Intermolecular Forces 2010 © Prem D. Sattsangi & Christopher L. Byers (programmer)

KE States of Matter (A balance between IMF & KE) IMF hold molecules together KE allows them to separate Solid (ice) H 2 O Liquid H 2 O Gas (vapor) H 2 O Own Shape Container’s Shape in part Container’s Shape Definite Volume Definite Volume Container’s Volume Incompressible Incompressible Compressible Doesn’t Flow Flows Flows Negligible Diffusion Slow Diffusion Rapid Diffusion heating cooling heating cooling <<KE Intermolecular Force is equal. (Depends upon molecular structure)  H 2 O (g) Kinetic Energy depends upon temperature. (s)(l)(g)

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. Less Velcro surface area Less Weight can be held More Velcro surface area More Weight can be held Force   Too much weight (F grav ) will cause the velcro to break.

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 Dipole-Dipole Forces: The opposite ends of two or more dipoles attract each other. >> A. is Stronger than B. A.B. δ + δ - H Cl δ + δ - δ - 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.). SubstanceF.W.D.M.B.P. (K) Propane (C 3 H 8 ) Dimethyl ether (C 2 H 6 O) Methyl chloride (CH 3 Cl) Acetaldehyde (C 2 H 4 O) Acetonitrile (C 2 H 3 N)

Polar Molecules are Nonsymmetrical Asymmetrical Polar Tetrahedral 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) 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 water 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 ammonia H-Bond in HF is limited for the same reason.

H-Bonding in DNA G C A T C G H-Bonds G, C, A, & T are Bases

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-e- Na (g) atomCl (g) atomNa + (g) ionCl - (g) ion NaCl (s)

Sample Exercise 1 Consider the FW, Dipole Moment (DP), & Boiling Point (BP) to answer the following questions. SubstanceFW DMBP CH 3 CN K CH 3 I K Which has greater London Dispersion? CH 3 CN or CH 3 I Which has greater dipole attraction? CH 3 CN or CH 3 I Which has better overall IMF? CH 3 CN or CH 3 I

Sample Exercise 2 Which has greater London Dispersion force? (Periodic Table used to determine FW) Why? HF b.p. = 20 °C and HCl b.p. = -85 °C. Why does HF have a higher b.p.? Br 2 b.p. = 59 °C and ICl b.p. = 97 °C. Why does ICl have a higher b.p.? Xe (FW=131) or Ar (FW=40) Xe has greater FW & therefore more surface area. HF has H-bonding & HCl is only polar. (See IMF figure above.) FW ≈ 160 ICl is polar & Br 2 is non-polar. (Both have FW ≈ 160.) Weaker IMFStronger IMF London Dispersion << Polar << H-Bonding << Ion-Dipole << Ionic

Weaker IMFStronger IMF London Dispersion << Polar << H-Bonding << Ion-Dipole << Ionic Sample Exercise 3 List the following substances in order of increasing boiling points. BaCl 2, H 2, CO, HF, & Ne (Higher IMF = Higher BP) 1 st highest Ionic Bonding:BaCl 2 2 nd highest H-Bonding:HF 3 rd highest Polar Bond:CO (slightly polar & higher FW than Ne) 4 th highest Non-Polar:Ne (higher FW than H 2 ) 5 th lowest Non-Polar:H 2 5 th lowest Non-Polar:H 2 (lowest FW) Answer: H 2 << Ne << CO << HF << BaCl 2

Sample Exercise 4 List the substances N 2, NH 3, LiF, CO, & CH 4 in order of increasing b.p. LiF = Ionic Bond (strongest) NH 3 = H-Bond (2 nd strongest) CO = Polar (3 rd strongest) N 2 & CH 4 = Non-Polar (weakest) N 2 FW = 28 CH 4 FW = 16 CH 4 LiF Answer: CH 4 < N 2 < CO < NH 3 < 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. B. C. (FW = 94) (FW = 93) (FW = 92) LD A≈B≈C Comparable FW and Shape. H-Bonding A<B<C H-Bonding increases with the number of OH groups.