Intermolecular Forces 2010 © Prem D. Sattsangi & Christopher L. Byers (programmer) States of Matter (A balance between IMF & KE) IMF hold molecules together KE allows them to separate Kinetic Energy depends upon temperature. (s) heating cooling <<KE (l) heating cooling H2O (g) <<KE (g) Intermolecular Force is equal. (Depends upon molecular structure) Solid (ice) H2O Own Shape Definite Volume Incompressible Doesn’t Flow Negligible Diffusion Liquid H2O Container’s Shape in part Definite Volume Incompressible Flows Slow Diffusion Gas (vapor) H2O Container’s Shape Container’s Volume Compressible Flows Rapid Diffusion London Dispersion London recognized that the motion of electrons in an atom or molecule can cause instantaneous & momentary dipole. This force is known as London Dispersion (LD). LD is determined by the formula weight (FW). If FW is the same, LD is determined by the surface area. Too much weight (Fgrav) will cause the velcro to break. Force Force Less Velcro surface area Less Weight can be held More Velcro surface area More Weight can be held Dipole-Dipole Forces Dipole: a bond with a positive and a negative end. Strength depends on electronegativity difference (END). END for HCl, [Cl(3.5) - H(2.1)] = 1.4 δ+ H δCl Dipole-Dipole Forces: The opposite ends of two or more dipoles attract each other. A. B. δ+ δ- δ- δ+ >> δ+ A. is Stronger than B. δ- δ+ δ- Dipole Moments (DM) A Dipole moment is the measure of the polarity of a molecule. For compounds with similar FW, the DM determines the IMF. As the Dipole moment increases, so does the boiling point (B.P.). Substance F.W. D.M. B.P. (K) Propane (C3H8) Dimethyl ether (C2H6O) Methyl chloride (CH3Cl) Acetaldehyde (C2H4O) Acetonitrile (C2H3N) 44 46 50 44 41 0.1 1.3 1.9 2.7 3.9 231 248 249 294 355 Polar Molecules are Nonsymmetrical Asymmetrical Polar Tetrahedral Symmetrical Non-polar Symmetrical Non-polar Tetrahedral Hydrogen Bonding Hydrogen Bonding is a special bond between the H atom in a polar bond (H—N, H—F, or H—O) and an unshared electron pair on a neighboring F, O, or N atom. It is much stronger than Dipole-Dipole force or LD. Of those bonds, H—O bond is the strongest, like that in water. Both H—N and H—O bonds are used in bonding DNA molecules together. H—O E.N.D. = 1.4, H—N E.N.D. = 0.9 Comparison of H-Bonds (H—O & H—N) H-bond in water 2 Electron Pairs (δ-) & 2 H ends (δ+). After 1 H-bond, bonding sites increase. 3 Electron Pairs (δ-) & 3 H ends (δ+). Water forms endless H-Bonds. H-bond in ammonia 1 Electron Pair (δ-) & 3 H ends (δ+). After 1 H-bond, H ends increase. 1 Electron Pair (δ-) & 5 H ends (δ+). The (δ-) end is hard to find; limited H-Bonds. H-Bond in HF is limited for the same reason. H-Bonding in DNA H-Bonds G G, C, A, & T are Bases C A T A T C G H-Bonds in Silk H-Bonds Ionic Bonding Ions are formed by electron transfer. They are held together by electrostatic forces in a lattice, causing ionic bonding. E.g. NaCl - + e- Na+(g) (g)atom ion Cl-(g) (g)atom ion NaCl (s) Sample Exercise 1 Consider the FW, Dipole Moment (DP), & Boiling Point (BP) to answer the following questions. Substance CH3CN CH3I FW 41 142 DM 3.90 1.62 Which has greater London Dispersion? CH3CN or CH3I Which has greater dipole attraction? CH3CN or CH3I Which has better overall IMF? CH3CN or CH3I BP 354.8 K 315.6 K Sample Exercise 2 Weaker IMF Stronger IMF London Dispersion << Polar << H-Bonding << Ion-Dipole << Ionic Which has greater London Dispersion force? (Periodic Table used to determine FW) Why? Xe (FW=131) or Ar (FW=40) Xe has greater FW & therefore more surface area. HF b.p. = 20 °C and HCl b.p. = -85 °C. Why does HF have a higher b.p.? HF has H-bonding & HCl is only polar. (See IMF figure above.) Br2 b.p. = 59 °C and ICl b.p. = 97 °C. Why does ICl have a higher b.p.? ICl is polar & Br2 is non-polar. (Both have FW ≈ 160.) Sample Exercise 3 Weaker IMF Stronger IMF London Dispersion << Polar << H-Bonding << Ion-Dipole << Ionic List the following substances in order of increasing boiling points. BaCl2, H2, CO, HF, & Ne 1st highest Ionic Bonding: 2nd highest H-Bonding: 3rd highest Polar Bond: 4th highest Non-Polar: 5th lowest Non-Polar: (Higher IMF = Higher BP) BaCl2 HF CO (slightly polar & higher FW than Ne) Ne (higher FW than H2) H2 (lowest FW) Answer: H2 << Ne << CO << HF << BaCl2 Sample Exercise 4 List the substances N2, NH3, LiF, CO, & CH4 in order of increasing b.p. LiF = Ionic Bond (strongest) NH3 = H-Bond (2nd strongest) CO = Polar (3rd strongest) N2 & CH4 = Non-Polar (weakest) N2 FW = 28 CH4 FW = 16 Answer: CH4 < N2 < CO < NH3 < LiF Sample Exercise 5 Explain the nature of IMF present in each of the following components. A. B. C. LD only Non-polar molecule. LD and Dipole-Dipole Polar molecule. LD and H-Bonding Polarity is present but H-Bond is more dominant force. Sample Exercise 6 Rank the following components in terms of (a) LD and (b) H-Bonding. A. (FW = 94) B. (FW = 93) C. (FW = 92) LD A≈B≈C Comparable FW and Shape. H-Bonding A<B<C H-Bonding increases with the number of OH groups.