Ionic Lattices (HL IB Chemistry)

What is an ionic lattice?

• The ions form a lattice structure, known as an ionic lattice
  • This is an evenly distributed crystalline structure
• Ions in a lattice are arranged in a regular repeating pattern so that positive charges cancel out negative charges
• Therefore, the final lattice is overall electrically neutral

What forces hold together an ionic lattice?

• The ionic lattice consists of alternating cations and anions
  • Cations are positively charged ions and anions are negatively charged ions
• Therefore, there are strong electrostatic forces of attraction between the oppositely charged ions
  • Remember: This is one possible definition of ionic bonding

Giant ionic lattice structure diagram

 Ionic solids are arranged in lattice structures

Lattice Enthalpy

• The lattice dissociation enthalpy (ΔH_latt^ꝋ) is defined as the standard enthalpy change that occurs on the formation of 1 mole of gaseous ions from the solid lattice
• Since this is always an endothermic process, the enthalpy change will always have a positive value
• The ΔH_latt^ꝋ is always endothermic as energy is always required to break any bonds between the ions in the lattice

NaCl (s) → Na⁺ (g) + Cl^– (g)     ΔH_lat^ꝋ = +790 kJ mol ^-1

• Different types of structure and bonding have different effects on the physical properties of substances such as their melting and boiling points, electrical conductivity and solubility

Ionic bonding & giant ionic lattice structures

• Ionic compounds are strong
  • The strong electrostatic forces in ionic compounds keep the ions held strongly together
• They are brittle as ionic crystals can split apart
• Ionic compounds have high melting and boiling points
  • The strong electrostatic forces between the ions in the lattice act in all directions and keep them strongly together
  • Melting and boiling points increase with the charge density of the ions due to the greater electrostatic attraction of charges
  • Mg²⁺O^2– has a higher melting point than Na⁺Cl^–
• Ionic compounds are not volatile
  • Volatility refers to the vapourisation of a chemical
  • Large amounts of energy are required to overcome the strong electrostatic forces of attraction, which means that ionic compounds are not volatile
• Ionic compounds are soluble in water as they can form ion-dipole bonds
• Ionic compounds only conduct electricity when molten or in solution
  • When molten or in solution, the ions can freely move around and conduct electricity
  • As a solid, the ions are in a fixed position and unable to move around

Table comparing the characteristics of giant ionic lattices with other structure types