Intermolecular Forces

1. Intermolecular Forces • The physical state of a substance (gas, liquid or solid) can be extremely important in many systems.This includes living organisms. Water, for example, is an important solvent that is the basis for life on Earth. Individual water molecules are held together in the solid and liquid phases by very strong intermolecular forces. These forces account for the fact that water is a liquid at ambient planetary temperatures.

2. Classification of Substances • In any consideration of intermolecular forces we need to be able to look at a molecular formula and determine whether a substance is made up of atoms, molecules or ions. Covalent substances and ionic substances, for example, have very different physical properties. Ionic substances have very high melting points while covalent/molecular substances have relatively low melting points.

3. Physical Properties – Examples:

4. Physical Properties – Trends: • 1. Atomic substances (Noble gas elements) have very low melting and boiling points. • 2. Covalent substances consisting of discrete molecules have “moderate” melting and boiling points. • 3. Ionic substances and network covalent substances (no discrete molecules) have very high melting and boiling points.

5. Physical Properties –Trends: • The strong coulombic forces of attraction between oppositely charged ions are easily understood. Similarly, the forces of attraction between molecules with permanent electric dipole moments are easily understood. The weak attractive forces that must exist (why?) between electrically neutral atoms (He, Ne, Ar…) and electrically neutral and nonpolar molecules are more difficult to appreciate.

6. Physical Properties – Weak Intermolecular Forces: • The next two slides show melting and boiling points for the Noble gas elements and the carbon tetrahalides. The attractive forces here are due to transient atomic and molecular electrical polarity. The magnitude of the transient polarity (fluctuating dipoles) is related to atomic size. Larger atoms have electron clouds which are more readily deformed (atoms are more polarizable).

7. Melting and Boiling Points (Noble Gas Elements)

8. Aside: Liquid Ranges • The Noble Gas elements listed on the previous slide have extremely small liquid ranges. It is fortunate that water, for example, has a much larger liquid range. Nonpolar molecules can have much larger liquid ranges than the Noble Gases – next slide.

9. Melting and Boiling Points of methane and Carbon Tetrahalides

10. Intermolecular Forces • Van der Waals Forces • A collection of weak attractive forces between groups of atoms or molecules. • Instantaneous and Induced Dipoles • Displacement of electrons cause polarization giving rise to an instantaneous dipole. This dipole can affect neighbouring molecules causing induced dipoles. • Dispersion or London forces. • Instantaneous dipole – induced dipole attraction. • Related to polarizability. General Chemistry: Chapter 12

11. The phenomenon of induction FIGURE 12-1 General Chemistry: Chapter 12

12. Instantaneous and induced dipoles FIGIURE 12-2 General Chemistry: Chapter 12

13. Melting and Boiling Points (Noble Gas Elements)

14. Noble Gases – Physical Properties • The melting points and boiling points of the Noble Gas elements increase as one moves to larger atoms. This is not due directly to the atoms becoming heavier. The key point is that the larger electron “clouds” in the larger/heavier Noble Gas atoms are much more readily polarizedthan the electron “cloud” in an atom such as He.

15. Melting and Boiling Points of Methane and Carbon Tetrahalides

16. General Chemistry: Chapter 12

17. Molecular shape and polarizability FIGURE 12-3 General Chemistry: Chapter 12

18. Molecules with Permanent Polarity • Many covalently bonded molecules have a net non-zero molecular electric dipole moment. Clearly, a positive (coulombic) attractive force – between the positive “end” of one molecule and the negative end of another molecule will make it more difficult to, for example, vaporize electrically polar molecules.

19. Dipole-Dipole Interactions Permanent Polarity Dipole-dipole interactions General Chemistry: Chapter 12

20. Melting and Boiling Points of Fluoromethanes and Chloromethanes • On the next two slides the melting and boiling points of several fluoromethanes and chloromethanes are presented. Some of these molecules are electrically nonpolar but all have London dispersion forces (of attraction). The dispersion forces are more important in the chloromethanes. Why?

21. Melting Points, Boiling Points and Dipole Moments of Fluoromethanes 1 D = 1 Debye = 3.3 x 10-30 coulomb.meter. Why is this unit small?

22. Melting Points, Boiling Points and Dipole Moments of Chloromethanes

23. Physical Properties of Halomethanes • On the previous two slides how can you rationalize the boiling points of the tetrahalomethanes in comparison to the data for the trihalomethanes?

24. Physical Properties of N2, NO and O2 • The next slide gives the boiling points of N2, NO and O2. Two of these molecules are nonpolar while NO is weakly polar. NO has the highest boiling point. (Aside: There are a number of other complications here – including the fact that the free radical NO (unpaired electron) can dimerize (partially) to form the N2O2 molecule).

25. General Chemistry: Chapter 12

26. Extremely Polar Bonds – Hydrogen Bonding • With H bonded to the very electronegative F, N and O the bonding electrons in, for example, an O-H bond are very unequally shared. The extreme electrical polarity that results leads to unusual physical properties as molecules such as these “stick together” by forming intermolecular hydrogen bonds.

27. Hydrogen Bonding • Look carefully at the plots on the next slide. At what temperatures might you expect liquid water and liquid ammonia hydrogen bonding was not important for these two substances?

28. Hydrogen Bonding FIGURE 12-5 Comparison of boiling points of some hydrides of the elements of groups 14, 15, 16, and 17 General Chemistry: Chapter 12

29. Hydrogen bonding in gaseous hydrogen fluoride FIGURE 12-6 Electrostatic potential map of HF General Chemistry: Chapter 12

30. Hydrogen bonding between H2O molecules General Chemistry: Chapter 12

31. Hydrogen bonding in water FIGURE 12-7 around a molecule in the solid in the liquid General Chemistry: Chapter 12

32. Class Examples • We will look at a number of molecular formulas for different substances and identify the strongest type of intermolecular force present. We’ll use a knowledge of intermolecular forces to predict relative boiling points for simple molecules.