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

Transcription

1 1 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 NaCl crystals little cubes?

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3 3 Have studied INTRAmolecular forces the forces holding atoms together to form molecules. Now turn to forces between molecules INTERmolecular forces. INTER Forces between molecules, between ions, or between molecules and ions.

4 !! " 4 Na + Cl - in salt These are the strongest forces. Lead to solids with high melting temperatures. NaCl,, mp = 800 o C MgO,, mp = 2800 o C

5 #$% & ' 5 water -δ O dipole H H +δ Water is highly polar and can interact with positive ions to give ( ions in water.

6 #$% & ' 6 water -δ O dipole H H +δ Water is highly polar and can interact with positive ions to give ( ions in water.

7 #$% & ' 7 Many metal ions are hydrated. This is the reason metal salts dissolve in water.

8 #$% & ' 8 δ- H O H δ+ O O H δ+ Mg 2+ Na + Cs + δ- H δ- H H δ kj/mol -405 kj/mol -263 kj/mol Attraction between ions and dipole depends on ion charge and ion-dipole distance. Measured by H for M n+ + H 2 O --> > [M(H 2 O) x ] n+

9 9 ' ' Such forces bind molecules having permanent dipoles to one another.

10 10 ' ' Influence of dipole-dipole forces is seen in the boiling points of simple molecules. Compd Mol. Wt. Boil Point N C CO C Br o C ICl o C

11 ( "$ " 11 A special form of dipole-dipole attraction, which enhances dipole-dipole attractions. ) " "% * + +,

12 $ "$% δ +δ H-bond -δ

13 $ "$%.% - - -δ +δ 13 -δ H-bond

14 $ "$% # 14 -δ +δ -δ H-bond This H-bond H leads to the formation of NH + 4 and OH -

15 ( "$ " /, 15 H-bonding is especially strong in water because the O H O H bond is very polar there are 2 lone pairs on the O atom Accounts for many of water s unique properties.

16 ( "$ " /, 16 Ice has open lattice-like like structure. Ice density is < liquid. And so solid floats on water. Snow flake: /~atomic/snowcrystals/sno w3x.jpg

17 ( "$ " /, 17 Ice has open lattice-like like structure. Ice density is < liquid and so solid floats on water.

18 ( "$ " /, 18 H bonds ---> > abnormally high specific heat capacity of water (4.184 g/k mol). This is the reason water is used to put out fires, it is the reason lakes/oceans control climate, and is the reason thunderstorms release huge energy.

19 ( "$ " 19 H bonds leads to abnormally high boiling point of water. See Screen 13.7

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23 ( "$ "$"( H-bonding is especially strong in biological systems such as DNA. 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 23

24 Double helix of DNA 24 Portion of a DNA chain

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26 26 ')0!'+ #

27 ( "$ "$"( 27 Hydrogen bonding and base pairing in DNA. See Screen 13.6

28 ,12+ 3, '52''&,2 How can non-polar molecules such as O 2 and I 2 dissolve in water? 28 The water dipole + '52 a dipole in the O 2 electric cloud. Dipole-induced induced dipole

29 ,12+ 3, '52''&,2 Solubility increases with mass the gas 29

30 ,12+ 3, '52''&,2 30 Consider I 2 dissolving in alcohol, CH 3 CH 2 OH. -δ -δ R I-I O H +δ The alcohol temporarily creates or INDUCES a dipole in I 2. -δ R I-I +δ O H +δ

31 ,12+ 3, '52''&,2 31 Formation of a dipole in two nonpolar I 2 molecules. Induced dipole- induced dipole

32 ,12+ 3, '52''&,2 The induced forces between I 2 molecules are very weak, so solid I 2 sublimes (goes from a solid to gaseous molecules). 32

33 ,12+ 3, '52''&,2 The magnitude of the induced dipole depends on the tendency to be distorted. Higher molec.. weight ---> > larger induced dipoles. Molecule CH 4 (methane) C 2 H 6 (ethane) C 3 H 8 (propane) C 4 H 10 (butane) Boiling Point ( o C) 33

34 $"&!( ) 34 C 4 H 10 C 3 H 8 C 2 H 6 CH 4 Note linear relation between bp and molar mass.

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37 37 6 In a liquid molecules are in constant motion there are appreciable intermolec.. forces molecules close together Liquids are almost incompressible Liquids do not fill the container

38 38 The two key properties we need to describe are EVAPORATION and its opposite CONDENSATION evaporation > LIQUID Add energy VAPOR break IM bonds make IM bonds Remove energy <---condensation

39 To evaporate, molecules must have sufficient energy to break IM forces. Breaking IM forces requires energy. The process of evaporation is endothermic.

