Intermolecular Forces, Liquids, and Solids

Transcription

1 Slide 1 / 92 Slide 2 / 92 Intermolecular Forces Intermolecular forces are the piece we need to add to the puzzle to explain the world around us. Intermolecular Forces, Liquids, and Solids We first explained atoms, and how to build up the periodic table from quantum numbers. Then we explained how atoms combine to form molecules: the most common way we find most atoms in nature. Now, we're going to use intermolecular forces to combine molecules to create the common states of matter. Without intermolecular forces, we wouldn't have tables, lakes, wall...or even our bodies. Intermolecular forces shape our world. Slide 3 / 92 States of Matter Slide 4 / 92 States of Matter The fundamental differences between states of matter is: While there are many states of matter, the three common states that dominate our world are gases, liquids and solids. These are the states of matter we'll be studying. We won't be discussing more exotic states such as plasma, nuclear matter, etc. the distance between particles the particles' freedom to move Particles are far apart, total freedom, much of empty space, total disorder cool or increase pressure heat or decrease pressure disorder, freedom, free to move relative to each other, close together cool heat Gas Liquid rystalline solid orderd arrangement, particles are in fixed positions, close together Slide 5 / 92 haracteristics of the States of Matter Slide 6 / 92 ondensed Phases Gas Liquid Solid ssumes the shape of its container xpands to the volume of its container Is compressible Flows easily iffusion within a gas is rapid ssumes the shape of the part of a container it occupies oes not expand to the volume of its container Is virtually incompressible Flows easily iffusion within a liquid is slow Retains its own shape, regardless of container oes not expand to the volume of its container Is virtually incompressible oes not flow iffusion within a solid is very very slow In the solid and liquid states particles are closer together, we refer to them as condensed phases. cool or increase pressure heat or decrease pressure Gas Liquid rystalline solid Particles are far apart, total freedom, much of empty space, total disorder disorder, freedom, free to move relative to each other, close together cool heat orderd arrangement, particles are in fixed positions, close together

2 Slide 7 / 92 1 Which of the following is not a type of solid? 2 Which of the below is a characteristic of a gas? Slide 8 / 92 ionic molecular covalentnetwork supercritical metallic It fills only a portion of its container. Its molecules are in relatively rigid positions. It takes on the shape of its container. It is not compressible. iffusion is very slow within it. Slide 9 / 92 3 Which of the below is a characteristic of a liquid? Slide 10 / 92 4 Which of the below is a characteristic of a solid? It fills only a portion of its container. It fills all of its container. Its molecules are in relatively rigid positions. Its molecules are in relatively rigid positions. It takes on the shape of its container. It takes on the shape of its container. It is compressible. It is compressible. iffusion is very rapid within it. iffusion is very rapid within it. Slide 11 / 92 Slide 12 / 92 5 Together, liquids and solids, constitute phases of matter. the compressible the fluid the condensed all of the above the disordered States of Matter The state of a substance at a particular temperature and pressure depends on two opposing properties: Intermolecular Forces: the strength of the attractions between the particles, which pulls them together the kinetic energy of the particles, which pulls them apart Without intermolecular forces, all molecules would be ideal gases...there would be no liquids or solids.

3 Slide 13 / 92 Intermolecular Forces & oiling Points oiling represents a transition from a liquid to a gas. To make that transition, molecules in the liquid must break free of the intermolecular forces that bind them. The kinetic energy of the molecules is proportional to the temperature: as temperature rises, so does kinetic energy. The temperature where the molecules' energy overcomes intermolecular forces is called the boiling point. The boiling point is a measure of the strength of the intermolecular forces: the higher the boiling point the stronger the intermolecular forces. Slide 14 / 92 Intermolecular Forces l l ovalent bond (strong) Intermolecular attraction ( week) The attractions between molecules, intermolecular forces, are not nearly as strong as the intramolecular attractions that hold compounds together. They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities. Slide 15 / 92 Intermolecular Forces Slide 16 / 92 ipoleipole Interactions There are three types of Intermolecular Forces: they are sometimes called van der Waals Forces Molecules that have permanent dipoles are attracted to each other. The interaction between any two like charges is repulsive (black) ipoledipole interactions ydrogen bonding The positive end of one is attracted to the negative end of the other and viceversa. London dispersion forces These forces are only important when the molecules are close to each other. The interaction between any two opposite charges is attractive ( red) Slide 17 / 92 ipoleipole Interactions The polarity of a molecule is measured by its dipole moment, m. The more polar the molecule, the greater its dipole moment. The more polar the molecule, the higher its boiling point. That's because the attraction between the dipoles holds the molecules together, not letting them boil away. Substance Molecular ipole oiling Weight (amu) Moment u() Point(K) cetonitrile, 3N cetaldehyde, 3O Methyl chloride, 3l imethyl ether, 3O Propane, Which of the below molecules will have the highest boiling point? Slide 18 / O 3 Substance Molecular ipole Wt. Moment 3l 3O N 3O l O N

