CHEM 111 Unit 10

General Chemistry: Periodic Table of the Elements

  • Periodicity of elements based on atomic number and trends in properties.

  • Groups (vertical columns) signify elements that share similar chemical properties.

States of Matter

  • Solid

    • Shape: Fixed

    • Volume: Fixed

    • Compressible: No

    • Flow: No

    • Strength of Intermolecular Attractions: Very strong

  • Liquid

    • Shape: Indefinite

    • Volume: Fixed

    • Compressible: No

    • Flow: Yes

    • Strength of Intermolecular Attractions: Moderate

  • Gas

    • Shape: Indefinite

    • Volume: Indefinite

    • Compressible: Yes

    • Flow: Yes

    • Strength of Intermolecular Attractions: Very weak

Change of State

  • Phase Transition: A change of a substance from one state to another.

    • Boiling Point (bp): Temperature at which the vapor pressure of a liquid equals the pressure on the liquid. Example: Normal boiling point.

    • Freezing Point (fp): Temperature where a pure liquid changes to a crystalline solid.

    • Melting Point (mp): Temperature where a solid becomes a liquid.

Types of Molecular Forces

Intermolecular Forces vs. Intramolecular Forces

  • Intramolecular Forces: Strong forces within a molecule, such as covalent or ionic bonds (100-1000 kJ/mol).

  • Intermolecular Forces: Weaker forces between distinct particles (typically 1-50 kJ/mol).

    • Types of Intermolecular Forces:

    • Ion-Ion: Between charged ions, strongest forces (250 kJ/mol).

    • Ion-Dipole: Between ions and polar molecules.

    • Dipole-Dipole: Between polar molecules.

    • London Dispersion Forces: Weak forces due to temporary dipoles arising from electron distributions in all molecules. Strength varies from 1-10 kJ/mol.

    • Hydrogen Bonds: Special dipole-dipole interactions involving H atoms bonded to highly electronegative atoms (N, O, F).

Hydrogen Bonding

  • A specific type of dipole-dipole interaction where H is covalently bonded to N, O, or F, creating strong attraction forces.

  • Examples include interactions in water (H2O) leading to anomalously high boiling points compared to other molecules due to hydrogen bonding strength (~20 kJ/mol).

Factors Influencing Intermolecular Forces

  • Polarizability: Tendency of electron density to distort; larger atoms/molecules are more polarizable.

  • Shape of Molecule: Increased surface area results in higher interactions.

Vapor Pressure and Boiling Point

  • Vapor Pressure: Partial pressure of vapor above a liquid at equilibrium, depends on temperature and intermolecular forces.

  • Boiling Point: Temperature where vapor pressure equals external pressure, specifically atmospheric pressure (1 atm).

  • Higher vapor pressures indicate more volatile substances.

    • Example data:

    • CH4: Molar Mass 16.04, Boiling Point -161.5 °C

    • CH3OH: Molar Mass 32.04, Boiling Point 64.5 °C

Phase Diagrams

  • Graphs summarizing conditions for different states of a substance (solid, liquid, gas).

    • Triple Point: All three phases coexist.

    • Critical Point: Temperature/pressure where liquid and gas phases have the same density.

    • Equilibrium Lines represent phase changes between states under specific conditions.

Learning Objectives

  • Intermolecular Forces: Identify types based on molecular structure and predict properties.

  • Phase Diagrams: Recognize regions and phase transitions, with comparisons between different substances like water and carbon dioxide.

Applications of Intermolecular Forces

  • Surface Tension and Viscosity are affected by the strength of intermolecular forces; higher forces lead to greater viscosity (resistance to flow)

Conclusion

  • Understanding these properties and interactions is crucial for predicting behaviors in chemical reactions and product formations in various physical states.