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Ch. 12: Liquids, Solids, and Intermolecular Forces

Ch. 12: Liquids, Solids, and Intermolecular Forces. Dr. Namphol Sinkaset Chem 152: Introduction to General Chemistry. I. Chapter Outline. Introduction Liquids and Solids Surface Tension and Viscosity Types of Intermolecular Forces. I. Electrostatic Forces.

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Ch. 12: Liquids, Solids, and Intermolecular Forces

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  1. Ch. 12: Liquids, Solids, and Intermolecular Forces Dr. Namphol Sinkaset Chem 152: Introduction to General Chemistry

  2. I. Chapter Outline • Introduction • Liquids and Solids • Surface Tension and Viscosity • Types of Intermolecular Forces

  3. 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: attractive forces that exist between molecules. These forces determine physical properties.

  4. I. 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.

  5. I. Solid, Liquid, or Gas? • Whether a substance exists as a solid, liquid, or gas depends on the relationship between the intermolecular forces and the kinetic energy of the molecules. • It’s a battle – which dominates? The KE or the IM attractions? • Recall that the average KE of a sample is related to its temperature, called the thermal energy.

  6. I. KE vs. IM Forces • Gas: the kinetic energy of the molecules is much greater than the intermolecular attractions. • Liquid: the kinetic energy of the molecules is moderately greater than the intermolecular attractions. • Solid: the kinetic energy of the molecules is less than the intermolecular attractions.

  7. II. Gases, Liquids, and Solids • Major difference between gases and condensed phases is the distance between particles. • Major difference between liquid and solid is the freedom of motion.

  8. II. Properties of Liquids • High densities relative to gases. • Indefinite shape; liquids take the shape of their container. • Definite volume; liquids cannot be compressed easily.

  9. II. Properties of Solids • High densities compared to gases. • Definite shape; solids do not take shape of their container. • Definite volume; solids are not easily compressed. • May be crystalline (ordered) or amorphous (unordered).

  10. III. Effects of Intermolecular Forces • Without IM forces, only the gas phase would exist! • Before looking at different types of IM forces, we look at two of their macroscopic effects. • Surface tension • Viscosity

  11. III. Surface Tension • More dense objects float??

  12. III. Surface Tension

  13. III. Attractions Pull Surface In • Molecules on the surface are attracted to molecules in the bulk. • This attraction pulls the surface molecules inward, minimizing surface area. • The result is a “skin” which resists penetration.

  14. III. Viscosity • Viscosity is the resistance of a liquid to flow. • Viscosity is the result of intermolecular attractions.

  15. IV. 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”

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

  17. IV. Instantaneous Dipoles • Charge separation in one creates charge separation in the neighbors.

  18. IV. 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. • Large molecules tend to have stronger dispersion forces.

  19. IV. Noble Gas Boiling Points

  20. IV. Dipole-Dipole Force • Occurs in polar molecules which have permanent dipoles, so attraction between molecules is always present.

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

  22. IV. “Like Dissolves Like” • Polar liquids are miscible with other polar liquids, but not with nonpolar liquids. • Miscible: mix without separating into two phases.

  23. IV. 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.

  24. IV. H “Bonding” in Ethanol & Water

  25. IV. Effect of H “Bonding” • Hydrogen “bonding” is a very strong intermolecular force. • Molecules with H “bonding” have much higher than expected melting and boiling points.

  26. IV. H “Bonding” and Life

  27. IV. Summary of IM Forces

  28. IV. Sample Problem • Determine the kind(s) of intermolecular forces present in the following substances. • HBr • I2 • CO • NF3 • SiCl4

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