Intermolecular Forces, Liquids, & Solids

Transcription

1 , Liquids, & Solids Mr. Matthew Totaro Legacy High School AP Chemistry

2 States of Matter The fundamental difference between states of matter is the distance between particles.

3 States of Matter Because in the solid and liquid states particles are closer together, we refer to them as condensed phases.

4 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

5 The attractions between molecules are not nearly as strong as the intramolecular attractions that hold compounds together.

6 They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities.

7 Why Are Molecules Attracted to Each Other? attractions are due to attractive forces between opposite charges + ion to ion + end of polar molecule to end of polar molecule H-bonding especially strong even nonpolar molecules will have temporary charges Larger charge = stronger attraction Longer distance = weaker attraction However, these attractive forces are small relative to the bonding forces between atoms generally smaller charges generally over much larger distances 7

8 Trends in the Strength of Attraction The stronger the attractions between the atoms or molecules, the more energy it will take to separate them Boiling a liquid requires we add enough energy to overcome all the attractions between the particles However, not breaking the covalent bonds The higher the normal boiling point of the liquid, the stronger the intermolecular attractive forces 8

9 These intermolecular forces as a group are referred to as van der Waals forces.

10 van der Waals Dipole-dipole interactions Hydrogen bonding London dispersion forces

11 Dipole-Dipole Interactions 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 viceversa. These forces are only important when the molecules are close to each other.

12 Dipole-Dipole Interactions The more polar the molecule, the higher is its boiling point.

13 Effect of Dipole Dipole Attraction on Boiling and Melting Points 13

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15 Practice Choose the substance in each pair with the higher boiling point a) CH 2 FCH 2 F CH 3 CHF 2 b) or 15

16 Practice Choose the substance in each pair with the higher boiling point a) CH 2 FCH 2 F CH 3 CHF 2 more polar b) polar or nonpolar Tro: Chemistry: A Molecular Approach, 2/e

17 London Dispersion While the electrons in the 1s 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.

18 London Dispersion At that instant, then, the helium atom is polar, with an excess of electrons on the left side and a shortage on the right side.

19 London Dispersion 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.

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

21 London Dispersion 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.

22 Factors Affecting London 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.

23 Effect of Molecular Shape on Size of Dispersion Force the larger surface-tosurface contact between molecules in n-pentane results in stronger dispersion force attractions 23

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

25 Boiling Points of n-alkanes 25

26 Practice Choose the Substance in Each Pair with the Higher Boiling Point a) CH 4 C 4 H 10 b) C 6 H 12 C 6 H 12 26

27 Practice Choose the Substance in Each Pair with the Higher Boiling Point a) CH 4 CH 3 CH 2 CH 2 CH 3 Both molecules are nonpolar larger molar mass b) CH 3 CH 2 CH=CHCH 2 CH 3 cyclohexane 27 Both molecules are nonpolar, but the flatter ring molecule has larger surface-tosurface contact

28 Which Have a Greater Effect: Dipole-Dipole Interactions or Dispersion? 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.

29 How Do We Explain This? The nonpolar series (SnH 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.

30 Hydrogen Bonding The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong. We call these interactions hydrogen bonds.

31 Hydrogen Bonding Hydrogen bonding arises in part from the high electronegativity of nitrogen, oxygen, and fluorine. Also, when hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed.

32 Effect of H-Bonding on Boiling Point 32

33 Practice Choose the substance in each pair that is a liquid at room temperature (the other is a gas) a) CH 3 OH CH 3 CHF 2 can H-bond b) CH 3 -O-CH 2 CH 3 CH 3 CH 2 CH 2 NH 2 can H-bond 33

34 Ion-Dipole Interactions A fourth type of force, 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.

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36 Summarizing

37 Liquids Properties & Structure

38 Affect Many Physical Properties The strength of the attractions between particles can greatly affect the properties of a substance or solution.

