I ntermolecular forces

1. Chemistry 11 Resource: Chang’s Chemistry Chapter 9 Intermolecular forces

2. Objectives • Predict whether or not a molecule is polar from its molecular shape and bond polarities.

3. Objectives • Describe the types of intermolecular forces (attractions between molecules that have temporary dipoles, permanent dipoles or hydrogen bonding) and explain how they arise from the structural features of molecules. • Describe and explain how intermolecular forces affect boiling points of substances.

4. Polarity • Recall the compound HF. • How did we predict / know that this was a polar molecule? • Demonstration of symbols. • We can also predict the polarity of a molecule based on its geometry.

5. Polarity • A dipole moment is a quantitative measure of the polarity of a bond. • Molecules without dipole moments are nonpolar. • Molecules with dipole moments are polar. • Does HF have dipole moments? HCl? H2? O2?

6. Polarity • A dipole moment is a quantitative measure of the polarity of a bond. • Molecules without dipole moments are nonpolar. • Molecules with dipole moments are polar. • Does HF have dipole moments? HCl? H2? O2?

7. Polarity • Consider the compound BF3. • Based on electronegativity, the B-F bond polar? • Now consider the geometry of BF3. • Is the molecule (as a whole) polar?

8. Polarity • The difference in electronegativities of B-F is 2.0, so YES, the bond is polar. • The geometry, however is very symmetrical, so NO, the molecule is not polar. • It is important to consider both the electronegativities of the atoms and geometry of the molecule to determine polarity.

9. Polarity • Construct the following molecules and label them. Are they polar or not? • HBr • H2S • Cl2 • CBr4 • I2 • NO3 • NH3 • C6H6

10. Kinetic molecular theory • Compare solids, liquids, and gases in terms of volume, shape, density, compressibility, and molecular motion.

11. Intermolecular forces • What does intermolecular mean? • What does intramolecular mean? • Which one is involved in bonding?

12. Intermolecular forces • Intermolecular forces are the attractive forces between molecules. Also called van der Waals forces. • Are there intermolecular forces between molecules in: • solids? • liquids? • gases? • Why are intermolecular forces important?

13. Intermolecular forces • Molecular geometry and intermolecular forces are responsible for the gross properties of matter, such as: • physical appearance • melting point • boiling point

14. Intermolecular forces • It takes about 41 kJ to vaporize one mole of water and about 930 kJ to break the two O-H bonds in 1 mole of water. • What does this imply about the strengths of inter- and intramolecular forces?

15. Intermolecular forces • Generally, intermolecular forces are much weaker than intramolecular forces. • Which physical property (physical appearance, boiling point, or melting point) is a better measure of the strength of intermolecular forces?

16. Intermolecular forces • Boiling point is the best indication of the intermolecular forces in a substance. • Evaporation involves almost completely overcoming the attractive forces between molecules. • If the boiling point is high, what does it tell you about the intermolecular forces?

17. Intermolecular forces Types of intermolecular forces: • dipole-dipole • dipole-induced dipole • (London) dispersion forces

18. Intermolecular forces • Attractive forces between polar molecules (molecules with dipole moments). • How do you think the polar molecules would line up? • How would an ion interact with a polar molecule? • What kind of force is responsible? Dipole-dipole

19. Intermolecular forces • There is a specific dipole-dipole interaction called the hydrogen bond. • This is a misnomer because it is not actually a bond. • It is the interaction between the hydrogen atom in a polar bond and an electronegative atom. Dipole-dipole

20. Intermolecular forces • What would happen if you place an ion near a nonpolar molecule? Induced dipole - dipole

21. Intermolecular forces • Although a nonpolar molecule may not possess dipole moments, dipoles can be induced. • Ions and polar molecules can induce dipoles in nonpolar molecules. • The separation of positive and negative charges in a nonpolar molecule is due to the proximity or a polar molecule. Induced dipole - dipole

22. Intermolecular forces • Does this mean all substances with ions/polar molecules will have induced dipole – dipole forces? • What factors might affect the likelihood of an induced dipole? Induced dipole - dipole

23. Intermolecular forces • Because electrons are always moving, it is possible that a dipole can exist in an atom / nonpolar molecule for an instant. • This is called a temporary dipole. • A temporary dipole can induce dipoles in the surrounding atoms / molecules. (London) dispersion forces

24. Intermolecular forces • In gases, these temporary dipoles do not have much impact. • At low temperatures, however, they can cause nonpolar substances to condense. • Why is this so? (London) dispersion forces

25. Intermolecular forces Melting points of similar nonpolar compounds What trend do you notice? What is responsible for this? (London) dispersion forces

26. Intermolecular forces Identify the type of intermolecular forces that exist between: • HBr and H2S • Cl2 and CBr4 • I2 and NO3 • NH3 and C6H6

27. Intermolecular forces Identify the type of intermolecular forces that exist between: • HBr and H2S – dipole-dipole • Cl2 and CBr4 – dispersion • I2 and NO3 – ion-induced and dispersion • NH3 and C6H6 – dipole-induced dipole and dispersion

28. Intermolecular forces Relationship between intermolecular forces and physical properties

29. Homework • Water is a very common (and yet very unique) substance on Earth. • What are the properties of water? Explain its properties using what we’ve learned about molecular geometry and intermolecular forces. • Quiz next class on molecular geometry and intermolecular forces. • Long test on Wednesday, 25 February. Coverage: Atomic theory and bonding