Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: cliverlong on January 05, 2009, 04:20:29 PM
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Hi,
I saw an exam question to which I can't find the answer. The question was (among other things)
“why does graphite have a high boiling point?”
My answer is
Graphite is held together by two different types of force
1)Every carbon atom is covalently bonded to three other atoms in a “sheet”. These bonds are sp2 hybrids. The bond angle is 120 degrees
2)The remaining p-orbital electron in carbon is delocalised between the atoms and in fact move between the sheets of carbon
The force resulting from the delocalised electrons is the relatively weak Van der Waals force.
Melting point and boiling points are determined by the strength of bonds between atoms and molecules. One would expect VdW forces as weak to make m.p. Graphite to be low. However, in a sample of graphite there are billions of such bonds between the sheets of carbon. Hence their aggregate effect is large causing a high m.p.
Does that seem a reasonable (correct) argument?
Thanks
Clive
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Delocalized electrons don't sound like VDW forces to me. Sounds more like the bonding found in metals.
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Delocalized electrons don't sound like VDW forces to me. Sounds more like the bonding found in metals.
Yes, of course, doh!
Can I make any statement about the strength of such forces or do I just say that by analogy with metals, which have a (relatively) high melting point then so will graphite?
The whole argument seems a bit flaky because one also argues that the layers of carbon slip over each other therefore the bonds between the layers/sheets of carbon must be weak !!
Thanks
Clive
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I thought it was due to the giant molecular struture of graphite which have strong covalent bonding network throughout resulting in high boiling point. VDW forces are only between the layers. Am i missing any of your arguement? ???
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do I just say that by analogy with metals, which have a (relatively) high melting point then so will graphite?
That's the argument I'd use, but my inorganic knowledge is really minimal.
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I agree that it is the giant network of carbon atoms in which carbon atoms are held together by strong covalent bonds .It is the reason for high melting and boiling point because more heat is required to break covalent bonds.
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I agree that it is the giant network of carbon atoms in which carbon atoms are held together by strong covalent bonds .It is the reason for high melting and boiling point because more heat is required to break covalent bonds.
You have missed my point entirely, which I tried to be really clear about in my posts
1) Yes. The sheets of carbon are held together by covalent bonds
2) My quesiton is about the effect of the metallic bonds (per nj_bartel) between the sheets and whether they are strong enough to hold together the sheets. I suppose as they are strong enough for metals to cause realtively high melting point, they must be strong enough for graphite.
Clive