1.4.7Physical Chemistry
Properties of Ionic Compounds — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 23 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Melting and Boiling
High melting points
- High melting points Large amounts of energy are needed to overcome the many strong electrostatic attractions between ions throughout the giant lattice.
Charge effect
- Charge effect Ions with a higher charge attract more strongly,
- so magnesium oxide, made of 2+ and 2− ions, melts at a much higher temperature than sodium chloride.
Size effect
- Size effect Smaller ions can approach more closely, which gives stronger attraction and a higher melting point when charges are the same.
Solid at room temperature
- Solid at room temperature Because the lattice is so strongly held, nearly all ionic compounds are solids with very low volatility under normal conditions.
Part 2 of 4Electrical and Solubility Behaviour
Electrical and Solubility Behaviour
Solid
- Solid An ionic solid does not conduct electricity, since the ions are fixed in position and there are no mobile charged particles.
Molten
- Molten When melted, the ions can move and carry charge, so the liquid conducts electricity.
Aqueous
- Aqueous When dissolved in water the ions are free to move through the solution, and so the solution also conducts electricity.
Solubility
- Solubility Many ionic compounds dissolve in water because polar water molecules attract the ions and pull them out of the lattice.
| State | Conducts? | Reason |
|---|---|---|
| Solid | No | Ions fixed in the lattice |
| Molten | Yes | Ions free to move |
| Dissolved | Yes | Ions free to move |
Part 3 of 4Brittleness
1
When a force is applied, one layer of ions can be displaced slightly relative to the next layer.
2
This brings ions of like charge next to each other, so positive ions face positive ions and negative ions face negative.
3
The repulsion between these like charges pushes the layers apart with great force.
4
The crystal therefore splits along a plane, which makes ionic compounds hard but brittle rather than malleable.
Part 4 of 4Exam Checklist & Tips
1.4.7 – Key Points for the Exam
- Brittle Breaks or shatters easily when struck instead of bending.
- Conductor A substance that allows charge to flow through it by means of mobile charged particles.
- Charge density
- The charge of an ion in relation to its size;
- higher density gives stronger attraction.
- The charge of an ion in relation to its size;
- Polar solvent A solvent whose molecules have a permanent dipole, such as water.
Common Mistakes
- Saying that ionic compounds contain free electrons which conduct;
- it is the movement of ions that carries current.
- Claiming a solid ionic compound conducts because it contains ions; the ions must be free to move.
- Stating that covalent bonds are broken when sodium chloride melts; it is ionic bonds that are overcome.
- Explaining solubility without mentioning the attraction between water molecules and ions.
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