You Ever Ask Why is water usually a

You Ever Ask? ? ? • Why is water usually a liquid and not a gas? • Why does liquid water boil at such a high temperature for such a small molecule? • Why does ice float on water? • Why do snowflakes have 6 sides? • Why is I 2 a solid whereas Cl 2 is a gas? • Why are Na. Cl crystals little cubes? 1

Chapter 11: Liquids, Solids and Intermolecular Forces 2

Chapter Topics Chapter Objectives • Kinetic-Molecular Description of Liquids & Solids • Intermolecular (IMF) Attractions & Phase Changes • Properties of Liquids – Viscosity, Surface Tension, Capillary Action – Evaporation, Vapor Pressure, – Boiling Points & Distillation, Heat transfer • Phase Changes The following will NOT be covered in class Types of crystals 3

States of Matter • List all the differences between – Solids – Liquids – Gases • Kinetic Energy? 4

States of Matter (KE of Matter) The fundamental difference between states of matter is the distance between particles. Intermolecular attractions in liquids & solids are strong: KE of molecules << IMF 5

States of Matter Because in the solid and liquid states particles are closer together, we refer to them as condensed phases (depends on T and P). 6

The States of Matter • The state a substance is in at a particular temperature and pressure depends on two antagonistic entities: – The kinetic energy of the particles – The strength of the attractions between the particles 7

Examples 1. What is the difference between intermolecular forces and intramolecular forces? 2. List all the intermolecular forces you are familiar with. 3. List all the intramolecular forces you are familiar with. 4. Which forces are stronger? 8

Intermolecular Forces The attractions between molecules are not nearly as strong as the intramolecular attractions that hold compounds together. Intermolecular forces are weaker than intramolecular forces ( to break 2 O-H in bonds in water: 930 k. J/mol; to vaporize water: 43 k. J/mol) Responsible for the existence of condensed states (liquids, solids) 9

Intermolecular Forces They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, surface tension, and viscosities (reflect the strength of the bond). INTRAmolecular forces—the forces that holds atoms together o form molecules INTERmolecular forces: the forces between molecules, ions and molecules-ions. 10

Intermolecular Forces These intermolecular forces as a group are referred to as van der Waals forces (electrostatic forces). 11

IMF Problem For each of the molecules below, Determine the geometry of the molecule Determine the polarity of the molecule List the types of intermolecular force which act between pairs of these molecules. (a) CH 4 (b) PF 3 (c) CO 2 (d) HCN, (e) HCOOH (methanoic acid) 12

Types of IMF • Ion – Ion • Van der Waals Forces Dipole – dipole (for molecules with dipole moments) Dipole – induced dipole Dispersion forces (London) Hydrogen Bond (special case of dipole-dipole (IMF) London Dispersion Forces (induced dipole-dipole) • Ion – induced dipole • Ion – dipole • Total attraction between molecules may depend on more than one type of intermolecular force. 13

Ion – Ion Forces 08 M 16 VD 1 14

Ion-Ion Forces • Ion-ion forces: electrostatic forces of attraction between _________ of ionic compounds. Generally very strong → 250 k. J. (not a true intermolecular force) • Ionic compounds: metal and nonmetal or polyatomic anions (NH 4+) • Coulomb’s law & the attraction energy determine: – melting & boiling points of ionic compounds – the solubility of ionic compounds 15

IMF: Ionic Solids • Ion-ion interactions – force of attraction between two oppositely charged ions is determined by Coulomb’s law • Energy of attraction between two ions is given by: D: MediaMovies8 M 17 AN 1. MOV 17

Ion-Ion Forces for comparison of magnitude Na+—Cl- in salt These are the strongest forces. Lead to solids with high melting temperatures. Na. Cl (lattice energy = 788 k. J/mol), mp = 800 o. C Mg. O (lattice energy = 3890 k. J/mol) , mp = 2800 o. C 18

