question on IMF

[gangazi]

When ranking the IMF, should I not consider the degree of dipole-dipole before considering london dispersion forces?

Ie... among NH3, PH3, SbH3, and AsH3, NH3 has lowest viscosity due to hydrogen bonding.

PH3 has next lowest viscosity (In my opinion) than AsH3 because it has stronger dipole-dipole due to the fact that P is more electronegative than As... But the answer solution on the DATqVAULT says that SbH3 is next to NH3 because it has the most LDF among PH3 and SbH3 and AsH3( I agree that among the three SbH3 has the most LDF, but I also think that it has the least dipole-dipole....). Thank you so much!!

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[gangazi]

corrected typo on this post!

 

[gangazi]

Which of the following has the greatest intermolecular forces?

• H2S
  
  
• H2Se
  
  
• H2Te

These 3 compounds are very different in molecular weight and therefore the determining factor will be molecular weight rather than polarity. H2Te is significantly heavier than the other 2 choices and will therefore have significantly greater London Dispersion forces than the other 2 and the greatest overall intermolecular forces.

Got this from chad's quiz (coursesaver.com) in case anyone else needs it too!

 

[Lnguyen7]

Interesting...

When you are comparing two molecules that have comparable shape and size, dipole-dipole is likely to determine the physical properties of a molecule. If one molecule is larger than the other, dispersion is likely to determine the physical properties of a molecule.

Even though the general trend is dipole-dipole > dispersion, sometimes dispersion is greater than dipole-dipole. For example, CH3F boiling point (~ -78C) is lower than CCl4 (~ +76C) even though CH3F is polar!

Back to your questions. Did you mean NH3 has HIGHEST viscosity? One thing I know is the stronger IMF, the stronger BP, MP and viscosity.

So I'd rank this as
Highest viscosity: NH3 > SbH3 > AsH3 > PH3. NH3 first because it has HB, Dipole and Dispersion.

Until I found this:

Figure taken from General Chemistry by Raymond Chang

From the figure, it seems that NH3 has lower boiling point than SbH3. Whaaaattttt!? Does it mean SbH3 > NH3 > AsH3 > PH3 when ranking viscosity too? I'm not sure, but SbH3 > AsH3 > PH3 and H2Te > H2Se > H2S for sure in either BP, MP or viscosity. I'd say on the test, they would give you something that is clear for most people and not something quite like this.