Ch. 11: Liquids, Solids, and Intermolecular Forces

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