Covalent Bonding Forces for comparison of magnitude C=C, 610 k. J/mol C–C, 346 k. J/mol C–H, 413 k. J/mol CN, 887 k. J/mol 19

Ion – Dipole Force 20

Ion-Dipole Interactions • Ion-dipole interactions are an important force in solutions of ions. • The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents. Na+(g) + 6 H 2 O(l)→ [Na(H 2 O)6]+(aq) Hydrated Ions? ΔHrxn = -405 k. J Coordination Number? 21

Attraction Between Ions and Permanent Dipoles 22

Attraction Between Ions and Permanent Dipoles Attraction between ions and dipole depends on ion charge and ion-dipole distance. Measured by ∆H for Mn+ + H 2 O → [M(H 2 O)x]n+ -1922 k. J/mol -405 k. J/mol -263 k. J/mol 23

Dipole – Dipole Force D: MediaMovies13 M 04 AN 2. MOV 24

Dipole-Dipole Forces Molecules that have permanent dipoles are attracted to each other. The positive end of one is attracted to the negative end of the other and vice-versa. These forces are only important when the molecules are close to each other. Note the difference between solid and liquid. Liquid liquid Solid 25

Effect of Dipole Moment on BP Substance MM BP, K 44 Dipole moment 0. 1 C 3 H 8 46 1. 3 248 44 2. 7 294 41 3. 9 355 231 propane CH 3 OCH 3 dimethyl ether CH 3 CHO Acetyl aldehyde CH 3 CN acetonitrile 26

Dipole-Dipole Interactions The more polar the molecule (higher μ), the higher is its boiling point. • Basic attraction : electrostatic, Coulomb’s Law Examples: HCl, CO, SO 2, NF 3, etc 27

IMF: Dipole-Dipole • Dipole-dipole are of the order of 5 to 20 k. J/mol. (KE due to temp at 25 o. C about 4 k. J/mol). Cmpds that have these forces (dipole-dipole) are frequently solids and liquids at room temp. • The stronger the forces, the ______ the melting and boiling points of the compounds. 28

Hydrogen Bond: type of dipole-dipole force 13 M 07 AN 2 29

Boiling Points of Simple Hydrogen-Containing Compounds The nonpolar series (Sn. H 4 to CH 4) follow the expected trend. The polar series follows the trend from H 2 Te through H 2 S, but water is quite an anomaly. EXPLAIN! 30

Intermolecular Forces: H-bond Which of these are capable of forming hydrogen bonds among themselves? a) b) c) d) e) f) g) CH 3 OH C 2 H 4 CH 3 NH 2 HCN NH 4+ KF CH 3 COOH 31

Hydrogen Bonding • The dipole-dipole interactions experienced when H is bonded to N, O, or F (HIGH ELECTRONEGATIVITY) are unusually strong. • Hydrogen nucleus is exposed. • We call these interactions hydrogen bonds. 32

Hydrogen Bonding 33

H-Bonding Between Methanol and Water - H-bond + - 34

Hydrogen Bonding in H 2 O H-bonding is especially strong (40 k. J/mol) in water because • the O—H bond is very polar • there are 2 lone pairs on the O atom Accounts for many of water’s (and other molecules such as DNA, proteins) unique properties such as anomalous high BP and high viscosity. 35

Hydrogen Bonding in H 2 O Ice has open lattice-like structure. Ice density is < liquid. And so solid floats on water. Snow flake: www. snowcrystals. com 36

Hydrogen Bonding in H 2 O Ice has open lattice-like structure. Ice density is < liquid and so solid floats on water. One of the VERY few substances where solid is LESS DENSE than the liquid. 37

Hydrogen Bonding H bonds leads to abnormally high boiling point of water. D: MediaMovies13 M 07 AN 1. MOV See Screen 13. 7 38

Boiling Point of Hydrides in ºC Group VIA Group VIIA Group IVA Group VA H 2 O 100 HF 20 NH 3 -33 CH 4 -161 H 2 S -65 HCl -85 PH 3 -87 Si. H 4 -112 H 2 Se -45 HBr -69 As. H 3 -60 Ge. H 4 -90 H 2 Te -15 HI -35 Sb. H 3 -25 39

