2.1.3Additional lessons
Ionic Lattices — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 24 of 172
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.
Section 1 of 5Structure of the Lattice
Ionic solids exist as giant lattices rather than small molecules.
Regular array
- Regular array Ions are arranged in a repeating three-dimensional pattern held together by attractions in every direction.
Sodium chloride
- Sodium chloride Each Na⁺ is surrounded by six Cl⁻ ions and each Cl⁻ by six Na⁺ ions.
Alternating charges
- Alternating charges Oppositely charged ions sit next to each other so attraction outweighs repulsion overall.
Empirical formula
- Empirical formula A lattice has no separate molecules, so the formula gives only the ratio of ions.
Section 2 of 5Physical Properties
Physical Properties
High melting point
- High melting point Many strong electrostatic attractions must be overcome, so a lot of energy is needed.
Brittle
- Brittle Shifting a layer brings like charges together, and the repulsion splits the crystal.
Solubility
- Solubility Many dissolve in water because polar water molecules attract and surround the ions.
Electrical conduction
- Electrical conduction Solids do not conduct as ions are fixed, but molten and aqueous ionic compounds do.
| State | Conducts? | Reason |
|---|---|---|
| Solid | No | Ions fixed in position |
| Molten | Yes | Ions free to move |
| Aqueous | Yes | Ions free to move |
Section 3 of 5Lattice Enthalpy
1
The lattice enthalpy is the energy needed to separate one mole of a solid ionic compound into its gaseous ions.
2
It is always endothermic, and a larger value means stronger ionic bonding.
3
Higher ion charges increase lattice enthalpy because the attraction between ions is greater.
4
Smaller ionic radii increase it because the ions can approach more closely.
5
This is why magnesium oxide, made of 2+ and 2− ions, melts at a far higher temperature than sodium chloride.
Section 4 of 5Comparing Compounds
Comparing Compounds
NaCl versus MgO
- NaCl versus MgO MgO has doubly charged ions, giving a much larger lattice enthalpy and melting point than NaCl.
NaCl versus KCl
- NaCl versus KCl K⁺ is larger than Na⁺, so KCl has the weaker attraction and a lower value.
LiF versus LiI
- LiF versus LiI Fluoride is much smaller than iodide, so LiF has the larger lattice enthalpy.
Section 5 of 5Exam Checklist & Tips
2.1.3 – Key Points for the Exam
- Ionic lattice A giant regular arrangement of oppositely charged ions.
- Lattice enthalpy The enthalpy change when one mole of solid ionic compound is separated into gaseous ions.
- Brittle Shatters when force displaces layers of ions.
- Coordination number The number of oppositely charged ions that surround one ion.
Common Mistakes
- Saying ionic compounds contain molecules or that NaCl 'molecules' melt.
- Claiming ionic solids conduct because electrons move in them.
- Explaining high melting point by strong covalent bonds.
- Forgetting that both charge and radius affect lattice enthalpy.
Last updated:
\n