Giant covalent structures
4 marks · NormalThe table shows the melting points and hardness for three forms of carbon.
| Allotrope | Melting point/°C | Hardness |
|---|---|---|
| Diamond | 3550 | Very hard |
| Graphite | 3600 | Soft |
| C60 fullerene | 280 | Soft |
Using only the data, identify which allotrope(s) have a giant covalent structure. Explain your reasoning.
Using the data and your knowledge, explain why C60 fullerene has a lower melting point than graphite.
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(a) Diamond and graphite have giant covalent structures because they both have high melting points above 3500 °C, suggesting strong covalent bonds in a large network. C60 fullerene, with a much lower melting point, likely does not have a giant structure.
(b) C60 fullerene has a lower melting point because it consists of separate molecules held together by weak intermolecular forces, which are easier to overcome than the strong covalent bonds holding together the giant structures in graphite.
Common mistakes: not explaining using data
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Marking points
M1 (a) · 1 mark · Identifies diamond and graphite as giant covalent (using high melting point).
A1 (a) · 1 mark · Relates high melting point to strong covalent bonds.
M2 (b) · 1 mark · States C60 fullerene is simple molecular structure/no giant network.
A2 (b) · 1 mark · Explains weak intermolecular forces in C60 mean less energy needed to melt, giving a lower melting point than graphite.
Worked solution
a) Diamond and graphite have giant covalent structures shown by their very high melting points, which are much greater than C60 fullerene’s. High melting points indicate strong covalent bonds extending throughout the crystal. C60 fullerene’s much lower melting point suggests it is not a giant structure.
b) C60 fullerene melts at a much lower temperature because it is made up of separate molecules held together by weak intermolecular forces. These forces require much less energy to overcome than the giant network of strong covalent bonds in graphite.