Hydrogen Bonding in Biology H-bonding is especially strong in biological systems — such as proteins and DNA. D: MediaMovies9 S 03 AN 1. MOV DNA — helical chains of phosphate groups and sugar molecules. Chains are helical because of tetrahedral geometry of P, C, and O. Chains bind to one another by specific hydrogen bonding between pairs of Lewis bases. —adenine with thymine —guanine with cytosine 40

Double helix of DNA Portion of a DNA chain 41

Base-Pairing through H-Bonds 42

Induced Dipole –Induced Dipole (London Dispersion Forces) 43

London Dispersion Forces While the electrons in the 1 s orbital of helium would repel each other (and, therefore, tend to stay far away from each other), it does happen that they occasionally wind up on the same side of the atom. 44

London Dispersion Forces Instantaneous dipole The helium atom becomes polar, with an excess of electrons on the left side and a shortage on the right side. Instantenous dipole forms (for an instant) 45

London Dispersion Forces Another helium nearby, then, would have a dipole induced in it, as the electrons on the left side of helium atom 2 repel the electrons in the cloud on helium atom 1. 46

London Dispersion Forces London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole. 47

London Dispersion Forces • These forces are present in all molecules, whether they are polar or nonpolar. • The tendency of an electron cloud to distort in this way is called POLARIZABILITY. 48

Forces Involving Dipole -Induced Dipole • Process of inducing a dipole is polarization • Degree to which electron cloud of an atom or molecule can be distorted in its polarizability. 49

IMF: London Dispersion Forces • Induced Dipoles: the temporary separation of positive and negative charges in a neutral particle due to the proximity of an ion, dipole, or another induced dipole. On average μ = 0. • London Dispersion Forces: attractive forces (electrostatic in origin) that arise as a result of temporary dipoles induced in atoms or molecules (instantaneous dipoles). Weak: 0. 1 - 5 k. J/mol • Dispersion forces allow non-polar molecules to condense. • Exist in all molecules!!!!! Importance depends on the type of intermolecular forces. 50

IMF: London Dispersion Forces • London Forces very weak only attractive force in nonpolar molecules Ar atom Cluster of Ar atoms 51

Factors Affecting London Forces • The shape of the molecule affects the strength of dispersion forces: long, skinny molecules (like n-pentane tend to have stronger dispersion forces than short, fat ones (like neopentane). • This is due to the increased surface area in n-pentane. 52

Effect of Geometry (shape) on BP of Molecules Compound N-butane Isobutane MM 58 58 N-pentane 72 2 -methyl butane 72 2, 2 methyl propane 72 BP, ºC -0. 45 -12. 0 36. 1 27. 8 9. 5 • Given the same molecular mass, GEOMETRY is important 53

Factors Affecting London Forces • The strength of dispersion forces tends to increase with increased molecular weight. • Larger atoms have larger electron clouds, which are easier to polarize (larger polarizability. 54

Boiling Points of Hydrocarbons Molecule CH 4 (methane) C 2 H 6 (ethane) C 3 H 8 (propane) C 4 H 10 (butane) MM 16 38 44 58 BP (o. C) - 161. 5 - 88. 6 - 42. 1 - 0. 5 C 4 H 10 Note: linear relationship between BP and MM (Polarizability increases). C 3 H 8 C 2 H 6 CH 4 55

Which Have a Greater Effect: Dipole-Dipole Interactions or Dispersion Forces? • If two molecules are of comparable size and shape, dipole-dipole interactions will likely be the dominating force. • If one molecule is much larger than another, dispersion forces will likely determine its physical properties. 56

Dipole – Induced Dipole 57

FORCES INVOLVING INDUCED DIPOLES How can non-polar molecules such as O 2 and I 2 dissolve in water? The water dipole INDUCES a dipole in the O 2 electric cloud. Dipole-induced dipole 58

