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 Slide 4 / 92 States of Matter 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. the distance between particles the particles' freedom to move These are the states of matter we'll be studying. We won't be discussing more exotic states such as plasma, nuclear matter, etc. cool or increase pressure cool heat or decrease pressure heat Gas Liquid Slide 5 / 92 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 orderd arrangement, particles are in fixed positions, close together Slide 6 / 92 haracteristics of the States of Matter Gas rystalline solid disorder, freedom, free to move relative to each other, close together Particles are far apart, total freedom, much of empty space, total disorder ondensed Phases In the solid and liquid states particles are closer together, we refer to them as condensed phases. Gas Particles are far apart, total freedom, much of empty space, total disorder cool or increase pressure cool heat or decrease pressure heat Liquid disorder, freedom, free to move relative to each other, close together rystalline solid orderd arrangement, particles are in fixed positions, close together

2 Slide 7 / 92 1 Which of the following is not a type of solid? ionic molecular covalentnetwork Slide 8 / 92 2 Which of the below is a characteristic of a gas? It fills only a portion of its container. Its molecules are in relatively rigid positions. It takes on the shape of its container. supercritical It is not compressible. metallic iffusion is very slow within it. Slide 9 / 92 3 Which of the below is a characteristic of a liquid? 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. Together, liquids and solids, constitute phases of matter. 4 Slide 11 / 92 5 Slide 10 / 92 the compressible the fluid the condensed all of the above the disordered Slide 12 / 92 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 Slide 14 / 92 Intermolecular Forces & oiling Points Intermolecular Forces 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. Slide 15 / 92 Slide 16 / 92 Intermolecular Forces These forces are only important when the molecules are close to each other. Slide 17 / 92 The interaction between any two opposite charges is attractive ( red) Slide 18 / 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 London dispersion forces The positive end of one is attracted to the negative end of the other and viceversa. ydrogen bonding The interaction between any two like charges is repulsive (black) Molecules that have permanent dipoles are attracted to each other. ipoledipole interactions ipoleipole Interactions There are three types of Intermolecular Forces: they are sometimes called van der Waals Forces cetonitrile, 3N cetaldehyde, 3O Methyl chloride, 3l imethyl ether, 3O3 Propane, 323 Intermolecular attraction ( week) They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities. The boiling point is a measure of the strength of the intermolecular forces: the higher the boiling point the stronger the intermolecular forces. ipole Moment u ( ) l The attractions between molecules, intermolecular forces, are not nearly as strong as the intramolecular attractions that hold compounds together. The temperature where the molecules' energy overcomes intermolecular forces is called the boiling point. Molecular Weight (amu) ovalent bond (strong) The kinetic energy of the molecules is proportional to the temperature: as temperature rises, so does kinetic energy. Substance l oiling Po i n t ( K) Which of the below molecules will have the highest boiling point? 323 3O3 3l Substance Molecular ipole Moment Wt. 3O N 3O l O N

4 Slide 19 / 92 7 Slide 20 / 92 London ispersion Forces Which of the below molecules will have the lowest boiling point? 323 3O3 3l 3O 3N London ispersion Forces occur between all molecules. Substance Molecular ipole Moment Wt O l O N 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 Slide 22 / 92 London ispersion Forces London ispersion Forces e While the electrons in helium repel each other, they occasionally wind up on the same side of the atom. 2 e elium atom t that instant, the helium atom is polar, with an excess of electrons on one side and a shortage on the other. 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. 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 δ That means, the higher the molecular weight of a molecule, the more London ispersion Force it experiences. electrostatic attraction e e 2 2 e e elium atom 1 δ elium atom 2 δ δ δ Slide 23 / 92 Slide 24 / 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. alogen Molecular Weight ( amu) oiling Noble Point (K) gas F e l Ne r r I Kr Xe Molecula oiling rweight point (K) (amu) 8 Which of the molecules below will have the highest boiling point? F2 l2 r2 I2

