O Level Chem Qn

EiN930 SG

Hi I just want to clarify whether the explanation is correct.

Explain why graphite has a lower melting point than diamond. (1 mark).

The answer is "Graphite has fewer strong bonds to break."

What if the student wrote this:

"Graphite has weak van der wals forces between the layers. Lesser energy is required to overcome such forces. As a result, it has a lower melting point than diamond".

Is this answer acceptable?

From my own understanding, diamond has 4 covalent bonds between the carbon atoms while graphite has 3 covalent bonds between the carbon atoms and weak van der wals forces. More energy is required to break the extra covalent bond in diamond than the weak van der wals forces between the layers of atoms in graphite.

UltimaOnline SG

Edited :

If the question were to ask, "why is diamond harder than graphite?"

To order to mechanically break apart a piece of diamond, a lot more energy is required to overcome the much stronger covalent bonds in diamond. In contrast, in order to mechanically slice away layers of graphite (which we do when we write using pencils), less energy is required to overcome the weaker van der Waals attraction between the graphene layers of graphite.

A comparison (note the use of comparative adjectives more / less / higher / lower / stronger / weaker in my answer above) must be made to contrast the requirements for mechanically breaking apart pieces of both allotropes, rather than just describing the bonding in only either allotrope.

EiN930 SG

Hi. Yup I am.

Actually the given answer was provided by cambridge mark scheme (from CIE). But my student gave the other answer. I felt the student' concept wasn't wrong. That's why I want to clarify it.

Yup noted about the use of comparion words in answers. Thank you for the clarification :)

UltimaOnline SG

Ok, here's an update and an erratum :

Our previous answers, would still be correct if the question were to ask why is graphite soft, but why is diamond hard.

Turns out that to completely melt graphite, you need to break the intramolecular covalent bonds within the graphene layers, as well as the van der Waals forces between the graphene layers.

Still, this question is rather unfair, because this is still not the full picture. Each C atom in graphite is sp2 hybridized and the covalent bonds within the graphene layers is actually stronger (due to partial double bond character) than the covalent bonds within diamond.

So you have greater number of covalent bonds within diamond, versus stronger covalent bonds within graphite. So who wins? Turns out the melting points of diamond and graphite are actually pretty close (despite graphite being soft and diamond being ultra-hard).

So, yes we made a mistake in our assumption earlier, but this is still not a fair question (so Cambridge is also arguably at fault), because students (especially at 'O' levels) can't be expected to be aware of, and compare the magnitude of energy required, to overcome the greater number of covalent bonds in diamond, versus the stronger covalent bonds in graphite.

EiN930 SG

Okay. Thanks for the update.

This is indeed tricky. So if such qn will to appear, which answer would be better? Or both answers are acceptable? But at this level (I mean O level), should we assume the the covalent bonds are similar in both diamond and graphite?

Or I should explain the ambuigity of the qn to the student?

UltimaOnline SG

Okay. Thanks for the update.

Or I should explain the ambuigity of the qn to the student?

1. If you feel your student is up to it, then show him the bigger picture, afterwhich you proceed to...

2. Instruct him to remember and regurgitate the simplified answer (but still incomplete picture) that "diamond has more covalent bonds than graphite" in the exams.

3. If your student is not up to it, then skip straight to step 2.

EiN930 SG

okay. Thank you!

UltimaOnline SG

okay. Thank you!

No problem, EiN930.

Tell all your 'O' level students, "Be patient. You'll really start to understand stuff better at 'A' levels. 'O' levels is really too simplified to explain anything."

EiN930 SG

Thank you :)

Anyway I re-checked the Mark Scheme and the answer given by the mark scheme is:

"Graphite has fewer strong bonds to break."

And according to the examiner reports, it states:

"To compare the two, mention of both diamond and graphite was required. That graphite ‘has weak bonds’ is not true, as the diagram shows. The Examiners required the answer that graphite has some weak bonds but diamond has only strong bonds."

UltimaOnline SG

Thank you :)

Anyway I re-checked the Mark Scheme and the answer given by the mark scheme is:

"Graphite has fewer strong bonds to break."

And according to the examiner reports, it states:

Yes, the examiner report confirms what I said in my previous posts, about usage of comparative adjectives, difference in number of covalent bonds, and strength of covalent bonds (stronger in graphite, weaker in diamond).

The so-called 'weak bonds' (in the examiner's report) do not refer to covalent bonds, but van der Waals forces. This should be specified more clearly in the examiner's report, which are to be read by teachers, not students, so Cambridge shouldn't use oversimplified (and technically erroneous) terms like "weak bonds" when referring to van der Waals forces, lest it may confuse some people reading it.

atomos SG

A bit of sidetrack here, does anyone know the values for these melting points?

I'm of the impression that high pressures are needed for them to melt instead of sublime, but are the respective melting points compared at the same pressure?

If not, then is it a fair question to compare in the first place?

UltimaOnline SG

Indeed, this isn’t a straightforward matter (as Cambridge made it out to be asking it in an ‘O’ level exam).

Source 1 :
http://invsee.asu.edu/nmodules/carbonmod/point….

Melting point of Diamond : 3550 deg C
Melting point of Graphite : 3652 – 3697 (sublimes)

Source 2 :
http://www.preservearticles.com/201101022315/di…

The melting point of graphite is slightly higher than the melting point of diamond because in graphite C-C bonds have partial double bond character and hence are stronger and difficult to break. In fact, carbon as graphite has the highest melting point among all the elements.

Source 3 :
http://www.scienceforums.net/topic/18255-graphi…

“The melting point of the Graphite is higher than the diamond! Can anyone explain why?”

Really its not – its the varying pressures used. According to the CRC handbook of chemistry and physics:

Carbon (diamond) has a melting point of 4440°C at 12.4 GPa.
Carbon (graphite) has a melting point of 4489°C at 10.3 MPa.

Due to the varying pressures used I’d say the comparision is not really fair because we know that pressure affects the melting and boilling points of materials, e.g. water on Mount Everest boils well below 100°C.

We know diamond has more internal bonds, the maximum possible for carbon (4). So it must take more energy to break 4 then to break 3 even is we include the inermolecular attraction between the layers in the graphite we could not account for such a difference. Our only conclusion can be that the pressure is having a big effect on the results :-)

Cheers,
Ryan Jones