variation, as summarized in Table 1.1. So long as the structures of a series of molecules resemble each other, these linear correlations have fair to excellent correlation coefficients [9]. Except for hydrogen-bonded and apparently rod-shaped molecules, the main coherent forces of the liquids come from the electronic polarizability. 

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