Lecture Notes: OCRA Lattice Enthalpy

Introduction

• Presenter: Chris Harris from Allery Chemistry
• Focus: Lattice enthalpy for OCRA
• Resources: Videos and slides available on YouTube channel (Alloy Chemistry) for free

Key Definitions

Enthalpy Change of Formation

• Definition: Enthalpy change when one mole of compound is formed from its elements in standard states under standard conditions.
• Notation: Delta FH (ΔHᶥ)

Lattice Enthalpy of Formation

• Definition: Enthalpy change when one mole of solid ionic compound is formed from its gaseous ions under standard conditions.
• Notation: ΔHᶥ_lattice

Ionization Energy

• Definition: Enthalpy change when one mole of gaseous ions is made from one mole of gaseous atoms (removal of an electron).

Importance of Standard Conditions

• Standard states and conditions are vital in definitions.
• Ensure to include these terms in definitions.

Ionic Bonding

• Strength of ionic bond affected by charge size:
  • Bigger charge = stronger electrostatic attraction.
  • Higher melting/boiling points due to stronger bonds.
• Examples:
  • Sodium Chloride (NaCl) melting point: ~801 °C
  • Potassium Chloride (KCl) melting point: ~770 °C
  • Calcium Oxide (CaO) melting point: ~2572 °C

Charge Density

• Smaller ions with higher charges exhibit higher charge density, leading to stronger attractions and higher melting points.

Born-Haber Cycle

• Useful for calculating lattice enthalpies (cannot be measured directly).
• Methodical approach required.
• Key steps in Born-Haber cycle:
  1. Enthalpy of formation: Lithium and chlorine forming lithium chloride (exothermic).
  2. Atomization: Breaking Cl2 into Cl atoms (endothermic).
  3. Ionization: Removing an electron from lithium (endothermic).
  4. Electron affinity: Adding electron to chlorine (exothermic).
  5. Lattice enthalpy formation: Forming lithium chloride from gaseous ions.

Calculation of Lattice Enthalpy

• Follow the rules of Hess's Law (same signs for arrows, flip the sign when going against arrows).
• Example: Lattice enthalpy for lithium chloride calculated as -846 kJ/mol (exothermic).

Enthalpy Change of Solution

• Definition: Enthalpy change when one mole of ionic substance is dissolved in minimum solvent without further enthalpy change upon dilution.
• Process:
  1. Endothermic: Bonds in ionic lattice break (lattice dissociation).
  2. Exothermic: New bonds form between solute and solvent (hydration).

Factors Affecting Enthalpy Change of Hydration

• Charge: Higher charge attracts water molecules more strongly, thus higher hydration enthalpy.
• Size: Smaller ions have higher charge density, attracting water molecules more effectively.

Conclusion

• Importance of understanding lattice enthalpy for chemistry, especially in ionic compounds.
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