Exercises: Metals, lattices and intermolecular forces

Pure water has a boiling point of $100.0\,^\circ\mathrm{C}$ at a pressure of $1.0\,\mathrm{atm}$. When liquid water is heated to boiling, it is transformed into water vapor according to the physical process $\mathrm{H_2O}\,(\mathrm{l}) \rightarrow \mathrm{H_2O}\,(\mathrm{g})$.

1. a) Justify, with reasoning, which specific type of interactions or bonds are overcome during the boiling of liquid water.
2. b) Explain why this physical process does not break the intramolecular covalent bonds $\mathrm{O-H}$ nor decompose the water into hydrogen and oxygen gases.

Consider the homologous series of the diatomic molecular halogens: difluorine ($\mathrm{F_2}$), dichlorine ($\mathrm{Cl_2}$), dibromine ($\mathrm{Br_2}$) and diiodine ($\mathrm{I_2}$). At $25.0\,^\circ\mathrm{C}$ and $1.0\,\mathrm{atm}$, $\mathrm{F_2}$ and $\mathrm{Cl_2}$ are gases, $\mathrm{Br_2}$ is a liquid and $\mathrm{I_2}$ is a solid.

1. a) State the nature of the intermolecular bonding that predominates between these nonpolar homonuclear molecules.
2. b) Justify the variation observed in the state of matter and in the boiling points on descending the halogen group.

Two pure crystalline solids are available in the lab, labeled sample 1 (metallic copper, $\mathrm{Cu}$) and sample 2 (sodium chloride, $\mathrm{NaCl}$). The electrical conductivity of both is measured in the solid state and after melting them by heating.

1. a) Explain why copper conducts electric current in the solid and liquid states, based on the electron sea model.
2. b) Justify why $\mathrm{NaCl}$ is an electrical insulator in the solid state but becomes a good electrical conductor when it melts.

Diamond and graphite are two allotropic forms of pure carbon. Both are giant covalent lattices. At atmospheric pressure they do not show ordinary melting: at very high temperatures the transformation between forms and the sublimation of carbon must be considered. However, diamond is the hardest known mineral and is an electrical insulator, whereas graphite is a soft, flaking material and an excellent electrical conductor parallel to its planes.

1. a) Describe the hybridization and spatial arrangement of the carbon atoms in diamond and in graphite.
2. b) Justify in detail the distinctive electrical conductivity and the lubricating or flaking ability of graphite in terms of its bonding structure.

The following pure molecular substances are available: diiodine ($\mathrm{I_2}$), ethanol ($\mathrm{CH_3CH_2OH}$) and glucose ($\mathrm{C_6H_{12}O_6}$, which has five hydroxyl groups $-\mathrm{OH}$). Their solubility at $20.0\,^\circ\mathrm{C}$ is tested in two solvents: pure water ($\mathrm{H_2O}$) and hexane ($\mathrm{C_6H_{14}}$).

1. a) Justify why diiodine is sparingly soluble in water but very soluble in hexane.
2. b) Explain why ethanol is completely miscible in water and glucose is highly soluble in water, identifying the intermolecular interactions responsible.

Four pure substances are characterized in the lab after being subjected to various physical tests. The experimental results are collected below:

• Substance W: melting point $1710\,^\circ\mathrm{C}$; does not conduct electricity in the solid or liquid state; completely insoluble in water and in non-polar organic solvents.

• Substance X: melting point $1084\,^\circ\mathrm{C}$; excellent electrical and thermal conductor both as a solid and as a molten liquid; malleable and ductile.

• Substance Y: melting point $801\,^\circ\mathrm{C}$; does not conduct electricity in the solid state, but does when molten or dissolved in water; brittle solid.

• Substance Z: melting point $-23\,^\circ\mathrm{C}$, boiling point $77\,^\circ\mathrm{C}$; does not conduct appreciably when pure, solid or liquid; soluble in hexane and insoluble in water.

1. a) Classify, with reasoning, each substance (W, X, Y, Z) as one of the four types of solid: ionic, giant covalent (network covalent), metallic or simple molecular.
2. b) Suggest a real chemical substance consistent with each of the four types described, based on their qualitative properties; you do not need to memorize the exact melting values.

The normal boiling points at $1.0\,\mathrm{atm}$ and the molar enthalpies of vaporization of three organic compounds of similar molar mass ($M \approx 44 - 46\,\mathrm{g\times mol^{-1}}$) are compared:

• Propane ($\mathrm{CH_3CH_2CH_3}$): $M = 44.1\,\mathrm{g\times mol^{-1}}$; $T_\mathrm{eb} = -42.1\,^\circ\mathrm{C}$; $\Delta H_\mathrm{vap} = 19.0\,\mathrm{kJ\times mol^{-1}}$.

• Dimethyl ether ($\mathrm{CH_3OCH_3}$): $M = 46.1\,\mathrm{g\times mol^{-1}}$; $T_\mathrm{eb} = -24.8\,^\circ\mathrm{C}$; $\Delta H_\mathrm{vap} = 21.5\,\mathrm{kJ\times mol^{-1}}$.

• Ethanol ($\mathrm{CH_3CH_2OH}$): $M = 46.1\,\mathrm{g\times mol^{-1}}$; $T_\mathrm{eb} = 78.3\,^\circ\mathrm{C}$; $\Delta H_\mathrm{vap} = 38.6\,\mathrm{kJ\times mol^{-1}}$.

1. a) Analyze in detail the intermolecular forces present in each of the three pure liquids.
2. b) Justify the marked difference in boiling points and in the values of $\Delta H_\mathrm{vap}$ between the three compounds despite their almost identical molar masses.