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Hardness of diamond in relation to molecular model


ybk
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Hey, I have a question... okay first I had to find a real world application to this point:

 

perform a first-hand investigation, analyse information and use available evidence to model the differences in atomic arrangement of diamond, graphite and fullerenes

 

 

Well, the hardness of diamond (physical properties) is related to its molecular model...

 

 

Now, the question asks me to draw a graph and an equation; do you guys know of anyway to integrate the physical properties of a mineral for example diamond into an equation and graph...

 

for example, what equation can we include using diamond? What's the original reaction that takes place in order to create diamond?

 

thanks! :)

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I don't know exactly how to answer your question, but I can give you some information about diamond and graphite that might help you get to the next step...

 

Diamond is formed from purely covalent bonding. To have four unpaired electrons in its outer shell, diamond crystals create hybrid orbitals by placing one of the electrons in the 2s orbital into the vacant 2p orbital (Remember carbon has 2 electrons in the 2s subshell and 2 electrons in the 2p subshell). The covalent bonds formed between the new orbitals created (now termed sp^3 orbitals) are called sigma bonds. The strength of these bonds is a function of the degree to which the orbitals of neighboring atoms overlap, but I'm not sure how to quantify that into a number.

 

Graphite is also formed by covalent bonding of carbon atoms using hybrid orbitals, but they are bonded together with pi bonds in addition to sigma bonds. These bonds allow electrons to travel between atoms with greater ease than in diamond, resulting in graphite's electric conductivity. In fact, the pi and sigma bonding in graphite makes graphite's bonds stronger than diamond's. The reason graphite is flaky and used as lead in pencils is because of the weak van der Waals forces between the sheets of graphite.

 

Hope I've helped. I'll look for some more information on fullerene, but I think it possesses both pi and sigma bonds just as graphite does.

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The strength of these bonds is a function of the degree to which the orbitals of neighboring atoms overlap, but I'm not sure how to quantify that into a number.
Perhaps the bond energy ? The C - C single bond has an energy of 348 kJ/mol. Also, the bond energy varies roughly inversely with the bond length, which for the C - C bond is 154 pm.

 

http://www.science.uwaterloo.ca/~cchieh/cact/c120/bondel.html

 

Graphite is also formed by covalent bonding of carbon atoms using hybrid orbitals, but they are bonded together with pi bonds in addition to sigma bonds. These bonds allow electrons to travel between atoms with greater ease than in diamond, resulting in graphite's electric conductivity. In fact, the pi and sigma bonding in graphite makes graphite's bonds stronger than diamond's. The reason graphite is flaky and used as lead in pencils is because of the weak van der Waals forces between the sheets of graphite.
The in-plane bond length in graphite is about 142 pm and hence these sp2 bonds are stronger that the bonds in diamond. But the interplanar spacing is about 335 pm, and the van der waals energy holding the layers together is small (roughly about 10 kJ/mol).

 

Also, to say that "These [pi] bonds allow electrons to travel between atoms with greater ease" is like putting the cart before the horse. It is the delocalized 4th electron (of each C-atom) that gives rise to the distributed (meaning, the positions of the pi bonds are not fixed, just like in benzene) pi bonds in graphite.

 

Hope I've helped. I'll look for some more information on fullerene, but I think it possesses both pi and sigma bonds just as graphite does.
Correct. The C-atoms in fullerenes are also sp2 hybrid, but being non-planar, the bond angles are smaller than 120 deg.
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why dont u compare the bond strengths for various covalent substances- if this has any relation to the pracs

 

Yeah, that's exactly what i'm thinking. But that still doesn't solve the stupid equations crap :-(

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There are formulae used to calculate various hardness values based on expirimental data. For instance the Vickers hardness depends on the apex angle of the indenter, the applied load and the size of the indentation. You can probably find this online somewhere.

 

On the other hand, if the question wants you to derive (empirically) a relation between say, bond length and bond energy, that would be more interesting.

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Sure, Ophiolite, the question is:

 

"your presentation must include the use of chemical equations to help explain your chosen topic"

 

To fulfil the criteria it must be: "Related to the topic, novel, not over complicated, numerous concepts supported, well integrated and explained"

 

DQW, that sounds great! I'll look into that :)

 

thanks!!!

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Wait a minute : chemical equations are completely different from mathematical equations !!!

 

To all posters : please, PLEASE, post you problems/questions EXACTLY as they are given to you. What can you possibly gain by rewording it ?

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Wait a minute : chemical equations are completely different from mathematical equations !!!

 

To all posters : please' date=' PLEASE, post you problems/questions EXACTLY as they are given to you. What can you possibly gain by rewording it ?[/quote']

 

i forgot to write chemical before :embarass:

 

would it make a difference? Do I have any other choice?

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If you can find chemical equations for the combustion (burning in oxygen) of diamond/graphite/fullerene that specify the enthalpy change, then you have something. Since the in-plane C-C bonds in graphite (and fullerene) are a little stronger, I'd expect them to have slightly larger anthalpies.

 

Edit : found a link; scroll down on page - http://wine1.sb.fsu.edu/chm1045/notes/Energy/HessLaw/Energy04.htm

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If you can find chemical equations for the combustion (burning in oxygen) of diamond/graphite/fullerene that specify the enthalpy change' date=' then you have something. Since the in-plane C-C bonds in graphite (and fullerene) are a little stronger, I'd expect them to have slightly larger anthalpies.

 

Edit : found a link; scroll down on page - http://wine1.sb.fsu.edu/chm1045/notes/Energy/HessLaw/Energy04.htm[/quote']

 

 

cool, thanks!

 

i'll have to check how enthalpy relates to hardness.

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