IB Chemistry SL

Revision Notes

4.1.2 Ionic Compounds

Ionic Lattices

  • The ions form a lattice structure which 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

 

Chemical Bonding Ionic Lattice, downloadable AS & A Level Chemistry revision notes

Ionic solids are arranged in lattice structures

Properties of ionic compounds

  • 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 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 charge density of the ions due to the greater electrostatic attraction of charges
    • Mg2+O2- has a higher melting point than Na+Cl
  • 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
    • In the solid state they’re in a fixed position and unable to move around

Characteristics of giant ionic lattices compared to other structures

States of Matter Table_4, downloadable AS & A Level Chemistry revision notes

Worked Example

The table below shows the physical properties of substances X, Y and Z

Worked Example - Bonding and Structure, downloadable IB Chemistry revision notes

Which one of the following statements about X and Y is completely true?

Statement 1: X has a giant ionic structure, Y has a giant molecular structure, Z is a metal

Statement 2: X is a metal, Y has a simple molecular structure, Z has a giant molecular structure

Statement 3: X is a metal, Y has a simple molecular structure, Z has a giant ionic structure

Statement 4: X has a giant ionic structure, Y has a simple molecular structure, Z is a metal

Answer:

The correct answer is Statement 4

    • The relatively high melting point, solubility in water and electrical conductivity when molten suggest that X is an giant ionic structure
    • The low melting point of Y  suggests that little energy is needed to break the lattice which corresponds to a simple molecular structure. This is further supported by the low electrical conductivity and almost insoluble in water
    • Compound Z has a very high melting point which is characteristic of either metallic or giant molecular lattices, however since it conducts electricity, compound Z must be a giant metallic lattice
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