Enthalpy Change Definitions 🌡️

Introduction

• Presenter: Chris Harris from Alerts.com
• Topic: Enthalpy changes for the Born-Haber cycle in ionic compounds
• Focus: Different types of enthalpy changes, their definitions, examples, and whether they are endothermic or exothermic

Types of Enthalpy Changes

1. Enthalpy of Formation

   • Definition: When 1 mole of a compound is formed from its elements in their standard states under standard conditions
   • Example: Formation of water from H₂ and O₂
   • Characteristic: Exothermic (heat energy is released)

2. Enthalpy of Atomization

   • Definition: When 1 mole of gaseous atoms is formed from an element in its standard state
   • Example: Na(solid) -> Na(gas); 1/2 Cl₂(gas) -> Cl(gas)
   • Characteristic: Endothermic (energy is absorbed)
   • Note: Some exam boards refer to it as bond enthalpy

3. First Ionization Energy

   • Definition: When 1 mole of gaseous ions is formed from 1 mole of gaseous atoms (removing one electron)
   • Example: Na(gas) -> Na⁺(gas) + e⁻
   • Characteristic: Endothermic (energy is absorbed)

4. Second Ionization Energy

   • Definition: Energy change when one mole of gaseous 2+ ions is formed from one mole of gaseous 1+ ions
   • Example: Ca⁺(gas) -> Ca²⁺(gas) + e⁻
   • Characteristic: Endothermic (energy is absorbed)

5. Electron Affinity

   • Definition: When 1 mole of gaseous 1- ions is formed from 1 mole of gaseous atoms (adding an electron)
   • Example: Cl(gas) + e⁻ -> Cl⁻(gas)
   • Characteristic: Exothermic (energy is released)
   • Second Electron Affinity: Formation of gaseous 2- ions from gaseous 1- ions (e.g., O⁻(gas) + e⁻ -> O²⁻(gas)), which is endothermic due to repulsion forces

6. Lattice Enthalpy of Formation

   • Definition: When 1 mole of an ionic compound is formed from its gaseous ions
   • Example: Na⁺(gas) + Cl⁻(gas) -> NaCl(solid)
   • Characteristic: Exothermic (energy is released)

7. Enthalpy of Hydration

   • Definition: When 1 mole of aqueous ions is formed from 1 mole of gaseous ions
   • Example: Na⁺(gas) -> Na⁺(aq)
   • Characteristic: Exothermic (energy is released due to bond formation)

8. Enthalpy of Solution

   • Definition: When 1 mole of a solute dissolves in a solvent with no further enthalpy change upon further dilution
   • Example: KCl(solid) -> KCl(aqueous)
   • Characteristic: Can be exothermic or endothermic
     • NaOH in water: Exothermic
     • KCl in water: Slightly Endothermic

9. Lattice Dissociation Enthalpy

   • Definition: When 1 mole of an ionic compound separates into its gaseous ions
   • Example: NaCl(solid) -> Na⁺(gas) + Cl⁻(gas)
   • Characteristic: Endothermic (energy is absorbed)

Conclusion

• Importance of remembering the definitions and state symbols
• Application in Born-Haber cycles (additional resources available for further learning)

Hope this helps! Bye!