Resumen del examen
Estudiante: Domitila Quiroga | Asignatura: Compuestos Inorgánicos de Estructura Compleja | Idioma: EN
Pregunta 1
Structure and bonding of silicates
Respuesta del estudiante
Silicates are inorganic polymers based on the silicon-oxygen (Si-O) bond, forming the most abundant class of minerals in the Earth's crust. Their structure and bonding are governed by the following key features:
1. **Basic Structural Unit**: The fundamental building block is the [SiO₄]⁴⁻ tetrahedron, where a central silicon atom is coordinated to four oxygen atoms in a tetrahedral arrangement. The Si-O bond has significant covalent character due to the overlap of silicon sp³ hybrid orbitals with oxygen 2p orbitals, though ionic contributions also exist due to the electronegativity difference.
2. **Polymerization and Connectivity**: Silicate structures arise from the condensation of [SiO₄]⁴⁻ tetrahedra through shared oxygen atoms (bridging oxygens, BO), forming Si-O-Si linkages. The degree of polymerization determines the structural classification:
- **Nesosilicates**: Isolated [SiO₄]⁴⁻ tetrahedra (e.g., olivine, (Mg,Fe)₂SiO₄).
- **Sorosilicates**: Two tetrahedra sharing one oxygen (e.g., hemimorphite, Zn₄Si₂O₇(OH)₂·H₂O).
- **Cyclosilicates**: Rings of tetrahedra sharing two oxygens per unit (e.g., beryl, Be₃Al₂Si₆O₁₈).
- **Inosilicates**: Single or double chains of tetrahedra (e.g., pyroxenes (single chain) or amphiboles (double chain)).
- **Phyllosilicates**: Sheets of tetrahedra sharing three oxygens per unit (e.g., clays, micas).
- **Tectosilicates**: Three-dimensional frameworks where all four oxygens are shared (e.g., quartz, feldspars).
3. **Bonding Characteristics**:
- The Si-O bond length (~1.6 Å) and bond angle (Si-O-Si ~140–180°) vary with structure due to steric and electronic effects.
- Bridging oxygens (BO) exhibit stronger covalent bonding than non-bridging oxygens (NBO), which are often associated with cations (e.g., Na⁺, Ca²⁺) to balance charge.
- The bond strength and stability increase with polymerization, as seen in the high melting points of tectosilicates (e.g., quartz, ~1700°C).
4. **Role of Cations**: Alkali and alkaline earth metals (e.g., Na⁺, K⁺, Ca²⁺) or transition metals (e.g., Al³⁺, Fe³⁺) can substitute for Si⁴⁺ in tetrahedral sites (isomorphic substitution), altering charge balance and requiring additional cations for neutrality. Aluminosilicates (e.g., feldspars) are common examples where Al³⁺ replaces Si⁴⁺, creating a negative framework charge.
5. **Thermodynamic Stability**: Silicate structures are stabilized by the high bond energy of Si-O (~452 kJ/mol) and the efficient packing of tetrahedra in three dimensions. The polymerization degree correlates with thermodynamic stability, with tectosilicates being the most stable under ambient conditions.
Pregunta 2
What is a borane? Classification of boranes
Respuesta del estudiante
A borane is a chemical compound composed of boron and hydrogen, with the general formula BxHy. These compounds are characterized by their electron-deficient nature, as boron has only three valence electrons, leading to the formation of multicenter bonds (such as two-electron three-center bonds) to achieve stability. Boranes exhibit unique structural and bonding properties, including deltahedral geometries and cluster-like arrangements.
Boranes are classified based on their stoichiometry and structure into the following categories:
- **Closo-boranes**: Closed polyhedral structures with the general formula BnHn²⁻, where n ranges from 5 to 12. These are the most stable boranes, with all boron atoms forming a complete polyhedron (e.g., B₆H₆²⁻, B₁₂H₁₂²⁻).
- **Nido-boranes**: Derived from closo-boranes by the removal of one boron vertex, resulting in a nest-like structure with the formula BnHn+4 (e.g., B₂H₆, B₅H₉, B₁₀H₁₄).
- **Arachno-boranes**: Obtained by removing two boron vertices from a closo-borane, leading to a web-like structure with the formula BnHn+6 (e.g., B₄H₁₀, B₅H₁₁).
- **Hypho-boranes**: Derived from the removal of three boron vertices from a closo-borane, forming very open structures with the formula BnHn+8 (e.g., B₈H₁₆).
- **Conjuncto-boranes**: Formed by the fusion of two or more borane clusters through shared boron atoms (e.g., B₁₀H₁₆, which consists of two B₅H₈ units).
Additionally, boranes can be classified as neutral (e.g., B₂H₆, diborane) or anionic (e.g., B₁₂H₁₂²⁻), depending on their charge.
