How do i determine the higher boiling point between molecules?

Question

The boiling point of CH4 is lower than the boiling point of SiH4, whereas the boiling point of NH3 is higher than the boiling point of PH3.

All i have to work with is the periodic table, so is it even possible to explain this in terms of universal chemical properties

Answer 1

this has to do with electronegativity and size:

first of all
in comparing methane versus silane,
the electronegativity of hydrogen carbon and silicon are all relatively close to each other, so the carbon - hydrogen bonds and the silicon - hydrogen bonds are minimally polar bonds.
so dipole moment is not of concern here, but rather only van der waals forces which depend mainly on the size of the molecule.
so silicon is a much larger atom than carbon bc of another whole shell of electrons around it, and therefore has more induced dipole-induced dipole force interactions

bc of more force between the molecules, it boils at a higher point than methane


in the case of ammonia versus phosphine,
there is a difference in electronegativity between the three elements. there is a large difference between hydrogen and nitrogen but there is only a moderate diff between hydrogen and phophorus. so both N-H bonds and P-H bonds are polar, but N-H bonds are much more polar and so ammonia exhibits stronger dipole -dipole forces than phosphine and so requires more energy to boil

Answer 2

The strength is based on intermolecular interactions. Methane is nonpolar and have very weak interactions between the molecules resulting in a low boiling point. Ammonia forms hydrogen bonding which is harder for it to break apart because the bond are stronger. Resulting that the ammonia boiling point is much higher. Therefore, substances composed of polar molecules typically have higher boiling points than substances composed of nonpolar molecules.

NH3 exhibits hydrogen bonding whereas PH3 doesn't.
SiH4 has a higher mass than CH4, therefore, SiH4 has a higher boiling point.

Answer 3

In general, the heavier one molecule of a substance is, the higher its boiling point will be. There are exceptions to this, of course, but your example contains molecules which are easy to compare.

CH3 and SiH4 are very similar. They each have four hydrogens, so you would expect a similar amount of hydrogen attraction for each molecule. However, the silicon atom has an atomic mass of about 28, compared to 12 for carbon. You would therefore expect (in the absense of other data) that SiH4 would have the higher boiling point. That is, you would need to add more energy for the heavier SiH4 molecules to fly off into the gas phase.

NH3 and PH3 are also similar. They each have 3 hydrogens and one free electron pair. However, NH3 has a much higher dipole moment than PH3, which make be the reason it sticks together (stays a liquid) at higher temperatures. The higher dipole moment of NH3 is the result of more acute bond angles (a more pointy molecular structure). A higher dipole moment basically means the two ends of the molecule are more electrically charged, leading to greater forces of attraction between molecules, therefore it takes more energy (higher temperature) to rip them apart into the gas phase.

Answer 4

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RE:
How do i determine the higher boiling point between molecules?
The boiling point of CH4 is lower than the boiling point of SiH4, whereas the boiling point of NH3 is higher than the boiling point of PH3.

All i have to work with is the periodic table, so is it even possible to explain this in terms of universal chemical properties

Answer 5

How To Determine Boiling Point

Answer 6

I'm a bit rusty, but I believe you have to find out their electron configuration, which will tell you if the molecule is stable. If it is STABLE, then it will have a HIGH boiling point (needs more energy to break stable molecules). You'll have to compare each electronic configuration, which is a bit painful, I posted you the list of the atoms and their boiling points, I hope it does help you somehow.

Also, the smaller the molecule, the more stable it (usually) is

"Electron Configuration:
The electron configuration is the orbital description of the locations of the electrons in an unexcited atom. Using principles of physics, chemists can predict how atoms will react based upon the electron configuration. They can predict properties such as stability, boiling point, and conductivity. Typically, only the outermost electron shells matter in chemistry, so we truncate the inner electron shell notation by replacing the long-hand orbital description with the symbol for a noble gas in brackets. This method of notation vastly simplifies the description for large molecules. "

Answer 7

Highest Boiling Point

Answer 8

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1. Graphite = giant molecular covalent: Van Der Waals' Forces between "sheets". .....LiCl = (giant) ionic. b.p. can't be easily predicted - my guess is graphite has higher b.p. 2. H2O = simple molecular covalent .....NaCl = (giant) ionic So NaCl has higher b.p. order: giant > larger simple molecular/higher RMM e.g. octane > smaller simple molecular/smaller RMM e.g. methane prediction can't be made between giant covalent and ionic. polar covalent will have higher b.p. than simple molecular covalent of similar RMM So if you revise bonding you can put the rest in order.