Liquids, Solids, and Intermolecular Forces or. Why your Water Evaporates and your Cheerios Don t. Why are molecules attracted to each other?

Transcription

1 Liquids, Solids, and Intermolecular Forces or Why your Water Evaporates and your heerios Don t Why are molecules attracted to each other? 1

2 Intermolecular attractions determine how tightly liquids and solids pack The strength of intermolecular attractions determine many physical properties: vapor pressure viscosity surface tension density melting point 4 2

3 5 3

4 Freedom of Motion the molecules in a gas have complete freedom of motion so kinetic energy overcomes the attractive forces the molecules in a solid are locked in place, they cannot move around though they do vibrate the molecules in a liquid have limited freedom they can move around a little within the structure - have enough kinetic energy to overcome some of the attractive forces, but not enough to escape each other Properties of the 3 Phases of Matter State Shape Volume ompressible Flow Strength of Intermolecular l Attractions Solid Fixed Fixed No No very strong Liquid Indef. Fixed No Yes moderate Gas Indef. Indef. Yes Yes very weak Fixed = keeps shape when placed in a container Indefinite = takes the shape of the container 4

5 Kinetic - Molecular Theory the properties explained based on the kinetic energy of the molecules and the attractive forces bt between them kinetic energy is proportional freedom of motion degrees of freedom = translational, rotational, vibrational attractive forces keep the molecules together kinetic energy depends only on the temperature KE = 1.5 kt Gas Structure Gas molecules generally don t stick to each other. Why not? 5

6 Explaining the Properties of Solids solid particles packed close together and fixed in position retain their shape and volume when placed in a new container; and prevents the particles from flowing incompressible crystalline solids salt and diamonds Solids amorphous solids rubber and window glass 6

7 Explaining the Properties of Liquids close contact higher densities than gases limited freedom of movement indefinite shape but definite volume (no escape) ompressibility 7

8 Phase hanges intermolecular attractions are due to attractive forces between opposite charges + ion to - ion + end of polar molecule to - end of polar molecule -bonding especially strong even nonpolar molecules will have temporary charges larger the charge = stronger attraction longer the distance = weaker attraction 8

9 these attractive ti forces are small relative lti to the ionic, covalent and metallic bonding forces between atoms generally smaller charges generally over much larger distances Dipole-dipole attractions Polar molecules tend to align their partial charges The attractive force is about 1% of a covalent bond and drops off as 1/d 3 (d=distance between dipoles) 9

10 ydrogen bonds (5-10% of covalent bond) Very strong dipole-dipole attraction when is covalently bonded to to a highly electronegative atom (F, O, or N) Typically about ten times stronger than other dipoledipole attractions Are responsible for the expansion of water as it freezes Partly responsible for twisting of proteins in a helix (DNA) ydrogen bonds 10

11 London or dispersion forces The (very) weak attractions between nonpolar molecules Arise from the interactions of instantaneous dipoles on neighboring molecules London forces depend on the number of atoms in the molecule The boiling point of hydrocarbons demonstrates this trend Formula BP at 1atm ( o Formula ) BP at 1atm ( 36.1 o ) 11

12 Trends in the Strength of Intermolecular 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 the attractions between the molecules or atoms the higher the normal boiling point of the liquid, the stronger the intermolecular attractive forces Attractive Forces

13 Dispersion Forces fluctuations in the electron distribution in atoms and molecules result in a temporary dipole region with excess electron density has partial ( ) charge region with depleted electron density has partial (+) charge the attractive forces caused by these temporary dipoles are called dispersion forces aka London Forces all molecules l and atoms will have them as a temporary dipole is established in one molecule, it induces a dipole in all the surrounding molecules Dispersion Force 13

14 Size of the Induced Dipole the magnitude of the induced dipole depends on several factors polarizability of the electrons volume of the electron cloud larger molar mass = more electrons = larger electron cloud = increased polarizability = stronger attractions shape of the molecule more surface-to-surface contact = larger induced dipole = stronger attraction Effect of Molecular Size on Size of Dispersion Force Noble As the Gases molar are mass all nonpolar increases, atomic the number elements. of electrons increase. Therefore the strength of the dispersion forces increases. The stronger the attractive forces between the molecules, the higher the boiling point will be. 14

