CHEM1910 Lecture notes

• Covalent hydrides


  Binary hydrides formed with beryllium and p-block elements

  
• Bond polarity


  Difference in bond polarity in these compounds results in a range of reaction types and hydrogen can be classified as H+, H-, or H.
• Classification of covalent hydrides

  • Electron-deficient: not possible to describe the bonding of the structure in terms of 2-centre 2-electron bonds (e.g. BeH2 and group 13 hydrides)
  • Electron-precise: all the valence electrons of the central atom are involved in forming bonds (group 14)
  • Electron-rich: Not all the electrons on the central atom participate in bonding (groups 15-17)

  
• The s-Block Elements – What we Will Discuss
• Topics to be discussed

  • General Periodic trends – atomic radii, ionization energies, melting points, redox potential, etc.
  • The preparation and uses, and structures of the s-block elements.
  • The synthesis and reactions of the s-block elements with oxygen, water, halogens, nitrogen, sulphur, and Brønsted acids.
  • Why s-block compounds are predominantly ionic and explain the exceptions.
  • Trends in reactivity and thermal stability of selected s-block compounds using enthalpy cycles.
  • The changes in solubility of s-block compounds and explain them using the lattice Gibbs energies and hydration Gibbs energies.
  • Justification of why the chemistry of lithium and beryllium differ from those of the other groups of elements.
  • the diagonal relationship between lithium and magnesium

  
• Types of covalent hydrides

  • Numerous covalent hydrides of boron, e.g. B2H6, B4H10 and B5H11
  • The three-dimensional polymeric covalent hydride of BeH2
  • Neutral, binary XH4 compounds of group 14. e.g. CH4
  • Somewhat basic binary XH3 compounds of group 15, e.g. NH3 and PH3
  • Weakly acidic or amphoteric binary XH2 compounds of group 16, e.g. H2O and H2S
  • Strongly acidic, binary HX compounds of group 17, e.g. HF and HI
  • Hydridic, complex compounds of hydrogen, e.g. LiAlH4 and NaBH4

  
• 1. Block


  The collective name for elements found in Group 1 and Group 2 of the periodic table

  
• Valence shell configuration


  ns1 and ns2
• Beryllium hydride (BeH2)


  • Covalent molecule with a network structure
  • Hydrogen atoms bridge between beryllium atoms forming 3-centre 2-electron bonds
  • Relatively stable in water - evidence of covalency
  • Be2+ is a small cation with high charge density, affecting its ability to form a lattice
  • Be has high electronegativity compared to other s-block elements
  • Be2+ is strongly polarizing and draws electron pair towards itself

  
• Electron-deficient hydride


  Not possible to describe the bonding of the structure in terms of 2-centre 2-electron bonds

  
• Boron hydride (BH3)


  • Sp2 hybridised with an empty p-orbital
  • Only observed in the gas phase
  • Readily dimerises to form B2H6

  
• Diborane (B2H6)


  • Boron is sp3 hybridized
  • Forms two 2-centre 2-electron B-H bonds
  • Also forms three-centre 2-electron hydrogen bridge bonds

  
• Diborane combusts in air to produce boric acid
• Diborane hydrolyses in water to produce boronic acid and hydrogen

  
• Abundant elements in the Earth's crust

  • Sodium
  • Magnesium
  • Potassium
  • Calcium
• All s-block elements are metals
• Boron forms many other boranes including decaborane which is produced when B2H6 is heated to 100°C

  
• Oxidation states


  +1 for group 1 compounds, +2 for group 2 compounds
• CHEM1910 Lecture VI – Chemistry of the S-Block Elements
• Essential elements for life

  • Sodium
  • Magnesium
  • Potassium
  • Calcium
• Hydrides of other group 13 elements are not as stable as the boron hydrides
• Alane (AlH3)


  • Polymer with bridging hydrogens
  • Decomposes above 150°C

  
• Lecturer: Dr Nickeisha Stephenson
• Group 1


  The alkali metals
• Ga2H6 is similar in structure to B2H6, and is unstable at room temperature

  
• Indium and thallium hydrides are only stable at low temperatures
• Precise hydrides


  Group 14 hydrides where all the valence electrons of the central atom are involved in forming bonds

  
• Email: [e-mail], [e-mail] (if sending attachments)
• Office Hours: Book with Calendly
• What we Will Discuss

  • General Periodic trends – atomic radii, ionization energies, melting points, redox potential, etc.
  • The preparation and uses, and structures of the s-block elements.
  • The synthesis and reactions of the s-block elements with oxygen, water, halogens, nitrogen, sulphur, and Brønsted acids.
  • Why s-block compounds are predominantly ionic and explain the exceptions.
  • Trends in reactivity and thermal stability of selected s-block compounds using enthalpy cycles.
  • The changes in solubility of s-block compounds and explain them using the lattice Gibbs energies and hydration Gibbs energies.
  • Justification of why the chemistry of lithium and beryllium differ from those of the other groups of elements.
  • the diagonal relationship between lithium and magnesium

  
• Group 2


  The alkali earth metals
• Carbon forms an unlimited amount of hydrides (hydrocarbons)
• Metallic elements


  • Have closed-packed structures
  • Group 1 elements adopt a body-centred cubic (bcc) geometry
  • Group 2 elements except barium and radium adopt a hexagonal-closed packed (hcp) geometry
  • Barium and radium are bcc
• Other group 14 elements form stable tetrahydrides - SiH4, GeH4, and SnH4. Except plumbane (PbH4) which is unstable.

  
• Group 14 hydrides become less stable as the central atom becomes heavier
• Reaction with Oxygen


  1. 4Li (s) + O2 (g) → 2Li2O (s)

  2. 2Na (s) + O2 (g) → Na2O2 (s)

  3. K (s) + O2 (g) → KO2 (s)

  
• Properties of metals


  • High and uniform thermal and electrical conductivity
  • Malleable and ductile
  • Lustrous
  • Typically have high melting points
• All group 1 metals burn in air to form oxides
• Metallic bonding


  • Nondirectional covalent bonding
  • Electrons are shared among many atoms and are equally spread throughout the lattice
• Diborane


  • Gas at room temperature
  • Combusts in air to produce boric acid
  • Hydrolyses in water to produce boronic acid and hydrogen