Forces Involving Dipole -Induced DIPOLE Solubility increases with mass of the gas 59

Forces Involving Dipole -induced Dipole Consider I 2 dissolving in ethanol, CH 3 CH 2 OH. - I-I - O R H + I-I The alcohol temporarily creates or INDUCES a dipole in I 2. + - O R H + 60

Summary of Intermolecular Forces • Ion-dipole forces (very strong; solubility of ions in water) • Dipole-dipole forces (larger dipole moments) – Special dipole-dipole force: hydrogen bonds • Induced dipoles (occur in all substances; important for nonpolar molecules); increase with molar mass (glues); depend on geometry. For large molecules may exceed the force of dipole-dipole force (polymers, glues) In general: ionic forces the strongest; then hydrogen bonding; dipole-dipole; and lastly dispersion for species of similar molar mass. London Dispersion Forces exist in all molecules and ions. 61

Summarizing Intermolecular Forces 62

Summary of dipole forces 63

Intermolecular Forces Summary 64

Intermolecular Forces Figure 13. 13 65

Example What type of intermolecular forces exist between the following pairs? 1. HBr and H 2 S 2. Cl 2 and CBr 4 3. I 2 and NO 34. NH 3 and C 6 H 6 1. Dipole-dipole; dispersion; 2. dispersion; 3. ion-induced dipole; dispersion 4. dipole-induced dipole; dispersion 66

Intermolecular Forces: Examples 5. Order the following compounds in order of increasing MP and BP: a) N 2, O 2, H 2 b) Cl 2, F 2, I 2, Br 2 c) Si. H 4, Ge. H 4, Sn. H 4, CH 4 Order of MP and BP: (a) H 2< N 2<O 2 (b) F 2<Cl 2<Br 2<I 2 (c) CH 4<Si. H 4<Ge. H 4<Sn. H 4 • Always compare like species. 6. 7. 8. But what about HF, HCl, HBr, HI and H 2 O, H 2 Se, H 2 Te and NH 3, PH 3, As. H 3 and Sb. H 3. 67

Examples 1. List the IMF and arrange the substances Ba. Cl 2, H 2, CO, and Ne in order of increasing boiling points. 2. In which of the following substances is hydrogen bonding possible? a. b. c. d. e. f. Methane Methyl alcohol Hydrazine (H 2 NNH 2) Methyl fluoride Hydrogen sulfide Carboxylic acid 68

Intermolecular Forces: Examples 4. Arrange in order of increasing BP: CO 2 CH 3 OH CH 3 Br , Rb. F 5. Which one in each pair has the higher BP? a) b) c) d) e) CH 4 and C 2 H 6 H 2 S and H 2 Te NH 3 and PH 3 HCl and HF I 2 and ICl 69

Example (London Forces) 6. Arrange the following molecules in order of increasing strength of intermolecular forces. F 2 Br 2 Cl 2 I 2 7. Explain the trend in the normal boiling points of these liquids in terms of intermolecular forces. CH 4 normal boiling point: -161. 5°C CF 4 normal boiling point: -28°C CCl 4 normal boiling point: +77°C CBr 4 normal boiling point: +190°C 70

Intermolecular Forces: Examples 8. (11. 100) Which of the following substances has the highest polarizability? CH 4, H 2, CCl 4, SF 6, H 2 S 71

Identify “intermolecular” forces 9. H 2 O 10. CH 2 Cl 2 11. KBr 12. F- + H 2 O 13. I 2 14. CH 3 OH 15. PCl 3 16. C 6 H 6 17. Fe 18. CS 2 19. BCl 3 20. Na+ + NH 3 21. Dimethyl ether (CH 3 OCH 3) and ethanol (C 2 H 5 OH) have the same formula (C 2 H 6 O) but the BP of the ether is -25ºC and of the ethanol 78ºC. Explain. 72