Intermolecular Attractions & the Properties of Liquids & Solids CHAPTER 12

1. Intermolecular Attractions & the Properties of Liquids & Solids CHAPTER 12 Chemistry: The Molecular Nature of Matter, 6th edition By Jesperson, Brady, & Hyslop

2. CHAPTER 12 Intermolecular Attractions & the Properties of Liquids & Solids • Understand, describe, and rank in order of strength the types of intermolecular forces. • Difference between bonds and intermolecular forces • Changes of state: heat of vaporization, fusion, & sublimation • Clausius-Clapyron equation • Heating and cooling curves: ΔH, phase transition temperatures • Phase diagrams • Solids: Unit cell, stoichiometry, packing patterns, XRD, common types and their properties Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

3. CHAPTER 12 Intermolecular Attractions & the Properties of Liquids & Solids Lecture Road Map: Properties of gas, liquids, solids Intermolecular forces Changes of state Dynamic Equilibrium Structure & Characterization of a solid Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

4. CHAPTER 12 Intermolecular Attractions & the Properties of Liquids & Solids Properties of gases, liquids, & solids Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

5. Intermolecular Forces Important differences between gases, solids, and liquids: • Gases • Expand to fill their container • Liquids • Retain volume, but not shape • Solids • Retain volume and shape Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

6. Intermolecular Forces • Physical state of molecule depends on • Average kinetic energy of particles • Recall KE  Tave • Intermolecular Forces • Energy of Inter-particle attraction • Physical properties of gases, liquids and solids determined by • How tightly molecules are packed together • Strength of attractions between molecules

7. Intermolecular Attractions • Converting gas liquid or solid • Molecules must get closer together • Cool or compress • Converting liquid or solid gas • Requires molecules to move farther apart • Heat or reduce pressure • As T decreases, kinetic energy of molecules decreases • At certain T, molecules don’t have enough energy to break away from one another’s attraction

8. Inter vs. Intra-Molecular Forces • Intramolecular forces • Covalent bonds within molecule • Strong • Hbond(HCl) = 431 kJ/mol • Intermolecular forces • Attraction forces between molecules • Weak • Hvaporization(HCl) = 16 kJ/mol Intermolecular attraction (weak) Covalent Bond (strong)

9. Electronegativity Review Electronegativity: Measure of attractive force that one atom in a covalent bond has for electrons of the bond

10. Bond Dipoles • Two atoms with different electronegativity values share electrons unequally • Electron density is uneven • Higher charge concentration around more electronegative atom • Bond dipoles • Indicated with delta (δ) notation • Indicates partial charge has arisen

11. Net Dipoles • Symmetrical molecules • Even if they have polar bonds • Are non-polarbecause bond dipoles cancel • Asymmetrical molecules • Are polarbecause bond dipoles do not cancel • These molecules have permanent, net dipoles • Molecular dipoles • Cause molecules to interact • Decreased distance between molecules increases amount of interaction

12. POLAR COVALENT BOND COVALENT BOND IONIC BOND ✔ CHCl3 TiO2 ✔ ✔ F2 CaBr2 ✔

13. Group Problem Identify the overall dipole moment for CHCl3 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

14. Group Problem Identify the overall dipole moment for these molecules: Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

15. Solubility LIKE DISSOLVES LIKE polar molecules dissolve in polar solvents nonpolar molecules dissolve in nonpolar solvents Polar Solvents Water: H2O Methanol: CH3OH Ethanol: CH3CH2OH Acetone: (CH3)2CO Acetic Acid: CH3CO2H Ammonia: NH3 Acetonitrile: CH3CN Nonpolar Solvents Pentane: C5H12 Hexane: C6H14 Cyclohexane: C6H12 Benzene: C6H6 Toluene: CH3C6H5 Chloroform: CHCl3 Diethylether: (CH3CH2)2O

16. Which molecule will dissolve in water? Group Problem Vitamin A Vitamin B12 16

17. CHAPTER 12 Intermolecular Attractions & the Properties of Liquids & Solids Intermolecular Forces Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

18. Intermolecular Forces The forces of attraction or repulsion between neighboring particles (atoms or molecules). + / - charges attract one another - / - or + / + forces repel each other KE r r

19. Intermolecular Forces • When substance melts or boils • Intermolecular forces are broken, not covalent bonds • Responsible for non-ideal behavior of gases • Responsible for existence of condensed states of matter • Responsible for bulk properties of matter • Boiling points and melting points reflect strength of intermolecular forces

20. Types of Intermolecular Forces • London dispersion forces • Dipole-dipole forces • Hydrogen bonds • Ion-dipole forces • Ion-induced dipole forces

21. London-Dispersion Forces • When atoms near one another, their valence electrons interact • Repulsion causes electron clouds in each to distort and polarize • Instantaneous dipoles result from this distortion • Effect enhanced with increased volume of electron cloud size • Effect diminished by increaseddistance between particles and compact arrangement of atoms

22. London Dispersion Forces Affects ALL molecules, both polar & nonpolar Boiling Point (BP) is an indication of relative intermolecular force strength. Ease with which dipole moments can be induced and thus London Forces depend on • Distance between particles • Polarizabilityof electron cloud • Points of attraction • Number atoms • Molecular shape (compact or elongated)

23. Polarizability = Ease with which the electron cloud can be distorted • Larger molecules often more polarizable • Larger number of less tightly held electrons • Magnitude of resulting partial charge is larger • Larger electron cloud

24. Group Problem Which is more polarizable? F2 or I2?

25. Table 12.1 Boiling Points of Halogens and Noble Gases Larger molecules have stronger London forces and thus higher boiling points.

