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Understanding Intermolecular Forces: Liquids, Solids, and Properties

Explore the world of condensed states, discover the impact of intermolecular forces on properties, and delve into the fascinating interactions between molecules in liquids and solids.

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Understanding Intermolecular Forces: Liquids, Solids, and Properties

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  1. Ch. 11: Liquids, Solids, and Intermolecular Forces Dr. Namphol Sinkaset Chem 200: General Chemistry I

  2. I. Chapter Outline • Introduction • Intermolecular Forces

  3. I. Condensed States • Liquids and solids are the condensed states because of the close proximity of atoms/molecules to one another. • This proximity leads to much more frequent interactions than in gases. • Structure determines properties via the nature of the interactions that occur.

  4. I. Ethanol vs. Dimethyl Ether • How can the same 9 atoms form two compounds with such different boiling points? • It’s because of the structure of the molecules!

  5. I. Electrostatic Forces • Every molecule in a sample of matter experiences two types of electrostatic forces. • Intramolecular forces: the forces that exist within the molecule (bonding). These forces determine chemical reactivity. • Intermolecular forces: the forces that exist between molecules. These forces determine physical properties.

  6. II. Intermolecular Forces • IM forces originate from interactions between charges, partial charges, and temporary charges on molecules. • IM forces are relatively weak because of smaller charges and the distance between molecules.

  7. II. Types of IM Forces • There are different kinds of IM forces, each with a different level of strength. • Dispersion force • Dipole-dipole force • *Hydrogen “bonding” • Ion-dipole force

  8. II. Dispersion Force • Dispersion force (London force) is present in all molecules and atoms and results from changes in e- locations.

  9. II. Instantaneous Dipoles • Charge separation in one creates charge separation in the neighbors.

  10. II. Dispersion Force Strength • The ease with which e-’s can move in response to an external charge is known as polarizability. • Large atoms with large electron clouds tend to have stronger dispersion forces. • Larger molecules tend to have stronger dispersion forces.

  11. II. Noble Gas Boiling Points

  12. II. Dispersion Force and Shape • Molecular size is not the only factor…

  13. II. Dispersion Force and Shape • Shape influences how the molecules interact with one another…structure determines properties.

  14. II. Dispersion Force in a Family

  15. II. Dipole-Dipole Force • Occurs in polar molecules which have permanent dipoles, so attraction is always present.

  16. II. Effect of Dipole-Dipole Force • Polar molecules have dispersion forces and dipole-dipole forces. • Effects can be seen in boiling and melting points.

  17. II. Increasing Polarity • If we increase the polarity, but keep molar mass approximately the same…

  18. II. “Like Dissolves Like” • Polar liquids are miscible(mix without separating) with other polar liquids, but not with nonpolar liquids. • Can be explained with intermolecular forces.

  19. II. Hydrogen “Bonding” • This IM force is a misnomer since it’s not an actual bond. • Occurs between molecules in which H is bonded to a highly electronegative element (N, O, F), leading to high partial positive and partial negative charges. • It’s a “super” dipole-dipole force.

  20. II. H “Bonding” Ethanol

  21. II. Ethanol vs. Dimethyl Ether • We compared these earlier. • Hydrogen “bonding” is so much stronger than dipole-dipole that one is a liquid at room temp. while the other is a gas.

  22. II. Effect of H “Bonding” • Hydrogen “bonding” is a very strong intermolecular force. • Without hydrogen “bonding” life as we know it could not exist!

  23. II. Ion-Dipole Force • Present in mixtures of ionic compounds & polar compounds. • Example: NaCl(s) dissolved in water.

  24. II. Summary of IM Forces

  25. II. Sample Problem 11.1 • Which substance has the highest boiling point and why? • CH3OH • CO • N2

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