Topic 2.2: The Covalent Model Questions

The boiling points of the straight-chain alcohols increase steadily from methanol to butan-1-ol.

Explain this trend in terms of intermolecular forces.

Deduce the Lewis (electron-dot) structure of the carbon dioxide molecule, CO₂.

State the bond order of the nitrogen–nitrogen bond in N₂, and explain why nitrogen gas is relatively unreactive.

Deduce the molecular shape and bond angle of the ammonia molecule, NH₃, and explain your answer.

Hydrogen fluoride, HF, contains a polar covalent bond.

Deduce, using electronegativity, which atom carries the partial negative charge (δ−), and explain your reasoning. [2]

Methane (CH₄) boils at −162 °C, but water (H₂O) boils at +100 °C, even though they have a similar number of electrons.

Explain this large difference in terms of intermolecular forces.

Diamond and graphite are both allotropes of carbon, yet diamond does not conduct electricity while graphite does.

Explain this difference in terms of their structures. [3]

Tetrachloromethane, CCl₄, is a tetrahedral molecule containing four polar C–Cl bonds, yet the molecule is non-polar.

Explain this observation. [2]

The molecules BF₃ and NF₃ both have a central atom bonded to three other atoms.

Explain why BF₃ is trigonal planar (120°) but NF₃ is trigonal pyramidal (~102°).

Silicon dioxide (SiO₂) is a giant covalent solid that melts at over 1600 °C, whereas carbon dioxide (CO₂) is a gas at room temperature.

Both contain only covalent bonds.

Discuss this large difference in terms of structure and bonding. [3]