Giant covalent structures
2 marks · HardDiamond is used for industrial cutting tools, while graphite is used as a lubricant. Explain, in terms of structure and bonding, why graphite can be used as a lubricant but diamond cannot.
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Also accept: Graphite has layers held together by weak forces, so the layers can slide over each other, making it slippery. Diamond has a rigid three-dimensional structure with strong covalent bonds throughout, so it is hard and not slippery.
Common mistakes: Saying graphite is soft without mentioning layers sliding; not referring to the strong covalent bonds in diamond; confusing melting or boiling points with hardness.
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Marking points
M1 · 1 mark · States that graphite has layers that can slide over each other due to weak forces between layers.
M2 · 1 mark · States that diamond has a giant 3D structure with strong covalent bonds throughout, so layers cannot slide.
Worked solution
Graphite has a structure where carbon atoms form layers, and the forces between these layers are weak. This allows the layers to slide over each other, making graphite slippery and suitable as a lubricant. Diamond, however, has a giant rigid three-dimensional network of strong covalent bonds; there are no layers that can slide, making it hard and unsuitable as a lubricant.