CHM 215 Note

Chemistry of Boron

  • Boron belongs to group 3 of the periodic table.

    • Symbol: B

    • Electronic Configuration: 1s² 2s² 2p¹

Characteristics of Boron

  • Non-metal with 3 valence electrons.

  • Boron rarely forms B^+3 ions due to high energy requirement to remove its 3 valence electrons.

  • The effective nuclear charge felt by protons in the nucleus influences ion formation.

  • Example reaction: Na^+ + Cl^- → B^+3 NaCl

Occurrence of Boron

  • Found in the form of borates such as tetraborate (Na₂B₄O₇ · 10H₂O).

  • Obtained by heating borates or reducing boric acid (H₃BO₃).

Reactions of Boron

  • Boric acid can be reduced with magnesium at high temperatures:

    • Na₂B₄O₇ + 2HCl + 5H₂O → 2NaCl + 4H₃BO₃

  • Boric acid decomposes to produce boron oxide:

    • 12H₃BO₃ → B₂O₃ + 3H₂O

  • Boron oxide can be reduced to yield pure (amorphous) boron:

    • B₂O₃ + 3Mg → 2B + 3MgO

Forms of Boron

  • Amorphous boron is brown in color and has a large surface area.

  • Crystaline boron is obtained through thermal decomposition.

Lattice Energy Concept

  • Lattice energy is the energy released when an ionic solid forms from gaseous ions.

    • Important for understanding stability in ionic compounds.

Boron Compounds and Reactivity

  • Amorphous boron reacts with O₂, N₂, and halogens to form various oxides, nitrides, and halides:

    • E.g. 2B + O₂ → 2B₂O₃

    • E.g. B + X₂ → BX₃ (halides)

Halides of Boron

  • Boron halides (BX₃) exhibit covalent bonding and planar structures to minimize repulsion.

  • They are classified as Lewis acids due to their empty orbitals that can accept electron pairs.

Hydrolysis of Boron Halides

  • Hydrolysis of BX₃ (except BF₃) produces boric acid and HX:

    • E.g. BCl₃ + 3H₂O → H₃BO₃ + 3HCl

Group 4 Elements Characteristics

  • Elements include C, Si, Ge, Sn, and Pb.

  • Common oxidation states: +2 and +4, with +2 being more stable down the group.

  • Trends observed: increasing atomic radius, decreasing ionization energy, and decreasing melting points.

Reactivity Trends of Group 4 Elements

  • Reactivity generally increases down the group, with Si being more reactive than the other elements.

  • Reactions with acids and water vary, with Pb being more reactive due to protective oxide formation.

Carbon Allotropes

  • Carbon exhibits two principal allotropes: diamond and graphite.

  • Diamond: High melting point, hard, uses in industrial applications, tetrahedral structure.

  • Graphite: Soft, layered structure, good electrical conductor due to free electrons.

Chemical Properties of Carbon

  • Diamond burns in air at high temperatures while graphite burns at lower temperatures.

  • Preparation of carbon disulfide (CS₂) from natural gas and sulfur.

Uses of CS₂

  1. Manufacture of viscous artificial silk.

  2. Serves as a solvent for various applications.

Urea

  • Widely used as an inorganic fertilizer.

  • Preparation involves the reaction of CS₂ with ammonia under high temperature and pressure.

Carbides

  • Binary compounds of carbon with more electropositive elements.

  • Classification includes:

    • Ionic carbides (e.g. CaC₂)

    • Covalent carbides (e.g. SiC)

Silicon Basics

  • Silicon differs from carbon due to its larger atomic size leading to weaker bonds in silicates.

  • Occurs mainly as silica (SiO₂) and is obtained by reduction from silicon dioxide.

Silicates

  • Basic unit: tetrahedral SiO₄.

  • Found in various minerals, contributing to the earth's crust.

Nitrogen Properties

  • Colorless, odorless, and the most abundant gas in the atmosphere (around 78%).

  • Preparation typically by the fractional distillation of liquefied air.

  • Reacts with hydrogen and oxygen to form ammonia and nitric oxide, respectively.

Uses of Nitrogen

  • Fertilizers, explosives, inert gas for reactions, and as a refrigerant.