Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: XxslbabesxX on January 04, 2006, 12:28:34 PM
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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.
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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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The mass..?
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Continue... you may be on the right track.
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H bonding only happens F, Cl, O, N. Also, CH4 is purely covalent.
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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.
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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?
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Yup.
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Thank you :)
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It is also more polar...there for has greater intermolecular forces.
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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.
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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).
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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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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
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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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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.
This is true. In addition to increasing dispersion forces though, increasing molecular mass also means increasing the energy needed by molecules in a substance to achieve the gas state. This is given by Graham's law of effusion / the root-mean-square molecular speed equation which both state that the average molecular speed of the molecules in a substance is directly proportional to the squareroot of the substance's molar mass :-X
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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.
I am under impression that your impression is correct :( I must think it over.
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I'm not sure I buy that argument. Yes, it is true that to be a gas, the molecule must have a high enough velocity to be able to escape the attractive forces of neighboring gas molecules. But, since we're talking about forces and energy, it's not velocity that we need to consider, but kinetic energy. So smaller molecules will need to travel at a higher velocity in order to escape neighboring molecules attractive forces, while larger molecules can travel at slower speeds because their larger mass gives them more kinetic energy.