39 Viscosity Resistance of a liquid to flow is called viscosity. It is related to the ease with which molecules can move past each other. Viscosity increases with stronger intermolecular forces and decreases with higher temperature.

40 Surface Tension Surface tension results from the net inward force experienced by the molecules on the surface of a liquid.

41 Capillary Action The adhesive forces pull the surface liquid up the side of the tube, and the cohesive forces pull the interior liquid with it The liquid rises up the tube until the force of gravity counteracts the capillary action forces The narrower the tube diameter, the higher the liquid will rise up the tube 41

42 Meniscus The curving of the liquid surface in a thin tube is due to the competition between adhesive and cohesive forces The meniscus of water is concave in a glass tube because its adhesion to the glass is stronger than its cohesion for itself The meniscus of mercury is convex in a glass tube because its cohesion for itself is stronger than its adhesion for the glass metallic bonds are stronger than intermolecular attractions

43 Phase Changes

44 Energy Changes Associated with Changes of State Heat of Fusion: Energy required to change a solid at its melting point to a liquid.

45 Energy Changes Associated with Changes of State Heat of Vaporization: Energy required to change a liquid at its boiling point to a gas.

46 Energy Changes Associated with Changes of State The heat added to the system at the melting and boiling points goes into pulling the molecules farther apart from each other. The temperature of the substance does not rise during the phase change.

47 Vapor Pressure At any temperature, some molecules in a liquid have enough energy to escape. As the temperature rises, the fraction of molecules that have enough energy to escape increases.

48 Vapor Pressure As more molecules escape the liquid, the pressure they exert increases.

49 Vapor Pressure The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate.

50 Vapor Pressure The boiling point of a liquid is the temperature at which its vapor pressure equals atmospheric pressure. The normal boiling point is the temperature at which its vapor pressure is 760 torr.

51 Solids We can think of solids as falling into two groups: Crystalline particles are in highly ordered arrangement.

52 Solids Amorphous no particular order in the arrangement of particles.

53 Types of Crystalline Solids

54 Attractions in Ionic Crystals In ionic crystals, ions pack themselves so as to maximize the attractions and minimize repulsions between the ions.

55 Crystalline Solids Because of the order in a crystal, we can focus on the repeating pattern of arrangement called the unit cell.

56 Crystalline Solids There are several types of basic arrangements in crystals, such as the ones shown above.

57 Crystalline Solids We can determine the empirical formula of an ionic solid by determining how many ions of each element fall within the unit cell.

58 Ionic Solids What are the empirical formulas for these compounds? (a) Green: chlorine; Gray: cesium (b) Yellow: sulfur; Gray: zinc (c) Green: calcium; Gray: fluorine (a) (b) (c) CsCl ZnS CaF 2

59 Types of Bonding in Crystalline Solids

60 Covalent-Network and Molecular Solids Diamonds are an example of a covalent-network solid in which atoms are covalently bonded to each other. They tend to be hard and have high melting points.

61 Covalent-Network and Molecular Solids Graphite is an example of a molecular solid in which atoms are held together with van der Waals forces. They tend to be softer and have lower melting points.

62 Metallic Solids Metals are not covalently bonded, but the attractions between atoms are too strong to be van der Waals forces. In metals, valence electrons are delocalized throughout the solid.

63 Metallic Structure

64 Metallic Bonding Metal atoms release their valence electrons Metal cation islands fixed in a sea of mobile electrons called the electron sea model e - e e - e- e- e- e - e - e - e- e- e - e - e- e - e

65 Practice Pick the solid in each pair with the highest melting point a) KCl SCl 2 b) C(s, diamond) S 8 c) Kr K d) SrCl 2 SiO 2 (s, quartz)

66 Practice Pick the solid in each pair with the highest melting point a) KCl ionic SCl 2 molecular b) C(s, diamond) cov. network S 8 molecular c) Kr atomic K metallic d) SrCl 2 ionic SiO 2 (s, quartz) cov. network