Metals, Lattices and Intermolecular Forces

Two pure crystalline solids are available in the laboratory, labelled 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.

Band theory explains the electrical behaviour of materials in terms of the energy gap between the valence band and the conduction band. The following diagram shows schematically the band structures of three different materials called A, B and C.

1. a) Identify, with reasoning, which of the three materials behaves as a metallic conductor, which as an intrinsic semiconductor and which as an electrical insulator at room temperature ($25.0\,^\circ\mathrm{C}$).
2. b) Explain the effect that a moderate increase in temperature has on the electrical conductivity of material A compared with material B. The figure is schematic: the sizes of the bands and gaps are not drawn to an energy scale.

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 hybridisation 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.

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 vapour 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 non-polar homonuclear molecules.
2. b) Justify the variation observed in the state of matter and in the boiling points on descending the halogen group.

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.

The normal boiling points at $1.0\,\mathrm{atm}$ and the molar enthalpies of vaporisation 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) Analyse 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.

Four pure substances are characterised in the laboratory 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, 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 memorise the exact melting values.

The mechanical behaviour and electrical conductivity of solid materials depend on the directional or non-directional nature of their bonds and on the presence of charge carriers.

1. a) Compare the mechanical response to a shear stress or impact in a crystalline metal (such as iron, $\mathrm{Fe}$) and in an ionic crystal (such as calcium fluoride, $\mathrm{CaF_2}$), explaining why the metal is malleable and the ionic solid is brittle.
2. b) Analyse why quartz (giant $\mathrm{SiO_2}$ lattice) has great hardness and very low solubility in water and in common organic solvents that do not react with it, comparing the band model of a metal with the insulating character of quartz.

Topic challenge. As a technical adviser to Red Eléctrica de España (REE), you must evaluate the materials used in substations and high-voltage transmission lines. Write a technical report justifying their suitability on the basis of their chemical bonding.

1. Data collection: Go to the REE website (ree.es) and identify real materials used in conductors (cables of $Al$ or $Cu$), solid ceramic insulators ($SiO_2$) and electrical insulating gases (such as $CO_2$ or $SF_6$).

2. Molecular structure: For the chosen dielectric gas, draw its Lewis structure, analyse possible resonance forms and calculate the formal charges. Deduce its molecular geometry (VSEPR) and the hybridisation of the central atom, and justify whether the molecule is polar or non-polar.

3. Lattices and ionic compounds: Assume the use of a salt as an insulating additive (e.g. $NaCl$). Set up its Born–Haber cycle to deduce its lattice energy ($U$). Then, compare the electrical conductivity and melting points of the conducting metal, the covalent lattice of $SiO_2$ and the insulating gas in terms of their bonding and intermolecular forces.

4. Reasoned proposal: Summarise in a table the relationship between the microscopic structure and the technical function of each material, arguing for your final selection.

The following will be assessed:

• Accuracy of the Lewis structures, resonance, formal charges, geometry and hybridisation.

• Rigorous set-up of the Born–Haber cycle and qualitative justification of the lattice energy.

• Coherent explanation of the macroscopic properties in terms of bonding and intermolecular forces.

• Clarity of the report and appropriate integration of the REE data.