Topic 2.2: The Covalent Model Questions

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]

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

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

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

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.

Explain why ethane (C₂H₆) allows free rotation about its C–C bond, whereas ethene (C₂H₄) does not allow rotation about its C=C bond.

Refer to σ and π bonds in your answer.

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

Explain this observation. [2]

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]

Two Lewis structures are proposed for the cyanate ion, OCN^-: structure X is O=C=N (a double bond on each side) and structure Y is O–C≡N (single O–C, triple C≡N), with the terminal atoms completing their lone pairs.

Use formal charge to determine which structure is more plausible, showing your working for the terminal atoms.

Propenenitrile (acrylonitrile) has the structure CH₂=CH–C≡N.

Deduce the hybridization of each of the three carbon atoms, and state the total number of σ (sigma) and π (pi) bonds in the molecule.

Sulfur trioxide, SO₃, can be drawn with one S=O double bond and two S–O single bonds.

The three S–O bonds are found experimentally to be equal in length.

(a) Explain, using resonance, why the three bonds are equal.

(b) State one reason the octet rule cannot describe every covalent species, giving a named example.

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°).