Overview
This lecture introduces key physical properties of organic compounds—boiling point, melting point, and vapor pressure—explaining how intermolecular forces affect them and outlining a structured approach to answering comparison questions in exams.
Physical Properties of Organic Compounds
- Physical properties include boiling point, melting point, vapor pressure, and density.
- Differences in physical properties are mainly due to differences in intermolecular forces.
Key Definitions: Boiling Point, Melting Point, Vapor Pressure
- Boiling point: the temperature at which a substance’s vapor pressure equals atmospheric pressure.
- Melting point: the temperature at which a substance’s solid and liquid phases are at equilibrium.
- Vapor pressure: the pressure exerted by a vapor in equilibrium with its liquid in a closed system.
Relationship Between Intermolecular Forces and Physical Properties
- Stronger intermolecular forces result in higher boiling and melting points, and lower vapor pressure.
- Weaker intermolecular forces lead to lower boiling and melting points, and higher vapor pressure.
- Boiling point and vapor pressure are inversely related.
Factors Affecting Physical Properties
- Type and strength of intermolecular forces present (e.g., hydrogen bonding, London forces).
- Type and number of functional groups (homologous series).
- Structure: chain length (longer chains = stronger intermolecular forces).
- Structure: degree of branching (more branching = weaker intermolecular forces).
Structured Approach for Comparison Questions ("Recipe")
- State homologous series and intermolecular forces for each compound.
- Compare compounds: mention differences in chain length, branching, or types of forces.
- State which compound has stronger intermolecular forces.
- Link strength of forces to required energy to overcome them.
- Conclude with the effect on boiling/melting point or vapor pressure.
- Write "intermolecular forces" in full, not as "IMF."
- Use correct phrasing: "overcome intermolecular forces," not "break bonds."
Example: Ethane vs. Ethanol Boiling Point
- Ethane (alkane): only London forces.
- Ethanol (alcohol): hydrogen bonding and London forces.
- Ethanol has stronger intermolecular forces, needs more energy to overcome them, thus has a higher boiling point.
Key Terms & Definitions
- Boiling point — the temperature at which vapor pressure equals atmospheric pressure.
- Melting point — the temperature at which solid and liquid phases are at equilibrium.
- Vapor pressure — the pressure exerted by vapor at equilibrium with its liquid.
- Intermolecular forces — attractive forces between molecules (not bonds).
Action Items / Next Steps
- Memorize the official definitions for boiling point, melting point, and vapor pressure.
- Watch the sub-videos on intermolecular forces, chain length, and branching for deeper understanding.
- Practice applying the comparison "recipe" on past exam questions.