15 Relationship between Induced Dipole and Molecular Size Boiling Point, BP, Noble Gas 100 BP, alogens 50 BP, X Period Properties of Straight hain Alkanes Non-Polar Molecules Name Molar Mass BP, MP, Density, g/ml Methane Ethane Propane Bt Butane Pentane exane eptane Octane Nonane Decane Undecane Dodecane Tridecane Tetradecane Pentadecane exadecane

16 Boiling Points of n-alkanes 500 n-alkane Boiling & Melting Points Temperature, BP, n- alkane Molar Mass 16

17 Effect of Molecular Shape on Size of Dispersion Force iso- branched chains have lower BPs than straight chains the straight chain isomers have more surface-tosurface contact Alkane Boiling Points 140 Alkane Boiling Points Tempe erature, n Molar Mass

18 Practice hoose the Substance in Each Pair with the ighest Boiling Point a) b) 3 2 = 2 3 cyclohexane Practice hoose the Substance in Each Pair with the ighest Boiling Point a) both molecules are nonpolar larger molar mass b) 3 2 = 2 3 cyclohexane both molecules are nonpolar flat molecule larger surface-to-surface contact 18

19 Dipole-Dipole Attractions polar molecules have a permanent dipole because of bond polarity and shape dipole moment as well as the always present induced dipole the permanent dipole adds to the attractive forces between the molecules raising the boiling and melting points relative to nonpolar molecules of similar size and shape Effect of Dipole-Dipole Attraction on Boiling and Melting Points 19

20 Molar Boiling Dipole Mass Point Size D 3 -O D 3 - =O D 3 - N D Practice hoose the Substance in Each Pair with the ighest Boiling Point a) 2F 2F 3F 2 F F F F b) or 20

21 Practice hoose the Substance in Each Pair with the ighest Boiling Point a) 2F 2F 3F 2 F F F F more polar b) polar or nonpolar Attractive Forces and Solubility Solubility depends on the attractive forces of solute and solvent molecules Like dissolves Like miscible i ibl liquids id will always dissolve in each other polar substance dissolve in polar solvents hydrophilic groups = O, O, =O, OO, N 2, l nonpolar molecules dissolve in nonpolar solvents h d h bi hydrophobic groups = -, - Many molecules have both hydrophilic and hydrophobic parts - solubility becomes competition between parts 21

22 Immiscible Liquids Polar Solvents Dichloromethane (methylene chloride) Ethanol Water (ethyl alcohol) 22

23 n-hexane Nonpolar Solvents 3 toluene l l l l carbon tetrachloride ydrogen Bonding When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it O-, N-, or F- Since hydrogen has no other electrons, when it loses the electrons, the nucleus becomes deshielded i th t exposing the proton The exposed proton acts as a very strong center of positive charge, attracting all the electron clouds from neighboring molecules 23

24 -Bonding F -Bonding in Water 24

25 Relationship between -bonding and Intermolecular Attraction 2 O BP, X BP, 2X BP, 3X Boilin Point, F 1 N S 2 Se4 5 Sn 4 Ge Si Period 2 Te BP, X4 Practice hoose the substance in each pair that is a liquid at room temperature (the other is a gas) a) 3 O 3 F 2 b) 3 -O N 2 25

26 Practice hoose the substance in each pair that is a liquid at room temperature (the other is a gas) a) 3 O 3 F 2 can -bond b) 3 -O N 2 can -bond Practice hoose the substance in each pair that is more soluble in water a) 3 O 3 F 2 b) l 26

27 Practice hoose the substance in each pair that is more soluble in water a) 3 O 3 F 2 can -bond with 2 O b) l more polar Ion-Dipole Attraction in a mixture, ions from an ionic compound are attracted to the dipole of polar molecules the strength of the ion-dipole attraction is one of the main factors that determines the solubility of ionic compounds in water 27