4 Slide 19 / 92 7 Which of the below molecules will have the lowest boiling point? Slide 20 / 92 London ispersion Forces London ispersion Forces occur between all molecules O 3 3l 3O 3N Substance Molecular Wt O l O N ipole Moment They result from the fact that electrons are in constant motion and sometimes are the same side of the molecule. When they are on one side, the molecule is polarized: one side is negative and the other is positive; the molecule acts like a dipole. That creates an electric field that oppositely polarizes nearby molecules...leading to an attraction. Let's see how that works using elium as an example. Slide 21 / 92 London ispersion Forces While the electrons in helium repel each other, they occasionally wind up on the same side of the atom. t that instant, the helium atom is polar, with an excess of electrons on one side and a shortage on the other. nother helium atom nearby, has a dipole induced in it, as the electrons on the left side of the first atom repel the electrons in the second. London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole. δ e 2 e elium atom δ electrostatic attractio e e 2 2 e e elium atom elium 1 atom Slide 22 / 92 London ispersion 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. The larger the molecule, the more polarizable it is...and the stronger the London ispersion Force. That means, the higher the molecular weight of a molecule, the more London ispersion Force it experiences. δ δ δ δ Slide 23 / 92 London ispersion Forces The strength of dispersion forces tends to increase with increased molecular weight. Larger atoms and molecules have larger electron clouds which are easier to polarize. Slide 24 / 92 8 Which of the molecules below will have the highest boiling point? F 2 l 2 r 2 Molecular alogen Weight ( amu) oiling Noble Point (K) gas Molecula rweight (amu) F e oiling point (K) I 2 l Ne r r I Kr Xe

5 Slide 25 / 92 9 What intermolecular force is responsible for ice being less dense than liquid water? London ispersion Forces ipoleipole Forces Ionipole Forces ydrogen onding Ionic onding Slide 26 / Intermolecular force(s) responsible for the fact that 4 has the lowest boiling point in the set 4, Si 4, Ge 4, Sn 4 is/are. ydrogen onding ipoleipole Interactions London ispersion Forces Mainly 2 onding with some dipoledipole interactions mainly London ispersion Forces with dipoledipole interactions Slide 27 / Which of the below molecules will have the lowest boiling point? Slide 28 / Which of the below molecules will have the highest boiling point? F 2 l 2 r 2 I 2 e Ne r Kr Xe 13 Which of the below molecules will have the lowest boiling point? e Ne r Slide 29 / 92 Slide 30 / 92 Which ave a Greater ffect? ipoleipole Interactions or ispersion Forces If two polar molecules are of comparable size, dipoledipole interactions are the dominating force. Kr Xe If one molecule is much larger than another, dispersion forces will likely determine its physical properties. If molecules are nonpolar, dispersion forces will dominate, since all molecules experience dispersion forces.

6 Slide 31 / 92 ydrogen onding In these graphs, boiling points increase with the mass of the molecules...as we'd expect: larger dispersion forces due to larger masses. The nonpolar series ( 4 to Sn 4 ) follow the expected trend. The polar series follows the trend from 2Te through 2S, ut water defies the trend. It is the lightest in its series, but has the highest boliing point. The dipoledipole interactions experienced when is bonded to N, O, or F are unusually strong. Slide 32 / 92 ydrogen onding We call these interactions hydrogen bonds. These powerful bonds occur only when is bonded directly to N, O or F. O O N N O O Slide 33 / 92 ydrogen onding ydrogen bonding arises in part from the high electronegativity and small radius of nitrogen, oxygen, and fluorine. When hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed. Slide 34 / 92 ydrogen onding Ice is the only solid that floats in its liquid form. If it didn't, life on arth would be very different. For instance, lakes would freeze from the bottom and fish couldn't survive winters. ydrogen bonding creates the space in ice that explains its low density. WaterIce transition simulation.htm F l r I ll of the above. Slide 35 / Which of the following molecules has hydrogen bonding as one of its IM forces? Slide 36 / 92 Ionipole Interactions There is a fourth molecular force that will be important as we explore solutions later this year: Iondipole interactions are not considered a van der Waals force. The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents. _ nion dipole attractions ation dipole attractions