26. Number of Atoms in Molecule • London dispersion forces increase with the number atoms in molecule because more points of attraction

27. Which of the following molecules will have the highest boiling point? Group Problem Hexane, C6H14 BP 68.7 °C Propane, C3H8 BP –42.1 °C

28. Molecular Shape • Increased surface area available for contact = increased points of contact = increase in London Dispersion forces. • More compactmolecules: Less surface area to interact with other molecules • Less compact molecules: More surface area to interact with other molecules

29. Small area for interaction Larger area for interaction More compact – lower BP Less compact – higher BP

30. Group Problem Which of the following molecules experience the strongest Dispersion forces?

31. Types of Intermolecular Forces • London dispersion forces • Dipole-dipole forces • Hydrogen bonds • Ion-dipole forces • Ion-induced dipole forces

32. Dipole-Dipole Attractions • Occurs only between polar molecules • Proportional to distance between molecules • Polar molecules tend to align their partial charges: + / - • As dipole moment increases, intermolecular force increases +  +   +  + +  + 

33. Dipole-Dipole Attractions Tumbling molecules • Mixture of attractive and repulsive dipole-dipole forces • Attractions (- -) are maintained longer than repulsions(- -) • Get net attraction • ~1–4% of covalent bond

34. Group Problem In the liquid state, which species has the strongest intermolecular forces, CH4, Cl2, O2 or HF? HF The polar molecule

35. Types of Intermolecular Forces • London dispersion forces • Dipole-dipole forces • Hydrogen bonds • Ion-dipole forces • Ion-induced dipole forces

36. Hydrogen Bonds • Very strong dipole-dipole attraction: ~10% of a covalent bond • Occurs between H and highly electronegative atom (O, N, or F): H—F, H—O, and H—N bonds very polar • Electrons are drawn away from H giving atoms high partial charges • H only has one electron, so +H presents almost bare proton • –X almostfull –1 charge • Element’s small size, means high charge density

37. Examples of Hydrogen Bonding

38. Hydrogen Bonding in Water Hydrogen Bonds are strong! • Responsible for the high boiling point of water • Responsible for expansion of water as it freezes • Hydrogen bonding (dotted lines) betweenwater molecules in ice form tetrahedral configuration

39. Hydrogen Bonding in Water 1.97 Å 0.957 Å

40. Group Problem List all intermolecular forces for CH3CH2OH. Hydrogen-bonds, dipole-dipole attractions, London dispersion forces

41. Types of Intermolecular Forces • London dispersion forces • Dipole-dipole forces • Hydrogen bonds • Ion-dipole forces • Ion-induced dipole forces

42. Ion-Dipole Attractions • Attractions between ion and charged end of polar molecules • Ions have full charges, increasing the attraction (a) Negative ends of water dipoles surround cation (b) Positive ends of water dipoles surround anion

43. AlCl3·6H2O • Positive charge of Al3+ ion attracts partial negative charges – on O of water molecules • Ion-dipole attractions hold water molecules to metal ion in hydrate • Water molecules are found at vertices of octahedron around aluminum ion Attractions between ion and polar molecules

44. Ion-Induced Dipole Attractions • Attractions between ion and dipole it induces on neighboring molecules • Depends on • Ion charge and • Polarizability of its neighbor • Attractions can be quite strong as ion charge is constant, unlike instantaneous dipoles of London-dispersion forces

45. Group Problem How many water molecules would be attracted to this molecule by Ion-Dipole interactions?

46. Group Problem List the intermolecular forces and rank in order of strength for the liquids of each molecule.

47. Group Problem Strongest • Ion-Dipole • Hydrogen Bonding • Dipole-Dipole • London Forces • Larger, longer, and therefore heavier molecules often have stronger intermolecular forces • Smaller, more compact, lighter molecules have generally weaker intermolecular forces Weakest

48. Intermolecular Forces and Temperature Decrease with increasing temperature • Increasing kinetic energy overcomes attractive forces • If allowed to expand, increasing temperature increases distance between gas particles and decreases attractive forces

49. Group Problem GROUP PROBLEM SET 12.1

50. CHAPTER 12 Intermolecular Attractions & the Properties of Liquids & Solids Diffusion Compressibility Surface Tension More properties of gases, liquids, & solids Melting Point Boiling Point Wetting Viscosity Retention of Volume & shape Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E