Want a Tough Problem ????

orgoman22 said:

Why would H2 have a higher boiling point than He ? Not a DAT problem but will get you thinking !!!!! IF you get this you are GOOD !!!!! good night guys Dr Romano Good Luck to all !

Click to expand...

hmmmm.....

orgoman22 said:

Why would H2 have a higher boiling point than He ? Not a DAT problem but will get you thinking !!!!! IF you get this you are GOOD !!!!! good night guys Dr Romano Good Luck to all !

Click to expand...

It should be something related with molecular weight. Maybe as a gas H2 expands and becomes heavier than He? 😕 😕

orgoman22 said:

Why would H2 have a higher boiling point than He ? Not a DAT problem but will get you thinking !!!!! IF you get this you are GOOD !!!!! good night guys Dr Romano Good Luck to all !

Click to expand...

In this case, "London Forces" play the most important role. The larger the size of the molecule then the greater the London Forces therefore the higher the boiling point.
H:
Atomic radius: 0.79 A
Bonding radius: 0.32 A

He:
Atomic radium 0.49

H2 is larger than He

Gasedo said:

It should be something related with molecular weight. Maybe as a gas H2 expands and becomes heavier than He? 😕 😕

Click to expand...

"melting point" is usually a function of MW (assuming stronger intermolecular forces are absent & everything else is the same). 🙂

dat_student said:

In this case, "London Forces" play the most important role. The larger the size of the molecule then the greater the London Forces therefore the higher the boiling point.
H:
Atomic radius: 0.79 A
Bonding radius: 0.32 A

He:
Atomic radium 0.49

H2 is larger than He



"melting point" is usually a function of MW (assuming stronger intermolecular forces are absent & everything else is the same). 🙂

Click to expand...

If MW is not related with BP, then why in halogens for example, BP is increasing with the MW? Or in alkanes? 😕

Gasedo said:

If MW is not related with BP, ...

Click to expand...

In the case of melting points (MP), MW may be used as a good guesstimate when you don't know all the variables. Intermolecular forces determine MPs and BPs. In the case of MPs, you go from solid to liquid and molecules are much much closer to each other (in comparison with the gaseous state). That's why when everything else is about the same MWs are usually good MP predictors (when everything else is equal the more compact your substance the higher the MP of that substance). You can't always use MWs to guesstimate BPs because there are too many exceptions and molecules are not very close.

Gasedo said:

then why in halogens for example, BP is increasing with the MW? ...

Click to expand...

because size increases as you go down a column (so does MW)

P.S. Other factors also play important roles (e.g. branching, charges, hydrogen bonding, polarity,...)

dat_student said:

In the case of melting points (MP), MW may be used as a good guesstimate when you don't know all the variables. Intermolecular forces determine MPs and BPs. In the case of MPs, you go from solid to liquid and molecules are much much closer to each other (in comparison with the gaseous state). That's why when everything else is about the same MWs are usually good MP predictors (when everything else is equal the more compact your substance the higher the MP of that substance). You can't always use MWs to guesstimate BPs because there are too many exceptions and molecules are not very close.



because size increases as you go down a column (so does MW)

P.S. Other factors also play important roles (e.g. branching, charges, hydrogen bonding, polarity,...)

Click to expand...

👍 👍
So lets see if I got this right:

1.When need to find BP between halogens go for the size. High MW, high BP
2.For alkanes: bigger MW + unbranched = high BP
3.For different compounds, BP: ionic > metal > polar (H-bond) > polar (dipole) > nonpolar (London forces)

I don't even know if they could ask just to rank in order of increasing or decreasing BP. But I want to understand the concept, cause I am a bit confused right now.

Gasedo said:

👍 👍
So lets see if I got this right:

1.When need to find BP between halogens go for the size. High MW, high BP
2.For alkanes: bigger MW + unbranched = high BP
3.For different compounds, BP: ionic > metal > polar (H-bond) > polar (dipole) > nonpolar (London forces)

I don't even know if they could ask just to rank in order of increasing or decreasing BP. But I want to understand the concept, cause I am a bit confused right now.

