[kissoftalons]

Couple questions...any pointers greatly appreciated!

1. Why is the boiling point of NH3 so much higher than that of NF3

My thinking: NH3 contains hydrogen bonds which would result in a higher boiling point (compared to NH3)..but then this thinking doesn't work for the next question.

2. Why is the boiling point of NH3 so much higher than PH3?

Both don't contain hydrogen bonds. A friend has mentioned that hydrogen bonds don't have to occur with hydrogen atoms...can someone please explain this?

I've reasoned it out to having something to do with electronegativity. NH3 is polar covalent (electroneg. 1.0) and PH3 is non-polar. Would this have anything to do with the boiling point?

3. Why is F2 a gas when I2 is a solid?

I don't really have an idea how to approach this! Pointers would be greatly appreciated.

[Borek]

You have to decide whether NH₃ contains hydrogen bonds, or not Right now there is some inconsistency in your post.

As for F₂ vs I₂ - think about general trends in periodic table, that should be enough.

[kissoftalons]

I'm a little iffy on the definition of hydrogen bond, but in NH3, would each H be attached to another N..or would the bonding capacity of N not allow that? Or I'm getting my concepts all mixed up!!!   Since NH3 and NF3 are both polar covalent...wouldn't the logical answer be "becuase NH3 contains hydrogen bonds"?

For F2 vs. I2 Reactivity increases as your going up the periodic table ...so F2 is more reactive? but also has a higher electronegativity...I can't seem to link this to answering the question!! *pulls hair out*

[Yggdrasil]

1 & 2)  A hydrogen bond is not a covalent bond.  Rather, a hydrogen-bond is an intermolecular bond, meaning it governs interactions between molecules (as compared to an intramolecular force, such as a covalent bond, which connects atoms within a molecule).

Hydrogen bonds require a hydrogen connected to one of the following atoms: F, O, N.  If you look at an electronegativity chart, these atoms are very electronegative and can very effectively pull electrons away from the hydrogen.  This creates a partial positive charge on the hydrogen and a partial negative on the F, O, or N.  It is the electrostatic attraction of the hydrogen's partial positive with the F/O/N's partial negative that creates the hydrogen bond.

So you need to think of whether NH₃, NF₃ and PH₃ are capable of forming hydrogen bonds.

3)  Think about how size affect boiling point.

[kissoftalons]

Thank you! Been waiting for a reply on this.

So Hydrogen bond MUST have a hydrogen...and therefore NF3 would not contain Hydrogen bonds...

I know that the existence of hydrogen bonds (like in water) would allow the substance to have a high boiling point (eg. 100 C with water). So in the case of NH3 vs. NF3, NH3 has hydrogen bonds and NF3 does not...which would allow for NH3 to have a much higher boiling point? Right?

I2 atoms would be much larger than F2 atoms...and are...subsequently...heavier?   causing ....F2 to be a gas...(totally muddling my way through that one)  

[Yggdrasil]

Yes, because NF₃ does not have any hydrogen atoms, it cannot hydrogen bond.  NH₃ has hydrogens and is capable of hydrogen bonding, which greatly increases its boiling point.  Hydrogens are the only atoms capable of forming a hydrogen bond because they consist of only a proton and electron and have some special properties because of this.

For the I₂ and F₂ problem, you must consider what intermolecular force is primarily responsible for holding the molecules together.  Since they have no hydrogens, they can't hydrogen bond.  Since the molecules are neither charged nor polar, there are no electrostacic/dipole-dipole interactions.  However, there are London dispersion forces.  Since disperison forces are greater for molecules with a greater polarizability, and larger molecules have larger polarizabilities, the London dispersion forces between I₂ molecules are greater than the dispersion forces between F₂ molecules.

In general, the larger the compound, the greater the boiling point (unless there are other factors, e.g. polar v. nonpolar or protic v. aprotic).