Resumen del examen
Estudiante: Isidoro Bermúdez | Asignatura: Compuestos Inorgánicos de Estructura Compleja | Idioma: EN
Pregunta 1
Structure and bonding of silicates
Respuesta del estudiante
Silicates are inorganic compounds characterized by a fundamental structural unit consisting of a silicon atom tetrahedrally coordinated to four oxygen atoms (SiO4 tetrahedron), with Si-O bond lengths of approximately 1.6 Å and O-Si-O bond angles close to 109.5°. The overall structure depends on the degree of polymerization through shared oxygen atoms (bridging oxygens), leading to classifications such as nesosilicates (isolated tetrahedra, e.g., olivine), sorosilicates (two tetrahedra sharing one oxygen), cyclosilicates (cyclic arrangements), inosilicates (single or double chains, e.g., pyroxenes), phyllosilicates (layered sheets, e.g., micas), and tectosilicates (three-dimensional frameworks, e.g., quartz). Bonding within the silicate framework is predominantly covalent, with partial ionic character due to the electronegativity difference between silicon and oxygen. The negative charge from non-bridging oxygen atoms is balanced by cations such as Na⁺, K⁺, or Ca²⁺, while in fully polymerized tectosilicates, all oxygen atoms are bridging, resulting in a neutral framework. The stability of silicates arises from the strong covalent Si-O network, with cation coordination influencing structural diversity and physical properties.
Pregunta 2
What is a borane? Classification of boranes
Respuesta del estudiante
Boranes are electron-deficient compounds consisting solely of boron and hydrogen atoms, characterized by multi-center bonding, including three-center two-electron bonds. They are classified according to Wade's rules into structural types: closo (closed polyhedral structures with the general formula [BₙHₙ]²⁻, e.g., [B₆H₆]²⁻), nido (nest-like structures derived from closo by removal of one vertex, formula BₙHₙ₊₄, e.g., B₅H₉), arachno (web-like structures with two vertices removed, formula BₙHₙ₊₆, e.g., B₄H₁₀), and hypho (structures with three vertices removed, formula BₙHₙ₊₈, e.g., B₅H₁₁). Neutral boranes such as diborane (B₂H₆) and tetraborane (B₄H₁₀) are common examples, while anionic boranes include the closo-borane dianions.
Pregunta 3
Give examples of the use of inorganic heterocycles in the synthesis of the corresponding polymers
Respuesta del estudiante
Inorganic heterocycles serve as key precursors in the synthesis of various inorganic polymers through ring-opening polymerization or thermal processes. For example, hexachlorophosphazene (N₃P₃Cl₆), a phosphorus-nitrogen heterocycle, undergoes ring-opening polymerization to form polyphosphazenes, which are tailored for biomedical and flame-retardant applications by substituting chloride groups. Tetrasulfur tetranitride (S₄N₄), a sulfur-nitrogen heterocycle, thermally polymerizes to yield polythiazyl ((SN)ₓ), a one-dimensional metallic conductor. Additionally, cyclic siloxanes such as octamethylcyclotetrasiloxane (D₄) undergo ring-opening polymerization to produce polysiloxanes (silicones), widely used in sealants and insulating materials due to their thermal stability and flexibility. These examples illustrate the critical role of inorganic heterocycles in constructing functional polymeric frameworks.
Pregunta 4
Crystalline defects
Respuesta del estudiante
Crystalline defects are deviations from the ideal periodic arrangement of atoms in a crystal lattice, classified into four main categories based on dimensionality. Point defects (0D) include vacancies, interstitial atoms, substitutional impurities, and intrinsic defects such as Frenkel (a vacancy-interstitial pair) and Schottky (paired cation-anion vacancies in ionic compounds). Line defects (1D), or dislocations, consist of edge dislocations (extra half-plane of atoms) and screw dislocations (helical atomic displacement). Planar defects (2D) encompass grain boundaries (interfaces between crystal domains), stacking faults (irregular layer sequences), and twin boundaries (symmetrical misorientations). Volume defects (3D) involve voids, precipitates, or foreign inclusions. These defects critically influence material properties, including mechanical strength, electrical conductivity, and reactivity, and are fundamental to understanding non-stoichiometric compounds and solid-state reactions.
Pregunta 5
Explain the polymerization of vanadates, VO43- as a function of concentration and pH
Respuesta del estudiante
The polymerization of vanadates (VO₄³⁻) is highly dependent on pH and concentration. At high pH (alkaline conditions, pH > 12), VO₄³⁻ remains predominantly monomeric due to electrostatic repulsion and deprotonation. As pH decreases (pH 10–7), partial protonation forms HVO₄²⁻ and H₂VO₄⁻, enabling condensation reactions via dehydration to yield oligomeric species such as divanadate (V₂O₇⁴⁻) and trivanadate (V₃O₉³⁻). At pH 6–4 and moderate to high vanadate concentrations, decavanadate (V₁₀O₂₈⁶⁻) becomes the dominant species, stabilized by its cyclic structure. Further acidification (pH < 4) promotes the formation of larger polyvanadates or amorphous V₂O₅, especially at elevated concentrations. Higher vanadate concentrations favor polymerization by shifting the equilibrium toward condensed species, while dilute solutions inhibit extensive polymerization even at lower pH due to reduced ion collision frequency. The process is reversible, with depolymerization occurring upon alkalinization.