Forces and Boiling Points

How do the boiling points of the group 4A hydrides change as the molecular masses of the hydrides change  

What are the molecular structure and polarity of the four group 4A hydrides  

Predict the strength of the forces between group 4A hydride molecules. Explain how those forces affect the boiling points of group 4A hydrides. 

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Conceptl i i ection 11.4 Intermolecular Forces and Boiling Point... 

The increases in melting point and boiling point arise because of increased attraction between the free atoms these forces of attraction are van der Waal s forces (p. 47) and they increase with increase of size. These forces are at their weakest between helium atoms, and helium approaches most closely to the ideal gas liquid helium has some notable characteristics, for example it expands on cooling and has very high thermal conductivity. 

Alkanes and cycloalkanes are nonpolar and insoluble m water The forces of attraction between alkane molecules are induced dipole/induced dipole attractive forces The boiling points of alkanes increase as the number of carbon atoms increases Branched alkanes have lower boiling points than their unbranched isomers There is a limit to how closely two molecules can approach each other which is given by the sum of their van der Waals radii... 

The melting points and boiling points of carboxylic acids are higher than those of hydro carbons and oxygen containing organic compounds of comparable size and shape and indicate strong mtermolecular attractive forces... 

Thermodynamic Effects 4.16.2.4.1 Intermolecular forces (i) Melting points and boiling points... 

The degree of polarity has considerable influence on the physical properties of covalent compounds and it can also affect chemical reactivity. The melting point (mp) and boiling point (bp) are higher in ionic substances due to the strong nature of the interionic forces, whereas the covalent compounds have lower values due to the weak nature of intermolecular forces. 

The generally low melting and boiling points of molecular substances reflect the fact that the forces between molecules (intermolecular forces) are weak. To melt or boil a molecular... 

Referring to Tables 5-1 and 5-II, we find that both sodium chloride and copper have extremely high melting and boiling points. These two solids have little else in common. Sodium chloride has none of the other properties that identify a metal. It has no luster, rather, it forms a transparent crystal. It does not conduct electricity nor is it a good heat conductor. The kind of forces holding this crystal together must be quite different from those in metals. 

The normal melting points and boiling points generally increase as the intermolecular forces between the molecules in the compounds increase. 

The graphs are alike in that the boiling points and melting points increase with increasing size as set by the number of electrons in the species. Melting points and boiling points of these non-polar molecules increase with increasing size because London forces increase with molecular size. 

In molecular covalent compounds, intermolecular forces are very weak in comparison with intramolecular forces. For this reason, most covalent substances with a low molecular mass are gaseous at room temperature. Others, with higher molecular masses may be liquids or solids, though with relatively low melting and boiling points. 

For small molecules, the van der Waals force is weaker than dipole - dipole forces and hydrogen bonding. Thus, small nonpolar molecules have low melting and boiling points. 

Melting points and boiling points are related to the strength of the intermolecular forces between solvent molecules, and to the molecular weight of the solvent. Dispersive forces, hydrogen bonding and permanent dipole moments all contribute. Typically, for molecules of similar mass, nonpolar compounds which... 

As a result of these dipole-dipole forces of attraction, polar molecules will tend to attract one another more at room temperature than similarly sized non-polar molecules would. The energy required to separate polar molecules from one another is therefore greater than that needed to separate non-polar molecules of similar molar mass. This is indicated hy the extreme difference in melting and boiling points of these two types of molecular substances. (Recall that melting and boiling points are physical properties of substances.)... 

Atomic solids are made up of individual atoms that are held together solely by dispersion forces. The number of naturally occurring atomic solids is quite small. In fact, the noble gases in their solid state are the only examples. Since the only forces holding atomic solids together are dispersion forces, these solids have very low melting and boiling points. 

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