Covalent bonding and giant covalent structures – IGCSE Chemistry exam questions with mark scheme

1Grade 4–53 marks

Methane, CH₄, is a simple molecular substance with a boiling point of −162 °C.

Explain why methane has a low boiling point. [3]

weak intermolecular forces / weak forces (of attraction) between molecules(1)
little (heat) energy needed to overcome these forces / break these forces(1)
covalent bonds (within the molecules) are not broken (reject covalent bonds are weak / covalent bonds broken: max 2)(1)
Answer: weak intermolecular forces need little energy to overcome; covalent bonds are not broken
Examiner tip: Saying the covalent bonds are weak is a contradiction and loses the mark — it is the forces BETWEEN molecules that are weak.

2Grade 6–74 marks

Diamond and graphite are both forms of carbon with giant covalent structures.

(a) Explain why graphite conducts electricity but diamond does not. [2]

(b) Explain why graphite is soft and slippery, making it useful as a lubricant. [2]

(a) graphite has delocalised electrons that can move (through the structure / along the layers) (allow free / mobile electrons; allow delocalised electrons between the layers; reject ions moving; "each carbon forms only three bonds" on its own does not score)(1)
(a) diamond has no delocalised / free electrons (allow all four outer electrons of each carbon are used in covalent bonds)(1)
(b) layers (of carbon atoms) held together by weak forces (allow weak intermolecular forces between layers; reject weak covalent bonds)(1)
(b) so the layers can slide over each other(1)
Answer: (a) delocalised electrons in graphite; none in diamond (b) layers held by weak forces slide
Examiner tip: You must explain BOTH substances in (a): graphite has delocalised electrons (each carbon uses only three outer electrons in bonds), while diamond has none — describing graphite alone scores only 1.

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