Intermolecular Forces, Liquids, and Solids

Transcription

1 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. States of Matter While there are many states of matter, the three common states that dominate our world are gases, liquids and solids. States of Matter The fundamental differences between states of matter is: 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 heat or decrease pressure cool heat 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 orderd arrangement, particles are in fixed positions, close together haracteristics of the States of Matter Gas ssumes the shape of its container xpands to the volume of its container Is compressible Flows easily iffusion within a gas is rapid ondensed Phases In the solid and liquid states particles are closer together, we refer to them as condensed phases. cool or increase pressure cool Liquid Solid 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 heat or heat decrease pressure Gas Liquid rystalline solid Particles are far apart, disorder, freedom, orderd arrangement, free to move relative total freedom, particles are in fixed much of empty space, to each other, positions, total disorder close together close together 1

2 1 Which of the below is a characteristic of a gas? 2 Which of the below is a characteristic of a liquid? It fills only a portion of its container. It fills only a portion 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 not compressible. It is compressible. iffusion is very slow within it. iffusion is very rapid within it. 3 Which of the below is a characteristic of a solid? It fills all of its container. Its molecules are in relatively rigid positions. It takes on the shape of its container. It is compressible. iffusion is very rapid within it. 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. 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. H l H l The kinetic energy of the molecules is proportional to the temperature: as temperature rises, so does kinetic energy. ovalent bond (strong) Intermolecular attraction ( week) 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. 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. 2

3 Intermolecular Forces ipoleipole Interactions There are three types of Intermolecular Forces: they are sometimes called van der Waals Forces ipoledipole interactions Hydrogen bonding London dispersion forces 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. The interaction between any two like charges is repulsive (black) The interaction between any two opposite charges is attractive ( red) ipoleipole Interactions The polarity of a molecule is measured by its dipole moment, µ. The more polar the molecule, the greater its dipole moment. 4 Which of the below molecules will have the highest boiling point? The more polar the molecule, the higher its boiling point. H 3H 2H 3 H 3OH 3 That's because the attraction between the dipoles holds the molecules together, not letting them boil away. H 3l H 3HO H 3N Substance Molecular Wt. ipole Moment Substance Molecular ipole oiling Weight (amu) Moment u() Point(K) cetonitrile, H3N cetaldehyde, H3HO Methyl chloride, H3l imethyl ether, H3OH Propane, H3H2H H 3H 2H H 3OH H 3l H 3HO H 3N Which of the below molecules will have the lowest boiling point? H 3H 2H 3 H 3OH 3 H 3l H 3HO H 3N Substance Molecular Wt. H 3H 2H H 3OH H 3l H 3HO H 3N ipole Moment London ispersion Forces London ispersion Forces occur between all molecules. 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 Helium as an example. 3

4 London ispersion Forces 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. e 2 e Helium atom δ δ electrostatic attraction e e 2 2 e e Helium atom 1 Helium atom 2 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 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. 6 Which of the molecules below will have the highest boiling point? F 2 l 2 r 2 I 2 Halogen Molecular Weight ( amu) oiling Point (K) Noble gas Molecular Weight (amu) F He l Ne r r oiling point (K) I Kr Xe Which of the below molecules will have the lowest boiling point? 8 Which of the below molecules will have the highest boiling point? F 2 l 2 r 2 I 2 He Ne r Kr Xe 4

5 9 Which of the below molecules will have the lowest boiling point? He Ne r Kr Xe Which Have 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. Hydrogen 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 (H 4 to SnH 4 ) follow the expected trend. The polar series follows the trend from H 2Te through H 2S, ut water defies the trend. It is the lightest in its series, but has the highest boliing point. The dipoledipole interactions experienced when H is bonded to N, O, or F are unusually strong. We call these interactions hydrogen bonds. Hydrogen onding These powerful bonds occur only when H is bonded directly to N, O or F. Hydrogen onding Hydrogen 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. Hydrogen 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. Hydrogen bonding creates the space in ice that explains its low density. WaterIce transition simulation.htm 5

