Properties of ionic compounds — AQA GCSE Chemistry
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Properties of ionic compounds explained
Properties of ionic compounds covers 3 specification points, including Ionic compounds have regular structures (giant… and These compounds have high melting points and….
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Ionic compounds have regular structures (giant ionic lattices) in which there are strong electrostatic forces of attraction in all directions between oppositely charged ions.
Ionic compounds form when oppositely charged ions pack together in a regular three-dimensional arrangement called a giant ionic lattice. Each ion is surrounded by ions of opposite charge, and the electrostatic forces of attraction act in all directions, not just between one pair. For example, in sodium chloride, each Na⁺ is surrounded by six Cl⁻ ions and each Cl⁻ by six Na⁺ ions. The lattice is described as giant because it extends throughout the crystal. These strong forces hold the structure together, which explains the high melting and boiling points. The regularity also means ionic compounds often form crystals with flat faces.
These compounds have high melting points and high boiling points because of the large amounts of energy needed to break the many strong bonds.
Ionic compounds have high melting and boiling points because melting or boiling requires breaking the strong electrostatic forces of attraction between ions throughout the giant lattice. These forces are strong and act in all directions, so a large amount of energy is needed to overcome them. For example, sodium chloride melts at about 801 °C and boils at about 1413 °C. The energy is needed to separate the ions from their fixed positions, not to break individual covalent bonds. The strength of the forces increases with the charge on the ions, so compounds such as magnesium oxide have even higher melting points than sodium chloride.
When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and so charge can flow.
Solid ionic compounds do not conduct electricity because the ions are fixed in the lattice and cannot move. When melted or dissolved in water, the lattice breaks down and the ions become free to move. These mobile ions carry charge through the liquid or solution, so the compound conducts electricity. For example, molten sodium chloride conducts electricity, and a solution of sodium chloride in water also conducts. The ions themselves move towards the oppositely charged electrodes, completing the circuit. This explains why ionic compounds are electrolytes when molten or aqueous but insulators when solid.
Your focus
- Describe the arrangement of ions in a giant ionic lattice.
- Explain how electrostatic forces of attraction operate in all directions.
- Relate the regular lattice structure to the physical appearance of ionic crystals.
Show all 9 objectives
- Explain why ionic compounds have high melting and boiling points.
- Describe the energy changes involved in breaking a giant ionic lattice.
- Compare the melting points of ionic compounds with different ionic charges.
- Explain why solid ionic compounds do not conduct electricity.
- Describe how melting or dissolving allows ionic compounds to conduct electricity.
- Identify the charge carriers in molten and aqueous ionic compounds.
Properties of ionic compounds exam tips
Marking Points
- Describes the structure as a giant ionic lattice with a regular arrangement of alternating positive and negative ions.
- States that electrostatic forces of attraction act in all directions between oppositely charged ions.
- Explains that the forces are strong because of the high charges and close packing of ions.
- Uses a named example such as sodium chloride to illustrate the lattice arrangement.
- Links the regular structure to the crystalline nature of ionic compounds.
- States that ionic compounds have high melting and boiling points.
- Explains that a large amount of energy is needed to overcome the strong electrostatic forces of attraction between ions.
- Refers to the many strong forces throughout the giant lattice, not just a few bonds.
- Links the energy required to the strength and number of electrostatic forces in the lattice.
- Uses a named example, such as sodium chloride or magnesium oxide, to illustrate the high melting point.
- States that solid ionic compounds do not conduct electricity because ions are not free to move.
- Explains that when melted or dissolved in water, the lattice breaks down and ions become mobile.
- Describes how mobile ions carry charge through the liquid or solution, allowing conduction.
- Uses a named example, such as molten sodium chloride or sodium chloride solution, to illustrate conduction.
- Links the ability to conduct to the presence of free-moving charged particles.
Examiner Tips
- 💡Use the phrase 'electrostatic forces of attraction in all directions' when describing ionic lattices.
- 💡Refer to a specific ionic compound to support your description.
- 💡If asked to explain properties, always link back to the strength and direction of the forces.
- 💡Use the term 'electrostatic forces of attraction' rather than 'bonds' to avoid confusion with covalent bonding.
- 💡Quantify the energy needed as 'a large amount' and link it to the strength of the forces.
- 💡Compare with a simple molecular substance to highlight the difference in melting point.
- 💡Always mention that ions must be free to move for conduction to occur.
- 💡Use the term 'electrolyte' for a molten or aqueous ionic compound that conducts electricity.
- 💡If asked to explain, refer to the movement of ions towards electrodes.
Common Mistakes
- Describing ionic bonding as sharing electrons; the correction is that ionic bonding involves transfer of electrons to form ions and electrostatic attraction between them.
- Saying forces act only between one pair of ions; the correction is that forces act in all directions throughout the lattice.
- Confusing a giant ionic lattice with a simple molecule; the correction is that it is a continuous three-dimensional structure, not discrete molecules.
- Saying that covalent bonds are broken when an ionic compound melts; the correction is that electrostatic forces between ions are overcome.
- Stating that only a few bonds need to be broken; the correction is that the whole lattice is held by many strong forces in all directions.
- Confusing melting with dissolving; the correction is that melting involves breaking the lattice, while dissolving separates ions by interaction with water molecules.
- Saying that electrons carry charge in ionic solutions; the correction is that ions are the charge carriers.
- Believing that solid ionic compounds conduct because they contain ions; the correction is that the ions must be free to move.
- Confusing melting with dissolving; the correction is that both processes free the ions, but melting produces a liquid and dissolving produces an aqueous solution.