Covalent Bonding
7 marks · HardThis question is about how the type of structure affects properties in covalent and giant covalent substances.
Explain what is meant by a 'simple molecular structure', using water (H₂O) as an example.
Compare the structure and bonding found in C₆₀ fullerene with that in graphite.
State which would have a higher melting point, graphite or C₆₀ fullerene. Explain your answer in terms of their bonding and structures.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) A simple molecular structure consists of small, discrete molecules held together by weak intermolecular forces. In water, each molecule contains atoms joined by strong covalent bonds, but the molecules are only weakly attracted to each other.
(b) C₆₀ fullerene is made up of separate molecules each with 60 carbon atoms arranged in a spherical shape, with strong covalent bonds within each molecule and weak intermolecular forces between molecules. Graphite is a giant covalent structure where each carbon atom is bonded to three others in flat layers, and the layers are held together by weak forces, but the covalent bonding extends in 2D across layers.
(c) Graphite has a higher melting point than C₆₀ fullerene, because its covalent bonding extends over large networks, requiring much more energy to break. In C₆₀ fullerene, only weak intermolecular forces need to be overcome to melt it, so less energy is needed.
Common mistakes: Confusing molecular structure with giant covalent; not linking melting point to breaking bonds versus intermolecular forces
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Marking points
M1 (a) · 1 mark · States that a simple molecular structure has discrete/small molecules with weak intermolecular forces between them.
A1 (a) · 1 mark · Mentions strong covalent bonds inside each molecule, e.g. H₂O as an example.
M2 (b) · 1 mark · Describes C₆₀ fullerene as a molecule (not a giant structure), with strong covalent bonds within and weak intermolecular forces between molecules.
M3 (b) · 1 mark · Describes graphite as a giant covalent structure with covalent bonds extending in layers.
A2 (b) · 1 mark · Compares the two correctly (C₆₀ molecules vs graphite layers/network).
M4 (c) · 1 mark · States graphite has a higher melting point than C₆₀ fullerene.
A3 (c) · 1 mark · Explains this is because graphite has a giant structure with many strong covalent bonds to break, but in C₆₀ only weak intermolecular forces must be overcome.
Worked solution
a) A simple molecular structure is one where there are individual molecules with strong covalent bonds holding the atoms together inside each molecule, but only weak forces (intermolecular) between molecules. Water is an example: each H₂O molecule has strong O–H covalent bonds, while the forces between molecules are much weaker.
b) C₆₀ fullerene is made up of separate molecules of 60 carbon atoms forming a sphere, with covalent bonds holding the atoms together inside each molecule, but with only weak forces between molecules. In graphite, each carbon atom is joined to three others forming flat layers with strong covalent bonds within layers and weak forces between layers. Graphite is a giant covalent structure, C₆₀ is not.
c) Graphite has a higher melting point than C₆₀ fullerene because to melt graphite, strong covalent bonds must be broken throughout the structure, which needs a lot of energy. Only weak intermolecular forces need to be broken to melt C₆₀ fullerene, so it melts at a much lower temperature.