Question

hardess, thermal conductivity , or solbiltiy in water explain how the property relates to the molecular structer silicon dioxide, Naphrhalene, cobalt (l) chloride

Answer 1

Below I explain how hardness, thermal conductivity, and water solubility follow from each substance’s molecular/solid structure for silicon dioxide (SiO2), naphthalene, and cobalt(II) chloride (CoCl2).

Silicon dioxide (SiO2)
- Structure: a 3‑D covalent network of SiO4 tetrahedra in which each O bridges two Si atoms (quartz/crystalline) or the same connectivity in an amorphous glass.
- Hardness: high. The continuous network of very strong Si–O covalent bonds produces a rigid lattice that resists deformation (quartz is relatively hard).
- Thermal conductivity: moderate for a non‑metal. Heat is carried by phonons (lattice vibrations); the stiff bonds and ordered lattice of crystalline SiO2 give better phonon transport than molecular solids, but no free electrons means conductivity is far lower than metals. Amorphous silica (glass) has lower thermal conductivity than crystalline quartz because disorder scatters phonons.
- Solubility in water: extremely low in neutral water. The strong Si–O network is not easily broken by water — silica only dissolves appreciably under high pH (alkaline) or with reactive agents (e.g., HF). Dissolution proceeds by formation of silicic acid (Si(OH)4), but the process is slow.

Naphthalene (C10H8)
- Structure: discrete neutral planar aromatic molecules (two fused benzene rings) packed in a molecular crystal held together mainly by London dispersion forces (and weak π–π interactions).
- Hardness: very low/soft. The intermolecular forces are weak compared with covalent or ionic bonds, so the solid is brittle and easily deformed or sublimed.
- Thermal conductivity: very low. Heat transport is inefficient because there are no free electrons and weak intermolecular coupling; phonons are strongly scattered in a molecular solid.
- Solubility in water: essentially insoluble (very low solubility). Naphthalene is nonpolar and cannot form favorable ion–dipole or hydrogen‑bonding interactions with water; it dissolves readily in nonpolar organic solvents instead.

Cobalt(II) chloride (CoCl2)
- Structure: an ionic solid of Co2+ cations and Cl– anions (in the anhydrous form) that forms a crystalline lattice; Co2+ also readily forms hydrated coordination complexes (e.g., CoCl2·6H2O).
- Hardness: moderate but brittle. Ionic crystals are held by strong electrostatic interactions, so they are harder than molecular solids but typically more brittle than network covalent solids.
- Thermal conductivity: low to moderate (typical of ionic insulators). Heat is carried by phonons; without mobile electrons, thermal conduction is poorer than metals and often comparable to other salts/oxides.
- Solubility in water: readily soluble. Water molecules stabilize separated Co2+ and Cl– by strong ion–dipole hydration (and Co2+ has a high hydration enthalpy), so the lattice energy is overcome and the salt dissolves. Hydration also leads to characteristic color changes (anhydrous vs hydrated Co2+ species).

Overall trend: hardness and thermal conductivity increase when strong, extended bonding networks (covalent or ionic) provide rigid, well‑connected lattices (SiO2 > CoCl2 > naphthalene). Solubility in water is governed by polarity/ionicity and ability to form favorable interactions with water (ionic salts like CoCl2 dissolve; nonpolar molecular solids like naphthalene do not; network covalent solids like SiO2 are effectively insoluble except under special chemical conditions).