5.3.2 Energy Changes and Intermolecular Forces
ChemistryChapter 5: States and Phases of Matter
5.3.2 Energy Changes and Intermolecular Forces
- As a result of melting of a solid, a small change
- As a result of melting of a solid, a small change in intermolecular distance and potential energy takes place in atoms, molecules, or ions.
- On the other hand, on evaporation of a liquid, atoms, molecules, or ions undergo large changes in their intermolecular distance and potential energy.
- Therefore, heat of vaporization is much greater than heat of fusion.
Comparison Table
| Process | ΔH (kJ mol⁻¹) | Change in Intermolecular Distance |
|---|
| Fusion (melting) | +6.02 | Small |
| Vaporization | +40.7 | Large |
Key Points
- Enthalpy change (ΔH) is the heat change at constant pressure.
- Molar heat of fusion: Solid → Liquid.
- Molar heat of vaporization: Liquid → Gas.
- Temperature remains constant during phase change.
- Heat of vaporization > Heat of fusion.
Definitions
| Term | Definition | Example |
|---|
| Molar Heat of Fusion | Heat to melt 1 mole of solid | Ice: +6.02 kJ/mol |
| Molar Heat of Vaporization | Heat to vaporize 1 mole of liquid | Water: +40.7 kJ/mol |
Energy Changes and Intermolecular Forces
- Melting: Small change in intermolecular distance → Small energy change.
- Vaporization: Large change in intermolecular distance → Large energy change.
- Therefore, ΔHvap > ΔHfus.
Key Points
- Enthalpy change (ΔH) is the heat change at constant pressure.
- Temperature remains constant during phase change.
- Heat of vaporization > Heat of fusion.
Important Points
- 1. Enthalpy change (ΔH): Heat change at constant pressure.
- 2. Molar heat of fusion: Solid → Liquid.
- 3. Molar heat of vaporization: Liquid → Gas.
- 4. ΔHfus for ice = +6.02 kJ/mol.
- 5. ΔHvap for water = +40.7 kJ/mol.
- 6. ΔHvap > ΔHfus because evaporation causes larger changes in intermolecular distance.
- 7. Temperature remains constant during phase change.
Remember
- - ΔH_fus: Solid → Liquid
- - ΔH_vap: Liquid → Gas
- - ΔH_fus (ice): +6.02 kJ/mol
- - ΔH_vap (water): +40.7 kJ/mol
- - ΔH_vap > ΔH_fus
- - Temperature constant during phase change
Common Mistakes
- 1. Thinking temperature changes during phase change: It remains constant.
- 2. Confusing ΔH_fus and ΔH_vap: Fusion = melting; Vaporization = boiling.
- 3. Forgetting the units: kJ mol⁻¹.
- 4. Thinking ΔH_vap is smaller than ΔH_fus: It is larger.
Quick Revision
- Enthalpy change (ΔH): Heat change at constant pressure.
- Molar heat of fusion: Solid → Liquid.
- Molar heat of vaporization: Liquid → Gas.
- ΔHfus for ice = +6.02 kJ/mol.
- ΔHvap for water = +40.7 kJ/mol.
- ΔHvap > ΔHfus because evaporation causes larger changes in intermolecular distance.
- Temperature remains constant during phase change.
60-Second Revision
- ΔH_fus: Solid → Liquid
- ΔH_vap: Liquid → Gas
- ΔH_fus (ice): +6.02 kJ/mol
- ΔH_vap (water): +40.7 kJ/mol
- ΔH_vap > ΔH_fus
- Temperature constant during phase change
QUALITY VALIDATION
- Topic: 5.3 Energetics of Phase Changes
- Chapter: 5 (States and Phases of Matter)
- Source re-read: PASS
- Source scope followed: PASS
- Knowledge level matched: PASS
- Important concepts covered: PASS
- Definitions checked: PASS
- Equations checked: PASS
- Derivations checked: N/A
- Examples checked: PASS
- Unsupported knowledge added: NO
- Unnecessary repetition: NO
- Notes format maintained: PASS
- Important source content omitted: NO
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Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.- ΔH_fus (ice): +6.02 kJ/mol
- ΔH_vap (water): +40.7 kJ/mol
- Temperature constant during phase change
