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.

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.

Physical Properties – Examples: Chemical Formula Type of Substance Melting Pt. (oC) Boiling Pt. (oC) He Atomic -272 -269 Ar -189 -186 Br2 Molecular -7 59 CH3OH -98 65 H2O 100 NaCl Ionic 801 1413 MgO 2852 3600 Si Network Covalent 1687 3538

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.

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.

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).

Melting and Boiling Points (Noble Gas Elements) Melting Point (oC) Boiling Point (oC) Helium -272 -269 Neon -249 -246 Argon -189 -186 Krypton -157 -153 Radon -71 -62

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.

Melting and Boiling Points of methane and Carbon Tetrahalides Compound Melting Point (oC) Boiling Point (oC) Methane (CH4) -187 -161 Carbon tetrafluoride (CF4) -185 -128 Carbon tetrachloride (CCl4) -23 76 Carbon tetrabromide (CBr4) 93 190 Carbon tetraiodide (CI4) 171 Decomposes

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 Copyright © 2011 Pearson Canada Inc.

The phenomenon of induction Chemistry 140 Fall 2002 The attraction of a balloon to a surface is a commonplace example of induction. The balloon is charged by rubbing, and the charged balloon induces an opposite charge on the surface. (See also Appendix B.) FIGURE 12-1 The phenomenon of induction General Chemistry: Chapter 12 Copyright © 2011 Pearson Canada Inc.

Instantaneous and induced dipoles Chemistry 140 Fall 2002 In the normal condition, a nonpolar molecule has a symmetrical charge distribution. In the instantaneous condition, a displacement of the electronic charge produces an instantaneous dipole with a charge separation represented as δ+ and δ- . In an induced dipole, the instantaneous dipole on the left induces a charge separation in the molecule on the right. The result is an instantaneous dipole–induced dipole attraction. FIGIURE 12-2 Instantaneous and induced dipoles General Chemistry: Chapter 12 Copyright © 2011 Pearson Canada Inc.

Melting and Boiling Points (Noble Gas Elements) Melting Point (oC) Boiling Point (oC) Helium -272 -269 Neon -249 -246 Argon -189 -186 Krypton -157 -153 Radon -71 -62

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 polarized than the electron “cloud” in an atom such as He.

Melting and Boiling Points of Methane and Carbon Tetrahalides Compound Melting Point (oC) Boiling Point (oC) Methane (CH4) -187 -161 Carbon tetrafluoride (CF4) -185 -128 Carbon tetrachloride (CCl4) -23 76 Carbon tetrabromide (CBr4) 93 190 Carbon tetraiodide (CI4) 171 Decomposes

General Chemistry: Chapter 12 Copyright © 2011 Pearson Canada Inc.

Molecular shape and polarizability Chemistry 140 Fall 2002 ≤10 kJ /mol FIGURE 12-3 Molecular shape and polarizability General Chemistry: Chapter 12 Copyright © 2011 Pearson Canada Inc.

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.

Dipole-Dipole Interactions Chemistry 140 Fall 2002 Dipole-Dipole Interactions 5-20 kJ /mol Permanent Polarity Dipole-dipole interactions General Chemistry: Chapter 12 Copyright © 2011 Pearson Canada Inc.

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?

Melting Points, Boiling Points and Dipole Moments of Fluoromethanes Compound Melting Pt. (OC) Boiling Pt. (OC) Molecular Electric Dipole Moment (D) Methane (CH4) -187 -161 Fluoromethane (CH3F) -142 -78 1.85 Difluoromethane (CH2F2) -136 -52 1.98 Trifluoromethane (CHF3) -155 -82 1.65 Tetrafluoromethane (CF4) -185 -128 1 D = 1 Debye = 3.3 x 10-30 coulomb.meter. Why is this unit small?

Melting Points, Boiling Points and Dipole Moments of Chloromethanes Compound Melting Pt. (OC) Boiling Pt. (OC) Molecular Electric Dipole Moment (D) Methane (CH4) -187 -161 Chloromethane (CH3Cl) -98 +24 1.86 Dichloromethane (CH2Cl2) -97 +40 1.14 Trichloromethane (CHCl3) -64 +61 1.15 Tetrachloromethane (CCl4) -23 +76

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?

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).

General Chemistry: Chapter 12 Copyright © 2011 Pearson Canada Inc.

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.

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?

Hydrogen Bonding FIGURE 12-5 Chemistry 140 Fall 2002 Hydrogen Bonding 15 to 40 kJ/mol The boiling points for NH3, H2O and HF are unusually high compared with those of other members of their groups. FIGURE 12-5 Comparison of boiling points of some hydrides of the elements of groups 14, 15, 16, and 17 General Chemistry: Chapter 12 Copyright © 2011 Pearson Canada Inc.

Hydrogen bonding in gaseous hydrogen fluoride Chemistry 140 Fall 2002 Electrostatic potential map of HF In gaseous hydrogen fluoride, many of the HF molecules are associated into cyclic (HF6) structures of the type pictured here. Each H atom is bonded to one F atom by a single covalent bond (-) and to another F atom through a hydrogen bond (…). FIGURE 12-6 Hydrogen bonding in gaseous hydrogen fluoride General Chemistry: Chapter 12 Copyright © 2011 Pearson Canada Inc.

Hydrogen bonding between H2O molecules General Chemistry: Chapter 12 Copyright © 2011 Pearson Canada Inc.

Hydrogen bonding in water Chemistry 140 Fall 2002 Each water molecule is linked to four others through hydrogen bonds. The arrangement is tetrahedral. Each H atom is situated along a line joining two O atoms, but closer to one O atom (100 pm) than to the other (180 pm). For the crystal structure of ice, H atoms lie between pairs of O atoms, again closer to one O atom than to the other. (Molecules behind the plane of the page are light blue.) O atoms are arranged in bent hexagonal rings arranged in layers. This characteristic pattern is similar to the hexagonal shapes of snowflakes. In the liquid, water molecules have hydrogen bonds to only some of their neighbours. This allows the water molecules to pack more densely in the liquid than in the solid. around a molecule in the solid in the liquid FIGURE 12-7 Hydrogen bonding in water General Chemistry: Chapter 12 Copyright © 2011 Pearson Canada Inc.

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.