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IBChemistry: SLDPRevision Notes4. Chemical Bonding & Structure4.1 Ionic & Covalent Bonding 4.1.2 Ionic Compounds

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First teaching 2014

Last exams 2024

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Ionic Compounds (DP IB Chemistry: SL)

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Stewart

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Stewart

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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

ball-and-stick-model-of-a-general-ionic-lattice

 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 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+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
    • 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

  Giant ionic Giant metallic Simple covalent Giant covalent
Melting / boiling point High Moderately high to high Low Very high
Electrical conductivity  Only when molten or in solution When solid or liquid Do not conduct electricity Do not conduct electricity (except graphite)
Solubility Soluble Insoluble but some may react Usually insoluble unless they are polar Insoluble
Hardness Hard, brittle Hard, malleable Soft Very hard (diamond and silica) or soft (graphite)
Physical state at room temperature Solid Solid Solid, liquid or gas Solid
Forces Electrostatic attraction between ions Delocalised electrons attracting positive ions Weak intermolecular forces and covalent bonds within a molecule Electrons in covalent bonds between atoms 
Particles Ions Positive ions in a sea of electrons Small molecules Atoms
Examples NaCl Copper  Br2  Graphite, silicon(IV) oxide

Worked example

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

Substance  Melting point (oC) Electrical conductivity when molten Solubility in water
X 839 Good Soluble
Y 95 Very poor Almost insoluble
Z 1389 Good Insoluble

 

Which one of the following statements about X, Y and Z 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:

  • Compound X has a relatively high melting point, is soluble in water and conducts electricity when molten
    • This suggests that X has a giant ionic structure
  • Compound Y has a low melting point which suggests that little energy is needed to break the lattice
    • This suggests that Y is a simple molecular structure
    • This is further supported by its low electrical conductivity and it being almost insoluble in water
  • Compound Z has a very high melting point, which is characteristic of either metallic, giant ionic lattices or giant covalent / molecular lattices
    • However since it is insoluble in water, compound Z must be a metal
  • Therefore, the correct answer is Statement 4

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    Previous:4.1.1 Forming IonsNext:4.1.3 Formulae & Names of Ionic Compounds
    Stewart

    Author: Stewart

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.