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 or structural units in the synthesis of various inorganic polymers. Examples include:
- **Cyclic phosphazenes (e.g., hexachlorocyclotriphosphazene, [NPCl₂]₃)**: These heterocycles undergo ring-opening polymerization (ROP) under thermal or catalytic conditions to form linear polyphosphazenes, [N=PR₂]ₙ, which exhibit high thermal stability and flexibility. Substitution of chlorine atoms with organic or inorganic groups (e.g., alkoxy, aryloxy, or amino) tailors the polymer’s properties.
- **Cyclic siloxanes (e.g., octamethylcyclotetrasiloxane, D₄, [Si(CH₃)₂O]₄)**: These silicon-oxygen heterocycles undergo anionic or cationic ROP to produce polysiloxanes (silicones), [Si(CH₃)₂O]ₙ, which are widely used for their hydrophobicity, thermal resistance, and biocompatibility.
- **Borazines (e.g., B₃N₃H₆)**: Analogous to benzene, borazines can be polymerized via dehydrocoupling or thermolysis to form boron nitride (BN) polymers or ceramics, which exhibit high thermal and chemical stability.
- **Cyclic thiazyl compounds (e.g., S₄N₄)**: These sulfur-nitrogen heterocycles can be polymerized under controlled conditions to yield polythiazyl, (SN)ₙ, a metallic polymer with intrinsic conductivity and superconductivity at low temperatures.
- **Cyclic boroxines (e.g., trialkylboroxines, [RBO]₃)**: These boron-oxygen heterocycles can be used as precursors for borate-based polymers or ceramics through condensation reactions, often employed in flame-retardant materials.
- **Cyclic silazanes (e.g., hexamethylcyclotrisilazane, [Si(CH₃)₂NH]₃)**: These silicon-nitrogen heterocycles undergo ROP to form polysilazanes, which are precursors for silicon nitride (Si₃N₄) ceramics via pyrolysis.
Pregunta 4
Crystalline defects
Respuesta del estudiante
Crystalline defects are imperfections in the regular arrangement of atoms, ions, or molecules within a crystal lattice. These deviations from ideal periodicity can be classified based on their dimensionality:
- **Point defects (0D):**
- *Vacancies:* Missing atoms or ions at lattice sites.
- *Interstitial defects:* Atoms or ions occupying positions between regular lattice sites.
- *Substitutional defects:* Foreign atoms replacing host atoms in the lattice.
- *Frenkel defects:* A combination of a vacancy and an interstitial defect, where an atom or ion moves from its lattice site to an interstitial position.
- *Schottky defects:* Paired vacancies of cations and anions to maintain electrical neutrality in ionic crystals.
- **Line defects (1D):**
- *Dislocations:* Linear defects where atoms are misaligned, including edge dislocations (extra half-plane of atoms) and screw dislocations (spiral arrangement around a dislocation line).
- **Planar defects (2D):**
- *Grain boundaries:* Interfaces between crystallites with different orientations.
- *Twin boundaries:* Mirror planes where the crystal lattice is symmetrically reflected.
- *Stacking faults:* Errors in the sequence of atomic planes, common in close-packed structures.
- **Bulk defects (3D):**
- *Voids:* Three-dimensional empty spaces within the crystal.
- *Precipitates:* Clusters of impurity atoms or secondary phases.
- *Inclusions:* Foreign particles embedded in the crystal.
These defects influence physical properties such as mechanical strength, electrical conductivity, diffusion, and optical behavior, playing a crucial role in materials science and solid-state chemistry.
Pregunta 5
Explain the polymerization of vanadates, VO43- as a function of concentration and pH
Respuesta del estudiante
The polymerization of vanadate ions (VO₄³⁻) is highly dependent on both concentration and pH, leading to the formation of different polyoxovanadate species through condensation reactions.
At very low concentrations (below ~10⁻⁵ M) and high pH (pH > 13), the dominant species is the monomeric orthovanadate ion, VO₄³⁻. As the pH decreases (pH 12–9), protonation occurs, forming HVO₄²⁻ and H₂VO₄⁻, which can condense to form dimeric (V₂O₇⁴⁻) and tetrameric (V₄O₁₂⁴⁻) species through the elimination of water.
At intermediate pH (pH 9–6) and higher concentrations, further condensation leads to the formation of decavanadate ions (V₁₀O₂₈⁶⁻), which are stable in acidic conditions. The decavanadate structure consists of edge- and corner-sharing VO₆ octahedra.
At even lower pH (pH < 6), protonation continues, and the decavanadate species (HV₁₀O₂₈⁵⁻, H₂V₁₀O₂₈⁴⁻) dominate until precipitation of hydrated V₂O₅ occurs. In highly acidic conditions (pH < 2), the vanadium exists primarily as the cationic VO₂⁺ species.
In summary, increasing concentration and decreasing pH favor polymerization, progressing from monomeric to oligomeric and eventually to large polyoxovanadate clusters before precipitation.