28 Summary Dispersion forces are the weakest of the intermolecular attractions. Dispersion forces are present in all molecules and atoms. The magnitude of the dispersion forces increases with molar mass Polar molecules also have dipole-dipole attractive forces Summary (cont d) ydrogen bonds are the second strongest of the intermolecular attractive forces a pure substance can have ydrogen bonds will be present when a molecule has directly bonded to either O, N, or F atoms only example of bonded to F is F Ion-dipole attractions are present in mixtures of ionic compounds with polar molecules. Ion-dipole attractions are the strongest intermolecular l attraction Ion-dipole attractions are especially important in aqueous solutions of ionic compounds 28

29 Liquids properties & structure 29

30 Surface Tension the tendency of liquids to minimize their surface area liquids minimize their surface area spherical as long as there is no gravity molecules of the surface behave differently molecules of interior because the cohesive forces on the surface molecules have a net pull into the liquid interior the surface layer acts like an elastic skin Surface Tension surface molecules have fewer neighbors to attract them the surface is less stable than the interior have a higher potential energy surface tension is the energy required to increase the surface area a given amount at room temp, surface tension of 2 O = 72.8 mj/m 2 30

31 Factors Affecting Surface Tension intermolecular attractive forces, surface tension temperature, surface tension raising the temperature of the liquid increases the average kinetic energy of the molecules the increased molecular motion makes it easier to stretch the surface 80 Surface Tension of Water vs. Temperature 75 Surface Tension, mj/m Temperature, 31

32 Viscosity viscosity is the resistance of a liquid to flow 1 poise = 1 P = 1 g/cm s often given in centipoise, cp intermolecular attractions = viscosity temperature = viscosity Viscosity of Water vs. Temperature Viscosity, cp Temperature, deg 32

33 apillary Action ability of a liquid to flow up a thin tube against the influence of gravity the narrower the tube, the higher the liquid rises results from two forces working in conjunction, the cohesive and adhesive forces cohesive forces attract the molecules together adhesive forces attract the molecules on the edge to the tube s surface apillary Action adhesive forces pull the surface liquid up the side of the tube, while 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 33

34 Meniscus meniscus 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 stronger than intermolecular attractions Vaporization molecules are constantly in motion average KE α T some molecules have more kinetic energy than the average molecules at the surface may have enough energy to overcome the attractive forces to become a gas therefore the larger the surface area, the faster the rate of evaporation 34

35 Distribution of Thermal Energy only a small fraction of the molecules in a liquid have enough energy to escape As temperature the fraction of the molecules with escape energy the higher the temperature, the faster the evaporation ondensation some molecules of the vapor will lose energy through molecular collisions some of the molecules will get captured back into the liquid when they collide with it some may stick and gather together to form droplets of liquid (e.g., rain, fog) particularly on surrounding surfaces 35

36 Evaporation vs. ondensation opposite processes in an open container, the vapor molecules generally spread out faster than they can condense the net result is that the rate of vaporization is greater than the rate of condensation, and there is a net loss of liquid however, in a closed container, the vapor is not allowed to spread out indefinitely in a closed container at some time rate of vaporization = rate of condensation Effect of Intermolecular Attraction on Evaporation and ondensation the weaker the attractive forces, the less energy they will need to vaporize also, weaker attractive forces means that more energy will need to be removed from the vapor molecules before they can condense the net result will be more molecules in the vapor phase, and a liquid that evaporates faster the weaker the attractive forces, the faster the rate of evaporation 36

37 liquids that evaporate easily are said to be volatile e.g., g,g gasoline, fingernail polish remover liquids that do not evaporate easily are called nonvolatile e.g., motor oil Energetics of Vaporization when the high energy molecules are lost from the liquid, it lowers the average kinetic energy of the liquid if energy is not drawn back into the liquid, its temperature will decrease therefore, vaporization is an endothermic process and condensation is an exothermic process vaporization requires input of energy to overcome the attractions between molecules 37