40 7 ')!2" 40 lower T higher T Number of molecules 0 See Figure Molecular energy Distribution of molecular energies in a liquid. KE is propor- tional to T. Minimum energy req d to break IM forces and evaporate

41 ')!2"( 41 Figure 13.12

42 Number of molecules 0 lower T higher T Molecular energy minimum energy needed to break IM forces and evaporate At higher T a much larger number of molecules has high enough energy to break IM forces and move from liquid to vapor state. High E molecules carry away E. You cool down when sweating or after swimming. 42

43 When molecules of liquid are in the vapor state, they exert a VAPOR PRESSURE 29 5$15-3#&,1&12512 is the pressure exerted by a vapor over a liquid in a closed container when the rate of evaporation = the rate of condensation. 43

44 44 2) 3& Liquid in flask evaporates and exerts pressure on manometer. See Fig

45 3& CD, Screen

46 2) 3& " : 46

47 2) 3& 47 FIGURE 13.16: VP as a function of T. 1. The curves show all conditions of P and T where LIQ and VAP are in 29 5$15-2. The VP rises with T. 3. When VP = external P, the liquid boils. This means that BP s of liquids change with altitude.

48 48 $" Liquid boils when its vapor pressure equals atmospheric pressure.

49 49 $"& %& When pressure is lowered, the vapor pressure can equal the external pressure at a lower temperature.

50 !3 &" 50 When can cools, vp of water drops. Pressure in the can is less than that of atmosphere, so can is crushed.

51 " :;3&8. 4. If external P = 760 mm Hg, T of boiling is the +,1- #$,+ 4&,+. 5. VP of a given molecule at a given T depends on IM forces. Here the VP s are in the order 51 ether O H 5 C 2 C 2 H 5 dipoledipole alcohol O H 5 C 2 H H-bonds water O H H extensive H-bonds increasing strength of IM interactions

52 52 2#.,3#&,1<#.,+ is the heat req d (at constant P) to vaporize the liquid. LIQ + heat ---> > VAP Compd. H vap (kj/mol) IM Force H 2 O 40.7 (100 o C) H-bonds SO (-47( o C) dipole Xe 12.6 (-107( o C) induced dipole

53 53 Molecules at surface behave differently than those in the interior. Molecules at surface experience net INWARD force of attraction. This leads to SURFACE TENSION the energy req d to break the surface.

54 54!. SURFACE TENSION also leads to spherical liquid droplets.

55 55 Intermolec.. forces also lead to CAPILLARY action and to the existence of a concave meniscus for a water column. concave meniscus H 2 O in glass tube #'232,12 )% % ",232,12 )% %

56 56 (# Movement of water up a piece of paper depends on H-bonds H between H 2 O and the OH groups of the cellulose in the paper.

57 ".( 57 PROBLEM Al has density = g/cm 3 and Al radius = 143 pm. Verify that Al is FCC. SOLUTION 1. Calc. unit cell volume V = (cell edge) 3 Edge distance comes from face diagonal. Diagonal distance = 2 edge

58 ".( 58 PROBLEM Al has density = g/cm 3 and Al radius = 143 pm. Verify that Al is FCC. SOLUTION V = (cell edge) 3 and face diagonal = 2 edge (Diagonal) 2 = 2 (edge) 2 Therefore, Diag = 2 (edge)

59 ".( 59 PROBLEM Al has density = g/cm 3 and Al radius = 143 pm. Verify that Al is FCC. SOLUTION Here diagonal = 4 radius of Al = 572 pm Therefore, edge = 572 pm / 2 = 404 pm In centimeters, edge = 4.04 x 10-8 cm So, V of unit cell = (4.04 x 10-8 cm) 3 V = 6.62 x cm 3

60 ".( 60 PROBLEM Al has density = g/cm 3 and Al radius = 143 pm. Verify that Al is FCC. SOLUTION 2. Use V and density to calc. mass of unit cell from DENS = MASS / VOL Mass = density volume = (6.62 x cm 3 )(2.699 g/cm 3 ) = 1.79 x g/unit cell

61 ".( 61 PROBLEM Al has density = g/cm 3 and Al radius = 143 pm. Verify that Al is FCC. SOLUTION 3. Calculate number of Al per unit cell from mass of unit cell. Mass 1 Al atom = g mol 1 atom = x g, so 1 mol x atoms 1.79 x g unit cell 1 atom x g = 3.99 Al atoms/unit cell

62 + )!# 5 62 How can there be 4 atoms in a unit cell? 1. Each corner Al is 1/8 inside the unit cell. 8 corners (1/8 Al per corner) = 1 net Al 2. Each face Al is 1/2 inside the cell 6 faces (1/2 per face) = 3 net Al s