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I have previously read that the boiling point of Stibane(SbH3) is greater than Ammonia(NH3) as ammonia is gas at room temperature and no hydrogen bonding exists in the gaseous form of NH3. Hence, as SbH3 is greater in size, has greater molecular weight; its boiling pint is higher than NH3.
why does Stibane have a higher boiling point than Ammonia

In the case, of AsH3, even this molecule is gas at room temperature and its boiling point should be greater than NH3, but this is not the case and NH3 has a higher boiling point than AsH3.

I want to know what is the reason behind this trend. (NH3 having a higher boiling point than AsH3)

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There is the general trend in the groups 15, 16, 17 of raising of boiling points for the binary compounds with hydrogen, going down the groups.

But the first members of each group - $\ce{NH3, H2O, HF}$ - have anomally with their boiling points being exceptionally high, due hydrogen bonds. The strength of these bonds and the boiling point decreases in the order $\ce{H2O > HF > NH3}$.

See the figure 2 on wordpress page for an illustrative picture:

enter image description here

Hydrogen bonds play role in (not limited to):

  • evaporation and boiling, as molecules need to gain an extra energy to overcome these bonds and to escape liquid.
  • condensation, acting as cohesive force for molecules to stick to each other or to a condensation center.
  • gas pressure, as the inter-molecular cohesive force decreases the gas pressure, compared to the pressure of an ideal gas at the same conditions.

Note that the fact that both NH3 and AsH3 are gases at room temperature is irrelevant for the above.


Not having in hand means for quantitative analysis, so just thoughts:

  • $\ce{NH3}$ has the least polar bonds and their dipole moments in large extent mutually cancel each other due sterical orientation.
  • $\ce{HF}$ is the most polar with the biggest electronegativity difference, but this seems about beaten by 2 polar bonds of $\ce{H2O}$, both having very similar dipole moments. What may play role is better sterical availability ( 2 versus 1 ) of water hydrogen for mutual hydrogen bonding.
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  • $\begingroup$ Is there a reason why "The strength of these bonds and the boiling point decreases in the order H2O>HF>NH"? $\endgroup$ Oct 7, 2021 at 12:23
  • $\begingroup$ @AdilMohammed That is a wrong question. If there was no reason, the trend would not be there. $\endgroup$
    – Poutnik
    Oct 7, 2021 at 12:26
  • $\begingroup$ Fair point, I'll rephrase my question. Is there a good reason why the trend occurs? If so what is it? If Hydrogen bonding depends on the EN of the other atom, the order would have obviously been different. I am suspecting the small size of Fluorine is the culprit here. $\endgroup$ Oct 8, 2021 at 13:23

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