7 Slide 37 / 92 Summarizing Intermolecular Forces Interacting molecules or ions Slide 38 / Which of the following molecules has London dispersion as its only IM force? re polar molecules involved? NO YS NO re hydrogen atoms bonded to N,O,or F atoms? re ions involved? YS re polar molecules and ions both present? YS NO P 3 2S l Si 4 None of the above. NO YS ispersion forces only (induced dipoles) r, I2 ipoledipole forces 2S, 3l ydrogen bonding 2O, N3 Ion dipole forces Nal in 2O Ionic bonding Nal, KI van der Waals forces Slide 39 / 92 Slide 40 / ow many of these substances will have dipoledipole interactions? (ow many are polar molecules?) 2 O O 2 4 N Which of the following molecules will have the highest boiling point? 2 O O 2 4 N Slide 41 / Which liquid will have the lowest freezing point? Slide 42 / Of the following diatomic molecules, which has the highest boiling point? pure 2O aq M glucose aq M sucrose aq M FeI3 aq M KF N 2 r 2 2 l 2 O 2

8 Slide 43 / 92 Slide 44 / Of the following diatomic molecules, which has the lowest boiling point? 21 Which one of the following derivatives of methane ( 4 ) has the lowest boiling point? N 2 r 2 2 l 2 r 4 F 4 l 4 I 4 O 2 Slide 45 / 92 Slide 46 / Which one of the following derivatives of methane ( 4 ) has the highest boiling point? 23 For an aqueous solution of a nonvolatile compound, the vapor pressure will be, the boiling point will be, and the freezing point will be than pure water. r 4 F 4 l 4 I 4 lower, lower, lower lower, higher, lower lower, higher, higher higher, higher, lower higher, lower, higher Slide 47 / 92 Intermolecular Forces ffect Many Physical Properties Slide 48 / 92 Viscosity Resistance of a liquid to flow is called viscosity. It is related to the ease with which molecules can move past each other. The strength of the attractions between particles can greatly affect the properties of a substance or solution. Viscosity increases with stronger intermolecular forces and decreases with higher temperature. Substance Formula Viscosity ( kg/ms) exane x 10 4 eptane x 10 4 Octane x 10 4 Nonane x 10 4 ecane x

9 Slide 49 / 92 Surface Tension Slide 50 / 92 Phase hanges GS Vaporization ondensation Surface tension results from the net inward force experienced by the molecules on the surface of a liquid. nergy of system Melting LIQUI Freezing SOLI Sublimation eposition Slide 51 / 92 nergy hanges ssociated with hanges of State hemical and physical changes are usually accompanied by changes in energy. When energy is put into the system, the process is called endothermic. 24 Which of the following is not a phase change? vaporization effusion melting sublimation Slide 52 / 92 When energy is released by the system, the process is called exothermic. Slide 53 / Of the following, is an exothermic process? Slide 54 / The first molecules to evaporate from a liquid are. melting Those with the lowest K subliming freezing Those farthest from the surface of the liquid Those with the highest K boiling all are exothermic

10 Slide 55 / The direct change of a substance from a solid to a gas is called. boiling evaporation sublimation condensation Slide 56 / 92 oiling vs. vaporation oiling and evaporation are two ways in which a liquid can vaporize into a gas. owever, there are important distinctions between these processes. oiling occurs at a specific temperature, the boiling point (.P.) vaporation occurs below the boiling point occurs throughout the entire liquid occurs only at the surface of a liquid achieved when atmospheric pressure equals vapor pressure (Patm = Pvap) Slide 57 / 92 Vapor Pressure t any temperature some molecules in a liquid have enough energy to escape. Slide 58 / 92 Vapor Pressure s more molecules escape the liquid, the pressure they exert increases. s the temperature rises, the fraction of molecules that have enough energy to escape increases. The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate. Slide 59 / 92 Vapor Pressure Slide 60 / When an aqueous salt solution is compared to water, the salt solution will have The boiling point of a liquid is the temperature at which it s vapor pressure equals atmospheric pressure. The normal boiling point is the temperature at which its vapor pressure is 760 torr. (K 760 mm g = 1 atm) a higher boiling point, a lower freezing point, and a lower vapor pressure a higher boiling point, a higher freezing point, and a lower vapor pressure. a higher boiling point, a higher freezing point, and a higher vapor pressure a lower boiling point, a lower freezing point, and a lower vapor pressure a lower boiling point, a higher freezing point, and a higher vapor pressure

11 Slide 61 / What is the normal boiling point of ethanol? Slide 62 / What is the boiling point ( in 0 ) of diethyl ether at 200 torr? Slide 63 / What is the boiling point of water at 300 torr? Slide 64 / 92 Vapor Pressure liquid will boil when its vapor pressure equals atmospheric pressure. pressure cooker works by increasing the "atmospheric" pressure inside it, so water will not boil at 100 o ; instead, it may be heated up to 120 o before turning to steam. Raising the cooking temperature cuts cooking time drastically. Slide 65 / 92 Vapor Pressure Slide 66 / 92 Vapor Pressure Recall that boiling occurs when P vap = P atm. Since atmospheric pressure is so low at high altitudes, (e.g. top of Mount verest) water will boil at a much lower temperature than here at ergen Tech. P atm = 33 kpa on Mt. verest P atm = kpa at sea level

