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Topic: Why would SnH4 have a higher boiling point than CH4?  (Read 41934 times)

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Offline XxslbabesxX

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Why would SnH4 have a higher boiling point than CH4?
« on: January 04, 2006, 12:28:34 PM »
On the section about hydrogen bonding my text book has this chart which ranks the following according to their boiling point from highest to lowest.

1.SnH4  (higest bp)
2. GeH4
3. SiHS4
4. CH4   (lowest bp)

Why would SnH4 have a higher boiling point than CH4? Carbon is smaller than Sn and more electronegative so shouldn't that mean that the hydrogen bonding between Carbon and Hydrogen should be stronger and therefore it should have the higher boiling point. Thank you.
« Last Edit: January 04, 2006, 12:35:56 PM by Mitch »

Offline Borek

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Re:Why would SnH4 have a higher boiling point than CH4?
« Reply #1 on: January 04, 2006, 12:53:36 PM »
IMHO answer has nothing to do with hydrogen bonding.

Are there any hydrogen bonds in the CH4?

What other property of particles must be taken into account when looking for changes in bp?
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Offline XxslbabesxX

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Re:Why would SnH4 have a higher boiling point than CH4?
« Reply #2 on: January 04, 2006, 02:33:13 PM »
The mass..?

Offline Borek

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Re:Why would SnH4 have a higher boiling point than CH4?
« Reply #3 on: January 04, 2006, 03:07:14 PM »
Continue... you may be on the right track.
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Chrataxe

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Re:Why would SnH4 have a higher boiling point than CH4?
« Reply #4 on: January 04, 2006, 04:20:43 PM »
H bonding only happens F, Cl, O, N.  Also, CH4 is purely covalent.

americanstrat4

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Re:Why would SnH4 have a higher boiling point than CH4?
« Reply #5 on: January 04, 2006, 08:07:29 PM »
Think about the polarity and how the bonding exists.  Which ones have a greater difference in electronegativity?  Then you can decide which one is the strongest.

Offline XxslbabesxX

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Re:Why would SnH4 have a higher boiling point than CH4?
« Reply #6 on: January 04, 2006, 10:48:01 PM »
Continue... you may be on the right track.

Is it because SnH 4 has a larger molar mass so it takes more energy to break up the intermolecular forces and so it has a higher bp?

Offline Borek

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Re:Why would SnH4 have a higher boiling point than CH4?
« Reply #7 on: January 05, 2006, 02:17:34 AM »
Yup.
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Offline XxslbabesxX

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Re:Why would SnH4 have a higher boiling point than CH4?
« Reply #8 on: January 05, 2006, 08:55:33 PM »
Thank you  :)

Chrataxe

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Re:Why would SnH4 have a higher boiling point than CH4?
« Reply #9 on: January 06, 2006, 02:29:50 AM »
It is also more polar...there for has greater intermolecular forces.

p3t3rl1

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Re:Why would SnH4 have a higher boiling point than CH4?
« Reply #10 on: January 06, 2006, 02:43:42 PM »
um. I think the polarity of the covalent bond has more to do than the molecular mass. My textbook nor teacher ever mentioned molecular mass when it comes to strenghs of the bonds. It was always type of the bond and the EN values that determine the bond strength.

Offline Yggdrasil

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Re:Why would SnH4 have a higher boiling point than CH4?
« Reply #11 on: January 06, 2006, 05:11:59 PM »
All four of the molecules have tetrahedral geometry.  Therefore any net dipole from the C-H, Si-H, etc., bonds will be canceled by symmetry and the molecule will have no net dipole.  Since there are no dipole-dipole interactions, the only intermolecular force holding these molecules together are London dispersion forces.  The strenght of these dispersion forces depends on the polarizability (not polarity) of the molecules.  In general, larger molecules are more polarizable and therefore they have stronger dispersion forces holding them together.  For example, this explains the trend in melting points from I2 (solid), Br2 (liquid), Cl2 (gas), and F2 (gas).

Offline Borek

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Re:Why would SnH4 have a higher boiling point than CH4?
« Reply #12 on: January 06, 2006, 07:13:51 PM »
All four of the molecules have tetrahedral geometry.  Therefore any net dipole from the C-H, Si-H, etc., bonds will be canceled by symmetry and the molecule will have no net dipole.  Since there are no dipole-dipole interactions, the only intermolecular force holding these molecules together are London dispersion forces.  The strenght of these dispersion forces depends on the polarizability (not polarity) of the molecules.  In general, larger molecules are more polarizable and therefore they have stronger dispersion forces holding them together.  For example, this explains the trend in melting points from I2 (solid), Br2 (liquid), Cl2 (gas), and F2 (gas).

Two comments:

First, London forces are very weak. I don't think they are strong enough to explain the trend you have mentioned.

Second, how are you going to calculate what part of the boling/melting point change is due to London forces, and what part is due to increasing molecule mass?
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Offline plu

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Re:Why would SnH4 have a higher boiling point than CH4?
« Reply #13 on: January 06, 2006, 08:17:51 PM »
Two comments:

First, London forces are very weak. I don't think they are strong enough to explain the trend you have mentioned.

Second, how are you going to calculate what part of the boling/melting point change is due to London forces, and what part is due to increasing molecule mass?

Alas, molecular mass and the strength of our ever-present London forces increase proportionate to each other.  Qualitative calculations aside, the consideration of both these physical properties to describe a trend such as the one being discussed is thus, fortunately, not problematic  :)  slbabes, it would be wise to mention both in the explanation of your trend

Offline Yggdrasil

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Re:Why would SnH4 have a higher boiling point than CH4?
« Reply #14 on: January 07, 2006, 07:27:06 PM »
Two comments:

First, London forces are very weak. I don't think they are strong enough to explain the trend you have mentioned.

Second, how are you going to calculate what part of the boling/melting point change is due to London forces, and what part is due to increasing molecule mass?

I was under the impression that increasing molecular mass increases boling/melting point because molecules with greter molecular masses have greater dispersion forces.  For example, my inorganic text (Rayner-Canham and Overton, Descriptive Organic Chemistry) explains the boiling point trend of the Group 14 halides by talking about dispersion forces.

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