Covalent Compounds: Molecular and Network

Activity 1

1. Covalent molecular compound. The very low MP of −85°C indicates only weak IMFs between discrete molecules (not covalent bonds, which are strong). Gaseous state at room temperature confirms very weak IMFs. No conductivity is consistent with no free electrons or ions. Slight water solubility suggests mild polarity. This substance could be HCl (BP −85°C).

2. Covalent network solid. MP 2730°C is extreme, only substances where strong covalent bonds extend throughout the crystal can reach this temperature. Extreme hardness confirms a continuous covalent network (all bonds must break to deform). No conductivity eliminates metals and graphite. Insolubility in all solvents is characteristic of network covalent solids. This substance is silicon carbide (SiC).

3. Covalent molecular compound (water, H₂O). A BP of 100°C is high for a small covalent molecule, this is explained by strong hydrogen bonding between H₂O molecules (O is highly electronegative → O–H bonds are strongly polar → O–H···O hydrogen bonds form). Classification is covalent molecular because it consists of discrete H₂O molecules with hydrogen bonds between them. Conducts only when ionised (pure water doesn't conduct well).

Activity 2

Response 1, Error: Student A is incorrect. When water boils, the H–O–H covalent bonds do NOT break, the water molecules remain intact as individual H₂O molecules in the gas phase. What breaks is the intermolecular hydrogen bonds between adjacent water molecules. Water has a higher boiling point than many other molecules (such as H₂S) because it forms strong hydrogen bonds (O–H···O), not because of stronger intramolecular bonds. The strength of the H–O covalent bond is not relevant to the boiling point, it would only matter if you were breaking water molecules apart chemically.

Response 2, Error: Student B incorrectly treated SiO₂ as a molecular substance. SiO₂ does NOT consist of discrete molecules, it is a covalent network solid where Si–O covalent bonds extend throughout the entire crystal. CO₂, by contrast, forms discrete O=C=O molecules with only weak dispersion forces between them. The higher MP of SiO₂ is because melting requires breaking strong Si–O covalent bonds (rather than just IMFs as in CO₂). The reason has nothing to do with molecular mass or IMF strength, it is entirely about structural type (network vs molecular).

❓ Multiple Choice

Boiling breaks intermolecular hydrogen bonds. Covalent O–H bonds remain intact. Gas-phase water is still H₂O molecules.

CO₂ (molecular, BP −78°C) vs SiO₂ (network, MP 1713°C) is the canonical example of this contrast. A = both ionic; B = both metallic; D = both molecular (comparing IMF type, not structural category).

Low BP in molecular substances reflects weak IMFs between molecules. The covalent bonds within molecules are not broken on boiling and their strength is irrelevant to the BP.

Dispersion forces increase with molecular size (more electrons → stronger temporary dipoles → stronger dispersion). These are the only IMFs in non-polar diatomic halogens. No hydrogen bonding; covalent bond strength is irrelevant to BP.

SiO₂ is a covalent network solid with a continuous Si–O bond network. It does not consist of discrete molecules; it does not dissolve in water; it does not conduct when molten.

Short Answer Model Answers

Q6 (3 marks): Both F₂ and I₂ are non-polar diatomic molecules, the only IMF acting between their molecules is dispersion forces (1 mark). Dispersion forces increase in strength with the number of electrons in a molecule. I₂ has 106 electrons compared to F₂'s 18 electrons, I₂ has many more electrons and a much larger, more easily polarised electron cloud (1 mark). The stronger dispersion forces between I₂ molecules require significantly more energy to overcome during boiling → much higher BP (184°C vs −188°C). No covalent bonds are broken in either case, only IMFs (1 mark).

Q7 (5 marks): Substance X is a covalent molecular compound (1 mark). Its low MP of −22°C indicates only weak IMFs between discrete molecules, which are easily overcome, covalent bonds within molecules are not broken on melting (1 mark). No conductivity in any state confirms no free electrons or mobile ions (1 mark). Water solubility indicates the substance is polar, like-dissolves-like; polar molecular substances can interact with polar water molecules and dissolve. Substance Y is a covalent network solid (1 mark). MP 1713°C requires breaking enormous numbers of strong covalent bonds extending throughout the crystal, consistent with a network solid (this is SiO₂, quartz). No conductivity confirms no free electrons or mobile ions, the strong, directional covalent bonds hold all electrons in place. Insolubility: the Si–O network is too stable and strongly bonded to be disrupted by water molecules (1 mark across Y properties).

Q8 (4 marks): Propane (C₃H₈) is a non-polar hydrocarbon molecule, it cannot form hydrogen bonds or dipole-dipole interactions, so only weak dispersion forces act between propane molecules (1 mark). Despite its larger size (MW 44 vs 18), propane's BP is −42°C because these dispersion forces are still relatively weak (1 mark). Water (H₂O) contains two polar O–H bonds where oxygen's high electronegativity creates a large partial negative charge on O and partial positive charge on H. This allows water molecules to form strong intermolecular hydrogen bonds (O–H···O) with each other (1 mark). Hydrogen bonds are much stronger than the dispersion forces in propane, approximately 5–10× stronger, so far more energy is required to separate water molecules, resulting in a boiling point 142°C higher than propane despite water being the smaller molecule (1 mark).