The Ionic Lattice & Enthalpy
Lattice Enthalpy, Coulomb's Law, and Physical Properties.
1. The Giant Ionic Lattice
Ionic compounds form a crystal lattice—a regular, repeating 3D arrangement of ions.
Coordination Number
The number of nearest neighbors of opposite charge surrounding an ion.
- NaCl: 6:6 (Octahedral geometry around each atom).
- CsCl: 8:8 (Cubic geometry).
2. Lattice Enthalpy (\(H_{latt}\))
Definition:
The enthalpy change when one mole of a solid ionic compound is separated into its gaseous ions under standard conditions (Endothermic).
3. Trends (Coulomb's Law)
To explain trends in Melting Point and Lattice Enthalpy, use Coulomb's Law:
Factor A: Charge (Dominant)
Doubling the charge quadruples the force.
- Compare: MgO (+2/−2) vs NaCl (+1/−1)
- Product: MgO = 4, NaCl = 1
- Result: MgO (\(2850°C\)) ≫ NaCl (\(801°C\))
Factor B: Radius (Secondary)
Smaller ions pack closer (\(r\) decreases).
- Compare: LiF vs KBr
- Radii: Li⁺ & F⁻ < K⁺ & Br⁻
- Result: LiF has higher Lattice Enthalpy.
Paper 2 Strategy: Comparing Enthalpies
Problem: Compare the lattice enthalpies of CaO and KCl. Justify your answer.
1. Identify Charges:
- CaO: \( Ca^{2+}, O^{2-} \) (Product = 4)
- KCl: \( K^+, Cl^- \) (Product = 1)
2. Identify Radii:
\( Ca^{2+} \) is smaller than \( K^+ \).
Examiner's Markscheme:
- CaO has a higher lattice enthalpy. [1]
- Reason (Charge): The charge product of CaO (+2/−2) is greater. [1]
- Reason (Radius): \( Ca^{2+} \) is smaller, leading to stronger attraction. [1]
4. Physical Properties
| Property | Observation | Explanation |
|---|---|---|
| Melting Point | High | Large energy required to overcome strong electrostatic forces. |
| Conductivity (Solid) | No | Ions are held in fixed positions. |
| Conductivity (Molten) | Yes | Lattice dissociates; ions are mobile charge carriers. |
| Brittleness | High | Stress causes layers to slide → like charges align → repulsion shatters crystal. |
Chemistry in Action: Road Salting
Why use Calcium Chloride (\(CaCl_2\)) instead of Salt (\(NaCl\)) in extreme cold?
1. Exothermic Dissolution
\(CaCl_2\) has very high lattice enthalpy, but even higher hydration enthalpy. Dissolving releases heat, melting ice.
2. Van 't Hoff Factor
\(CaCl_2\) dissociates into 3 ions vs 2 for NaCl. Greater freezing point depression.
Check Your Understanding
Q1. Which compound has the highest Lattice Enthalpy?
Q2. Why does solid KBr NOT conduct electricity?