Chem Bonding IIb

• Lattice


  A regular, repeating arrangement of particles in a region of space
• Crystalline solids


  • Atoms, molecules or ions are highly ordered in a regular 3-dimensional arrangement that repeat over and over again in all directions
• Examples of crystalline solids

  • sodium chloride, diamond, iodine, ice and copper

  
• Types of lattices of crystalline solids

  • Giant ionic lattice (e.g. sodium chloride)
  • Giant molecular lattice (e.g. diamond, graphite and silicon dioxide)
  • Simple molecular lattice (e.g. iodine and ice)
  • Giant metallic lattice (e.g. copper)
• The physical properties of a compound are a consequence of its structure and bonding
• If the melting point is high


  It reflects strong bonding between the particles forming the lattice
• A low melting point


  Suggests weak bonding between the particles
• Conduction of electricity


  Indicates presence of mobile charge carriers such as delocalised electrons or mobile ions
• Comparison of the four different lattice structures

  • Giant metallic lattice
  • Giant ionic lattice
  • Giant molecular lattice
  • Simple molecular lattice
• Giant ionic lattice


  An ionic compound has a giant ionic lattice structure. The ions are arranged in a lattice such that there is maximum electrostatic forces of attraction between the oppositely charged ions (e.g., Na+ and Cl−) and minimum repulsion between similarly charged ions (e.g., Na+ and Na+, or Cl− and Cl−)
• Lattice energy


  The energy released when 1 mole of solid ionic compound is formed from its free gaseous ions (∆Hlatt is negative)
• Lattice energy = q+ q- / r+ + r-
• Factors affecting strength of ionic bonds

  • Higher ionic charge (greater electrostatic attraction)
  • Smaller radius/ size (shorter distance between ions results in greater attraction)
• The greater the magnitude of the lattice energy


  The stronger the ionic bond
• MgO and Al2O3 are used as refractories (solid that can resist high temperatures without melting or decomposing)
• Explain why the melting point of MgO is higher than that of CaO

  1. Both have giant ionic lattice structure

  2. Mg2+ has smaller ionic radius than Ca2+

  3. Magnitude of lattice energy of MgO is larger than CaO

  4. More energy required to overcome strong electrostatic forces in MgO

  5. Therefore, MgO has higher melting point
• MgO has the largest magnitude of lattice energy
• Hardness of ionic compounds

  Ionic bonds are strong and non-directional
• Brittleness of ionic compounds

  Ordered arrangement of oppositely charged ions in the crystal lattice. When stress is applied, ions of the same charge are brought close together. Repulsion between like charges occur, which results in fracture (cleavage) of the crystal lattice
• Solubility of ionic compounds

  When a solute dissolves in a solvent, the energy released from forming solute-solvent interactions must be sufficient to compensate for the energy needed to break up the attractive forces between solvent particles as well as those between the solute particles
• Solubility of ionic compounds in polar solvents (e.g. water)


  The giant ionic lattice is broken up. The cations and anions form ion-dipole attractions with the solvent molecules (water). Energy is released as a result of these interactions

  
• Ion-dipole attractions


  Interactions between an ion (e.g. Na+ or Cl−) and the partial charge on the end of a polar molecule (e.g. H2O)

  
• The magnitude of the ion-dipole attraction increases with the charge density of the ion and the size of partial charge of the polar solvent
• Not all ionic compounds are soluble in water. Lattice energy in such compounds is so exothermic (compared to ion-dipole attractions) that the hydration energy of the ions is insufficient to overcome the strong ionic bonds holding the ions in their lattice (e.g. MgO, PbCl2)
• Solubility of ionic compounds in non-polar / organic solvents

  The energy released from forming ion-solvent attractions is insufficient to compensate for the energy required to break up the relatively stronger ionic bonds in the lattice
• Polar solutes dissolve in polar solvents (like H2O) while non-polar solutes dissolve in non-polar solvents (like organic solvents e.g. hexane, benzene)
• Electrical conductivity of ionic compounds

  • Solid state: Ions are held in fixed positions, unable to move and conduct electricity
  • Molten state or in aqueous solution: Ions are free to move and can conduct electricity
• Not all ionic compounds are soluble in water
• Lattice energy


  Energy released when ions are arranged in a crystal lattice
• Lattice energy in ionic compounds can be so exothermic (compared to ion-dipole attractions) that the hydration energy of the ions is insufficient to overcome the strong ionic bonds holding the ions in their lattice (e.g. MgO, PbCl2)
• Ionic compounds do not dissolve in non-polar / organic solvents (e.g. hexane)
• General rule: "Like dissolves like"

  Polar solutes dissolve in polar solvents (like H2O), non-polar solutes dissolve in non-polar solvents (like organic solvents e.g. hexane, benzene)
• Ionic compounds conduct electricity when molten or in aqueous solution, but not in solid state
• Solid state


  Ions are held in fixed positions by strong ionic bonds in the lattice and are not mobile
• Molten or aqueous solution


  Ions can move freely, act as mobile charge carriers and can thus conduct electricity
• Ion-dipole attraction


  1. Label partial charges on water

  2. + H atom of water attracted to anion, - O atom of water attracted to cation

  3. Draw and label ion-dipole attractions

  
• Metallic bond is the electrostatic attraction between a lattice of positive ions and delocalised electrons
• Giant metallic lattice


  • Consists of a lattice of metal cations in a 'sea' of delocalised electrons, held together by strong metallic bonding
  • Metal cations are packed closely together (gaps between cations are kept to a minimum)
• Copper (Cu)


  • Solid Cu has a giant metallic structure, which consists of a lattice of copper cations surrounded by a sea of delocalised electrons with strong metallic bonds holding them together
• High melting points and boiling points of metals


  Large amount of energy needed to overcome the strong electrostatic forces of attraction between the positively charged cations and the mobile 'sea' of delocalised electrons (metallic bond) in the metallic lattice