12 Slide 67 / It will take longer to hardboil an egg on top of Mt. verest here at ergen Tech cooking times are equal Slide 68 / volatile liquid is one that. is highly flammable is highly viscous is highly hydrogenbonded is highly cohesive readily evaporates Slide 69 / If a liquid is sealed in a container and kept at constant temperature, how does its vapor pressure change over time? it rises at first, then remains constant it rises as first, then falls it remains constant Slide 70 / ased on this figure, the boiling point of diethyl ether under an external pressure of 1.32 atm is Slide 71 / 92 Phase iagrams Phase diagrams display the state of a substance at various pressures and temperatures and the places where equilibria exist between phases. Slide 72 / 92 Volatility The more volatile a liquid: The more quickly it evaporates The higher its vapor pressure at a given temperature The weaker its Intermolecular Forces

13 Slide 73 / 92 Phase iagrams The circled line is the liquidvapor interface. It starts at the triple point (T), the point at which all three states are in equilibrium. Slide 74 / 92 Phase iagrams It ends at the critical point (); above this critical temperature and critical pressure the liquid and vapor are indistinguishable from each other. Slide 75 / 92 Phase iagrams The circled line in the diagram below is the interface between liquid and solid. The melting point at each pressure can be found along this line. Slide 76 / 92 Phase iagrams elow the triple point the substance cannot exist in the liquid state. long the circled line the solid and gas phases are in equilibrium; the sublimation point at each pressure is along this line. Slide 77 / 92 Phase iagram of Water Note the high critical temperature and critical pressure. These are due to the strong van der Waals forces between water molecules. The slope of the solidliquid line is negative. This means that as the pressure is increased at a temperature just below the melting point, water goes from a solid to a liquid. Slide 78 / 92 Phase iagram of arbon ioxide The low critical temperature and critical pressure for O 2 make supercritical O 2 a good solvent for extracting nonpolar substances (like caffeine). arbon dioxide cannot exist in the liquid state at pressures below 5.11 atm; O 2 sublimes at normal pressures.

14 Slide 79 / 92 Slide 80 / 92 Phase iagram of 2 O and O 2 The slope of the solidliquid line is negative. If you increase the pressure at a given temperature, near the freezing point, the ice will melt. The Water slope of is the solidliquid only known line is positive, as it is for most of substance with this behavior. the substances. if you increase the pressure at a given temperature near 55 0, the liquid will freeze. Note that, since the Triple point is at a high pressure, you will see O 2 only subliming under normal atmospheric condition. Slide 81 / The phase diagram of a substance is shown to the right. The region that corresponds to the solid phase is. w x y x x and y Slide 82 / In this phase diagram, the substance is a at 25 and 1.0 atm 38 On a phase diagram, the melting point is the same as the solid liquid gas supercritical fluid crystal triple point critical point freezing point boiling point vaporpressure curve Slide 83 / On the phase diagram shown to the right, segment corresponds to the conditions of temperature and pressure under which the solid and the gas of the substance are in equilibrium Slide 84 / 92 Solids We can think of solids as falling into two groups rystalline, in which particles are in highly ordered arrangement. morphous, in which there is no particular order in the arrangement of particles.

15 Slide 85 / 92 Solids Some examples of amorphous solids are: rubber, glass, paraffin wax and cotton candy. Slide 86 / 92 Types of onding in rystalline Solids rystalline solids include ionic compounds, metals and another group called covalentnetwork solids. rystalline solids are categorized by bonding type as shown on the next slide. Slide 87 / In a saturated solution of salt water,. Slide 88 / 92 ovalentnetwork Solids the rate of crystallization > the rate of solution the rate of solution > the rate of crystallization seed crystal addition may cause massive crystallization rate of solution = rate of crystallization addition of more water causes massive crystallization iamonds are an example of a covalentnetwork solid, in which carbon atoms are covalently bonded to four other carbon atoms. They tend to be hard and have high melting points. Slide 89 / 92 ovalentnetwork Solids Slide 90 / 92 Metallic Solids Metals are not covalently bonded, but the attractions between atoms are too strong to be van der Waals forces. Graphite is another example of a covalentnetwork solid. ach carbon atom is covalently bonded to 3 others in layers of interconnected hexagonal rings. The layers are held together by weak dispersion forces. The layers slide easily across one another, so graphite is used as a lubricant as well as the "lead" in pencils. In metals, valence electrons are delocalized throughout the solid. This means that the "sea" of electrons moves freely around all the nuclei.

16 Slide 91 / 92 Properties of Metallic Solids Slide 92 / 92 The delocalized nature of the electrons in metals accounts for many physical properties. For example, metals are generally: good conductors of heat and electricity malleable ductile, (i.e. may be drawn into wires)