Ch 10: Intermolecular Forces and Types of Solids AP Chemistry Sequoyah High School

2 10.1: Intermolecular Forces (IMF)  IMF < intramolecular forces (covalent, metallic, ionic bonds)  IMF strength: solids > liquids > gases  Boiling points and melting points are good indicators of relative IMF strength.

3 10.1: Types of IMF 1.Electrostatic forces: act over larger distances in accordance with Coulomb’s law a.Ion-ion forces: strongest; found in ionic crystals (i.e. lattice energy)

4 b.Ion-dipole: between an ion and a dipole (a neutral, polar molecule/has separated partial charges)  Increase with increasing polarity of molecule and increasing ion charge.          Cl -          S 2- < Ex: Compare IMF in Cl - (aq) and S 2- (aq).

5 c.Dipole-dipole: weakest electrostatic force; exist between neutral polar molecules  Increase with increasing polarity (dipole moment) of molecule Ex: What IMF exist in NaCl (aq)?

6 d.Hydrogen bonds (or H-bonds):  H is unique among the elements because it has a single e - that is also a valence e -. –When this e - is “hogged” by a highly EN atom (a very polar covalent bond), the H nucleus is partially exposed and becomes attracted to an e - -rich atom nearby. ( ice and water simulation) ( H bond movie) ( H bonding in water)

7  H-bonds form with H-XX', where X and X' have high EN and X' possesses a lone pair of e-  X = F, O, N (since most EN elements) on two molecules: F-H O-H N-H :F:O:N:F:O:N

8  * There is no strict cutoff for the ability to form H-bonds (S forms a biologically important hydrogen bond in proteins).  * Hold DNA strands together in double-helix Nucleotide pairs form H- bonds DNA double helix

9  H-bonds explain why ice is less dense than water

10 Ex: Boiling points of nonmetal hydrides Boiling Points (ºC) Conclusions:  Polar molecules have higher BP than nonpolar molecules ∴ Polar molecules have stronger IMF  BP increases with increasing MW ∴ Heavier molecules have stronger IMF  NH 3, H 2 O, and HF have unusually high BP. ∴ H-bonds are stronger than dipole-dipole IMF

11 2.Inductive forces:  Arise from distortion of the e - cloud induced by the electrical field produced by another particle or molecule nearby.  London dispersion: between polar or nonpolar molecules or atoms –* Proposed by Fritz London in 1930 –Must exist because nonpolar molecules form liquids Fritz London ( )

12 How they form: 1.Motion of e- creates an instantaneous dipole moment, making it “temporarily polar”. 2.Instantaneous dipole moment induces a dipole in an adjacent atom * Persist for about or second Ex: two He atoms

13 * Geckos!  Geckos’ feet make use of London dispersion forces to climb almost anything.  A gecko can hang on a glass surface using only one toe.  Researchers at Stanford University recently developed a gecko-like robot which uses synthetic setae to climb walls

14 London dispersion forces increase with:  Increasing MW, # of e -, and # of atoms (increasing # of e - orbitals to be distorted) Boiling points: Effect of MW:Effect of # atoms: pentane36ºCNe–246°C hexane69ºCCH 4 –162°C heptane98ºC ??? effect: H 2 O 100°C D 2 O 101.4°C  “Longer” shapes (more likely to interact with other molecules) C 5 H 12 isomers: 2,2-dimethylpropane10°C pentane36°C

Summary of IMF Van der Waals forces

16 Ex: Identify all IMF present in a pure sample of each substance, then explain the boiling points. BP(⁰C ) IMFExplanation HF20 HCl-85 HBr-67 HI-35 Lowest MW/weakest London, but most polar/strongest dipole-dipole and has H-bonds Low MW/weak London, moderate polarity/dipole-dipole and no H-bonds Medium MW/medium London, moderate polarity/dipole-dipole and no H-bonds Highest MW/strongest London, but least polar bond/weakest dipole-dipole and no H-bonds London, dipole- dipole, H-bonds London, dipole- dipole

: Properties resulting from IMF 1.Viscosity: resistance of a liquid to flow 2.Surface tension: energy required to increase the surface area of a liquid

18 3. Cohesion: attraction of molecules for other molecules of the same compound 4. Adhesion: attraction of molecules for a surface

19 5.Meniscus: curved upper surface of a liquid in a container; a relative measure of adhesive and cohesive forces Ex: HgH2OH2O (cohesion rules)(adhesion rules)

: Phase Changes Processes:  Endothermic: melting, vaporization, sublimation  Exothermic: condensation, freezing, deposition I 2 (s) and (g)Microchip

21 Water: Enthalpy diagram or heating curve

: Vapor pressure A liquid will boil when the vapor pressure equals the atmospheric pressure, at any T above the triple point. wf Pressure cooker ≈ 2 atm Normal BP = 1 atm 10,000’ elev ≈ 0.7 atm 29,029’ elev (Mt. Everest) ≈ 0.3 atm

: Phase diagrams: CO 2  Lines: 2 phases exist in equilibrium  Triple point: all 3 phases exist together in equilibrium (X on graph)  Critical point, or critical temperature & pressure: highest T and P at which a liquid can exist (Z on graph)  For most substances, inc P will cause a gas to condense (or deposit), a liquid to freeze, and a solid to become more dense (to a limit.) Temp (ºC)

24 Phase diagrams: H 2 O For H 2 O, inc P will cause ice to melt.

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: Structures of solids  Amorphous: without orderly structure Ex: rubber, glass  Crystalline: repeating structure; have many different stacking patterns based on chemical formula, atomic or ionic sizes, and bonding

Types of crystalline solids (Table 11.6) TypeParticlesForces Notable properties Examples AtomicAtoms London dispersion  Poor conductors  Very low MP Ar (s), Kr (s)

Molecular Molecules (polar or non-polar) London dispersion, dipole- dipole, H- bonds  Poor conductors  Low to moderate MP CO 2 (s), C 12 H 22 O 11, H 2 O (s) Sucrose Carbon dioxide (dry ice) Ice

Ionic Anions and cations Electrostatic attractions  Hard & brittle  High MP  Poor conductors  Some solubility in H 2 O NaCl, Ca(NO 3 ) 2

Covalent (a.k.a. covalent network) Atoms bonded in a covalent network Covalent bonds  Very hard  Very high MP  Generally insoluble  Variable conductivity C (diamond & graphite) SiO 2 (quartz) Ge, Si, SiC, BN DiamondGraphiteSiO 2

Metallic Metal cations in a diffuse, delocalized e - cloud Metallic bonds  Excellent conductors  Malleable  Ductile  High but wide range of MP Cu, Al, Fe