Overview
This lecture focuses on comparing the properties of hydrocarbon molecules by examining their intermolecular forces (IMF) and how these forces influence boiling point, melting point, and volatility.
Hydrocarbons and Types of Forces
- Both example molecules are hydrocarbons, made of hydrogen and carbon only.
- Hydrocarbons with single carbon-carbon bonds are called alkanes.
- Alkanes are held together by dispersion forces (also called London dispersion forces).
- Dispersion forces are the weakest type of intermolecular force.
- Dispersion force strength increases as molecular weight increases (heavier molecules have stronger dispersion).
Comparing Molecular Properties
- Molecule 1 has 3 carbons; Molecule 2 has 4 carbons.
- Molecule 2, being heavier, has stronger dispersion forces (stronger IMF).
- Stronger IMF leads to higher boiling and melting points because more energy is required to separate the molecules.
- Volatility describes how easily a substance goes to the gas phase; lower volatility means it resists vaporizing.
- As IMF increases, boiling point and melting point increase, while volatility decreases.
Key Terms & Definitions
- Hydrocarbon — a molecule containing only hydrogen and carbon atoms.
- Alkane — a hydrocarbon with only single carbon-carbon bonds.
- Dispersion Forces (London Dispersion Forces) — weak intermolecular forces arising from temporary dipoles in molecules.
- Intermolecular Forces (IMF) — forces of attraction between molecules.
- Boiling Point — the temperature at which a liquid turns to gas.
- Volatility — the tendency of a substance to vaporize.
Action Items / Next Steps
- Review notes on intermolecular forces and their effects on physical properties.
- Practice comparing properties (boiling point, melting point, volatility) using molecular structure and IMF strength.