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, Si4, Ge4, Sn4 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? F2 l2 r2 I2 13 Which of the below molecules will have the lowest boiling point? e Ne r Kr Xe 12 Which of the below molecules will have the highest boiling point? Slide 29 / 92 Slide 28 / 92 e Ne r Kr Xe 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. 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 Slide 32 / 92 ydrogen onding 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 Sn4 ) 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. N O We call these interactions hydrogen bonds. O These powerful bonds occur only when is bonded directly to N, O or F. O N O Slide 33 / 92 Slide 34 / 92 ydrogen onding 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. 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 Slide 35 / 92 F l r I 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. Ionipole Interactions 14 Which of the following molecules has hydrogen bonding as one of its IM forces? Slide 36 / 92 ll of the above. _ nion dipole attractions ation dipole attractions

7 Slide 37 / 92 Slide 38 / 92 Summarizing Intermolecular Forces Interacting molecules or ions re polar molecules involved? NO re ions involved? YS YS NO 15 Which of the following molecules has London dispersion as its only IM force? re polar molecules and ions both present? YS NO re hydrogen atoms bonded to N,O,or F atoms? NO ispersion forces only (induced dipoles) r, I2 P3 2S l Si4 None of the above. YS ipoledipole forces 2S, 3l ydrogen bonding 2O, N3 Ion dipole forces Nal in 2O Ionic bonding Nal, KI Slide 39 / ow many of these substances will have dipoledipole interactions? (ow many are polar molecules?) 2O O2 4 N Slide 40 / Which of the following molecules will have the highest boiling point? 2O O2 4 N3 Slide 41 / Which liquid will have the lowest freezing point? pure 2O aq M glucose aq M sucrose aq M FeI3 aq M KF Slide 42 / Of the following diatomic molecules, which has the highest boiling point? N2 r2 2 l2 O2

8 Slide 43 / Of the following diatomic molecules, which has the lowest boiling point? Slide 44 / Which one of the following derivatives of methane (4) has the lowest boiling point? N2 r4 r2 F4 2 l4 l2 I4 O2 Slide 45 / Which one of the following derivatives of methane (4) has the highest boiling point? Slide 46 / 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. r4 F4 lower, lower, lower l4 lower, higher, lower I4 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 exane eptane Octane Nonane ecane Formula Viscosity ( kg/ms) x x x x x 103

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

10 Slide 55 / Slide 56 / 92 oiling vs. vaporation The direct change of a substance from a solid to a gas is called. boiling evaporation sublimation condensation oiling and evaporation are two ways in which a liquid can vaporize into a gas. owever, there are important distinctions between these processes. vaporation oiling occurs at a specific temperature, the boiling point (.P.) 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 Slide 58 / 92 Vapor Pressure Vapor Pressure s more molecules escape the liquid, the pressure they exert increases. t any temperature some molecules in a liquid have enough energy to escape. 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 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) Slide 60 / When an aqueous salt solution is compared to water, the salt solution will have 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? 45 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 100o; instead, it may be heated up to 120o before turning to steam. Raising the cooking temperature cuts cooking time drastically. 90 Slide 65 / 92 Vapor Pressure Slide 66 / 92 Vapor Pressure Recall that boiling occurs when Pvap = Patm. 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. Patm = 33 kpa on Mt. verest Patm = kpa at sea level

12 Slide 67 / It will take longer to hardboil an egg Slide 68 / volatile liquid is one that. is highly flammable on top of Mt. verest is highly viscous here at ergen Tech cooking times are equal 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? Slide 70 / ased on this figure, the boiling point of diethyl ether under an external pressure of 1.32 atm is. 10 it rises at first, then remains constant 20 it rises as first, then falls 30 it remains constant 40 0 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 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 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. 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 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 78 / 92 Phase iagram of arbon ioxide arbon dioxide cannot exist in the liquid state at pressures below 5.11 atm; O2 sublimes at normal pressures. 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. The low critical temperature and critical pressure for O2 make supercritical O2 a good solvent for extracting nonpolar substances (like caffeine).

14 Slide 79 / 92 Phase iagram of 2O and O2 Slide 80 / 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. Water is the only known substance with this behavior. The slope of the solidliquid line is positive, as it is for most of the substances. if you increase the pressure at a given temperature near 550, the liquid will freeze. Note that, since the Triple point is at a high pressure, you will see O2 only subliming under normal condition. Slide 81 atmospheric / In this phase diagram, the substance is a at 25 and 1.0 atm 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 / On a phase diagram, the melting point is the same as the solid triple point liquid critical point gas freezing point supercritical fluid boiling point crystal 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,. 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 Slide 88 / 92 ovalentnetwork Solids 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 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) Slide 92 / 92