Click to expand...

#1) I don't think you'll be asked to compare BPs. That's a bit tricky
#2) branching and MW are better indicators of MPs. Like I said, molecules are much closer to each other in the liquid and solid forms. So, compaction plays a more important role in determining MPs.

You have the correct order for both BP and MP:
Ionic > H-Bonds > Polar bonds > London Forces*

*Bigger molecues have more "London Forces" (and more polarizable) and therefore higher BP. Usually bigger molecules are also heavier but not always.

**Kaplan has a question about all of these (compaction vs polarity). I don't remember which test it is. One of their questions gives a trans form and a cis form and asks which has a higher boiling point and which has a higher melting point. The trans form has a higher melting point because compaction is more important for MPs and the cis form has a higher boiling point becuase it's more polar.

Boiling Points and melting points are related to how stable the molecule is with regards to the atoms shaking free of each other.

Two factors will affect how strong the molecular bond is.

1) The charges involved in the molcules. The higher the ionic charges of the individual ions involved the strongr the bond.

2) Atomic Radius. The bigger the atoms, the bigger the distance between them in the molecular bond. Trend wise, atomic size gets bigger from right to left, and from top to bottom. However for flourine the opposite is true. Flourine is the smallest atom (non-noble). Therfore logically it should have the strongest bond since the atoms are so close together. However fluorine is actually so small that its atoms elctron clouds repel one anothere making it weaker.

Helium is smaller than hydrogen. Nearly half the size actually.

Thus logically Helium should have a higher boiling/melting point since its atoms are closer together in the bond and thus attract more strongly, which means more energy needed to break up the bond.

BUT since you said that h2's boiling point is higher, I can only assume that Helium is in the same boat as flourine. Its bond strength is diluted by the fact that the atoms are so tiny that their electron clouds are repelling one another .

Quiet-Storm. said:

...
BUT since you said that h2's boiling point is higher, I can only assume that Helium is in the same boat as flourine. Its bond strength is diluted by the fact that the atoms are so tiny that their electron clouds are repelling one another .

Click to expand...

Nice hypothesis but unlike F2 He is unimolecular. No intramolecular bonds for He. It's all about London Forces. H2 is bigger than He.....(read above)
again:
size:
F2 < Cl2 < Br2 < I2
boiling points:
F2 (84.95 K) < Cl2 (239.1 K) < Br2 (332.25 K) < I2 (458.4K)

Note: the last two are so big and the strength of London Forces is so great that @standard pressure and room temp they're NOT in the gaseous state.

dat_student said:

#1) I don't think you'll be asked to compare BPs. That's a bit tricky
#2) branching and MW are better indicators of MPs. Like I said, molecules are much closer to each other in the liquid and solid forms. So, compaction plays a more important role in determining MPs.

You have the correct order for both BP and MP:
Ionic > H-Bonds > Polar bonds > London Forces*

*Bigger molecues have more "London Forces" (and more polarizable) and therefore higher BP. Usually bigger molecules are also heavier but not always.

**Kaplan has a question about all of these (compaction vs polarity). I don't remember which test it is. One of their questions gives a trans form and a cis form and asks which has a higher boiling point and which has a higher melting point. The trans form has a higher melting point because compaction is more important for MPs and the cis form has a higher boiling point becuase it's more polar.

Click to expand...

👍 👍
Hey thanks for the explanations!
😍

dat_student said:

.. It's all about London Forces...

Click to expand...

👍
I would also like to add that it is because van der Waals force will be larger for hydrogen molecules than for rare gas atoms. This includes forces that arise from fixed or angle-averaged dipoles and free or rotation dipoles as well as shifts in electron cloud distribution.

Charge fluctuation in a hydrogen molecule can cause the probability amplitude function of an adjacent molecule to shift with a wave function of only slightly different energy.

Because a helium atom has both its 2s states filled, such distortion requires superposition of 2s and 2p states in response to random charge fluctuation. Perturbation theory indicates that such higher order contribution to wavefunction distortion is not as great.