6 IM_Forces_Presentation_v_1.0.notebook 10 Which of the following molecules has hydrogen bonding as one of its IM forces? HF Hl Hr 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. HI ll of the above. _ nion dipole attractions ation dipole attractions Summarizing Intermolecular Forces Interacting molecules or ions 11 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 PH3 H2S Hl SiH 4 None of the above. NO YS ispersion forces only (induced dipoles) r, I2 ipoledipole forces H2S, H3l Hydrogen bonding H2O, NH3 Ion dipole forces Nal in H2O Ionic bonding Nal, KI van der Waals forces 12 How many of these substances will have dipoledipole interactions? (How many are polar molecules?) H 2 O O 2 H 4 NH 3 13 Which of the following molecules will have the highest boiling point? H 2O O 2 H 4 NH

7 14 Of the following diatomic molecules, which has the highest boiling point? 15 Of the following diatomic molecules, which has the lowest boiling point? N 2 N 2 r2 r2 H 2 H 2 l 2 l 2 O 2 O 2 16 Which one of the following derivatives of methane (H 4 ) has the lowest boiling point? 17 Which one of the following derivatives of methane (H 4 ) has the highest boiling point? r 4 F 4 r 4 F 4 l 4 l 4 I 4 I 4 Intermolecular Forces ffect Many Physical Properties 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) Hexane H3H2H2H2H2H x 10 4 Heptane H3H2H2H2H2H2H x 10 4 Octane H3H2H2H2H2H2H2H x 10 4 Nonane H3H2H2H2H2H2H2H2H x 10 4 ecane H3H2H2H2H2H2H2H2H2H x

8 Surface Tension 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 nergy hanges ssociated with hanges of State 18 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. When energy is released by the system, the process is called exothermic. vaporization effusion melting sublimation oiling vs. vaporation oiling and evaporation are two ways in which a liquid can vaporize into a gas. However, there are important distinctions between these processes. oiling occurs at a specific temperature, the boiling point (.P.) vaporation occurs below the boiling point Vapor Pressure 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. occurs throughout the entire liquid achieved when atmospheric pressure equals vapor pressure (Patm = Pvap) occurs only at the surface of a liquid 8

9 Vapor Pressure s more molecules escape the liquid, the pressure they exert increases. The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate. 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 Hg = 1 atm) Vapor Pressure 19 What is the normal boiling point of ethanol? What is the boiling point ( in 0 ) of diethyl ether at 200 torr? What is the boiling point of water at 300 torr? 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. 90 9

10 Vapor Pressure 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. Patm = 33 kpa on Mt. verest Patm = kpa at sea level 22 It will take longer to hardboil an egg on top of Mt. verest here at ergen Tech cooking times are equal Phase iagrams Phase diagrams display the state of a substance at various pressures and temperatures and the places where equilibria exist between phases. Volatility The more volatile a liquid: The more quickly it evaporates 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. The higher its vapor pressure at a given temperature The weaker its Intermolecular Forces 10

11 Phase iagrams It ends at the critical point (); above this critical temperature and critical pressure the liquid and vapor are indistinguishable from each other. 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. 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. 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. Phase iagram of H 2 O and O 2 Phase iagram of arbon ioxide arbon dioxide cannot exist in the liquid state at pressures below 5.11 atm; O 2 sublimes at normal pressures. 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 low critical temperature and critical pressure for O 2 make supercritical O 2 a good solvent for extracting nonpolar substances (like caffeine). 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 55 0, the liquid will freeze. Note that, since the Triple point is at a high pressure, you will see O2 only subliming under normal atmospheric condition. 11

12 Solids We can think of solids as falling into two groups Solids rystalline, in which particles are in highly ordered arrangement. morphous, in which there is no particular order in the arrangement of particles. Some examples of amorphous solids are: rubber, glass, paraffin wax and cotton candy. 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. Types of onding in rystalline Solids 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. ovalentnetwork Solids 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. 12

13 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) 13