1. © 2009, Prentice-Hall, Inc. Chapter 11 Intermolecular Forces, Liquids, and Solids

2. © 2009, Prentice-Hall, Inc. States of Matter The fundamental difference between states of matter is the distance between particles.

3. © 2009, Prentice-Hall, Inc. States of Matter Because in the solid and liquid states particles are closer together, we refer to them as condensed phases.

4. © 2009, Prentice-Hall, Inc. The States of Matter The state a substance is in at a particular temperature and pressure depends on two antagonistic entities: – the kinetic energy of the particles; – the strength of the attractions between the particles.

5. © 2009, Prentice-Hall, Inc. Intermolecular Forces The attractions between molecules are not nearly as strong as the intramolecular attractions that hold compounds together.

6. © 2009, Prentice-Hall, Inc. Intermolecular Forces They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities.

7. © 2009, Prentice-Hall, Inc. Intermolecular Forces These intermolecular forces as a group are referred to as van der Waals forces.

8. © 2009, Prentice-Hall, Inc. van der Waals Forces Dipole-dipole interactions Hydrogen bonding London dispersion forces

9. © 2009, Prentice-Hall, Inc. Ion-Dipole Interactions Ion-dipole interactions (a fourth type of force), are important in solutions of ions. The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents.

10. © 2009, Prentice-Hall, Inc. How Does a Solution Form? As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates, them.

11. © 2009, Prentice-Hall, Inc. Dipole-Dipole Interactions Molecules that have permanent dipoles are attracted to each other. – The positive end of one is attracted to the negative end of the other and vice- versa. – These forces are only important when the molecules are close to each other.

12. © 2009, Prentice-Hall, Inc. Dipole-Dipole Interactions The more polar the molecule, the higher is its boiling point.

13. © 2009, Prentice-Hall, Inc. London Dispersion Forces While the electrons in the 1s orbital of helium would repel each other (and, therefore, tend to stay far away from each other), it does happen that they occasionally wind up on the same side of the atom.

14. © 2009, Prentice-Hall, Inc. London Dispersion Forces At that instant, then, the helium atom is polar, with an excess of electrons on the left side and a shortage on the right side.

15. © 2009, Prentice-Hall, Inc. London Dispersion Forces Another helium nearby, then, would have a dipole induced in it, as the electrons on the left side of helium atom 2 repel the electrons in the cloud on helium atom 1.

16. © 2009, Prentice-Hall, Inc. London Dispersion Forces London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole.

17. © 2009, Prentice-Hall, Inc. London Dispersion Forces These forces are present in all molecules, whether they are polar or nonpolar. The tendency of an electron cloud to distort in this way is called polarizability.

18. © 2009, Prentice-Hall, Inc. Factors Affecting London Forces The shape of the molecule affects the strength of dispersion forces: long, skinny molecules (like n- pentane tend to have stronger dispersion forces than short, fat ones (like neopentane). This is due to the increased surface area in n-pentane.

19. © 2009, Prentice-Hall, Inc. Factors Affecting London Forces The strength of dispersion forces tends to increase with increased molecular weight. Larger atoms have larger electron clouds which are easier to polarize.

20. © 2009, Prentice-Hall, Inc. Which Have a Greater Effect? Dipole-Dipole Interactions or Dispersion Forces If two molecules are of comparable size and shape, dipole-dipole interactions will likely the dominating force. If one molecule is much larger than another, dispersion forces will likely determine its physical properties.

21. Sample Exercise 11.1 Comparing Intermolecular Forces The dipole moments of acetonitrile, CH 3 CN, and methyl iodide, CH 3 I, are 3.9 D and 1.62 D, respectively. (a)Which of these substances has greater dipole–dipole attractions among its molecules? (b) Which of these substances has greater London dispersion attractions? (c) The boiling points of CH 3 CN and CH 3 I are 354.8 K and 315.6 K, respectively. Which substance has the greater overall attractive forces? Of Br 2, Ne, HCl, HBr, and N 2, which is likely to have (a) the largest intermolecular dispersion forces, (b) the largest dipole– dipole attractive forces? Practice Exercise

22. Answer: (a) Br 2 (largest molecular weight), (b) HCl (largest polarity)

23. © 2009, Prentice-Hall, Inc. How Do We Explain This? The nonpolar series (SnH 4 to CH 4 ) follow the expected trend. The polar series follows the trend from H 2 Te through H 2 S, but water is quite an anomaly.

24. © 2009, Prentice-Hall, Inc. Hydrogen Bonding The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong. We call these interactions hydrogen bonds.

25. © 2009, Prentice-Hall, Inc. Hydrogen Bonding Hydrogen bonding arises in part from the high electronegativity of nitrogen, oxygen, and fluorine. Also, when hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed.

26. © 2009, Prentice-Hall, Inc. Summarizing Intermolecular Forces

27. Sample Exercise 11.2 Identifying Substances that Can Form Hydrogen Bonds In which of the following substances is hydrogen bonding likely to play an important role in determining physical properties: methane (CH 4 ), hydrazine (H 2 NNH 2 ), methyl fluoride (CH 3 F), or hydrogen sulfide (H 2 S)? Solution

28. Sample Exercise 11.2 Identifying Substances that Can Form Hydrogen Bonds In which of the following substances is significant hydrogen bonding possible: methylene chloride (CH 2 Cl 2 ), phosphine (PH 3 ), hydrogen peroxide (HOOH), or acetone (CH 3 COCH 3 )? Answer: HOOH Practice Exercise

29. Sample Exercise 11.3 Predicting the Types and Relative Strengths of Intermolecular Attractions List the substances BaCl 2, H 2, CO, HF, and Ne in order of increasing boiling points. Solution Questions to ask: Ionic character? Polarity? Hydrogen bonds (N,O, F)? Molecular weight? H 2 < Ne < CO < HF < BaCl 2 The actual normal boiling points are H 2 (20 K), Ne (27 K), CO (83 K), HF (293 K), and BaCl 2 (1813 K)—in agreement with our predictions.