1.4.4Physical Chemistry
Physical Properties of Ionic Compounds — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 13 of 42
Part 1 of 4Melting and Boiling Points
Almost every physical property of an ionic compound comes from the lattice of fixed, charged ions.
Strong forces
- Each ion is attracted to many oppositely charged neighbours throughout the lattice,
- so the overall electrostatic attraction is very strong.
High energy needed
- A large amount of energy must be supplied to separate the ions, so ionic compounds have high melting and boiling points.
Solid at room temperature
- Most ionic compounds are hard crystalline solids at room temperature for the same reason.
Bonds not molecules
- Melting does not break covalent bonds; it frees the ions from fixed positions so they can move.
Hardness
- The strong attractions between ions in the lattice also make many ionic solids hard, even though they shatter when struck.
Part 2 of 4Brittleness and Conductivity
Brittleness and Conductivity
Brittle
- A hit can shift a layer of ions so that like charges meet, and the repulsion splits the crystal.
Solid conductivity
- In the solid, ions are fixed in position and cannot carry charge, so solids do not conduct electricity.
Molten conductivity
- When molten, ions are free to move and carry charge through the liquid, so the compound conducts.
Aqueous conductivity
- When dissolved, ions are free to move in solution, so the solution conducts.
Electrolysis link
- Molten or dissolved ionic compounds can be decomposed by passing electricity through them,
- because the mobile ions move to the electrodes.
| State | Ions mobile? | Conducts? |
|---|---|---|
| Solid | No | No |
| Molten | Yes | Yes |
| Aqueous | Yes | Yes |
Part 3 of 4Solubility in Water
1
Water molecules are polar, with δ⁺ hydrogen atoms and a δ⁻ oxygen atom.
2
Water's δ⁻ oxygen atoms attract cations at the surface of the lattice, while δ⁺ hydrogen atoms attract anions.
3
These ion-dipole attractions release energy that helps to overcome the attractions within the lattice.
4
The ions become surrounded by water molecules in a process called hydration, and move into the solution.
5
Ionic compounds are generally insoluble in non-polar solvents, since those molecules cannot attract ions strongly.
Part 4 of 4Exam Checklist & Tips
1.4.4 – Key Points for the Exam
- Brittle Breaks easily when struck because layers of ions shift and like charges repel.
- Hydration Surrounding of ions by water molecules through attraction to their partial charges.
- Mobile ions Charged particles free to move, which are needed to conduct electricity.
- Polar solvent A solvent such as water whose molecules have partial positive and negative charges.
Common Mistakes
- Saying ionic solids conduct because of free electrons; their lack of mobile ions is why they do not.
- Stating that covalent bonds break when an ionic solid melts.
- Explaining solubility only by saying "like dissolves like" without mentioning ion-dipole attraction.
- Claiming that the lattice is easy to break because it is brittle;
- brittleness and melting point are different properties.
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