Intermolecular Forces, Then we explained how atoms combine to form - PDF document

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. We first explained atoms, and how to build up the periodic table from quantum numbers. Intermolecular Forces, Then we explained how atoms combine to form molecules: the most common way we find most atoms in Liquids, and Solids 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 · the distance between particles three common states that dominate our world are gases, liquids and solids. · the particles' freedom to move These are the states of matter we'll be studying. cool or cool increase pressure heat or heat We won't be discussing more exotic states decrease pressure such as plasma, nuclear matter, etc. Gas Liquid Crystalline solid Particles are far apart, disorder, freedom, orderd arrangement, free to move relative total freedom, particles are in fixed to each other, much of empty space, positions, total disorder close together close together Slide 5 / 92 Slide 6 / 92 Characteristics of the States of Matter Condensed Phases In the solid and liquid states particles are closer together, we refer to them as condensed phases. Gas Assumes the shape of its container Expands to the volume of its container Is compressible Flows easily cool or cool increase pressure Diffusion within a gas is rapid heat or heat decrease pressure Liquid Assumes the shape of the part of a container it occupies Does not expand to the volume of its container Gas Liquid Crystalline solid Is virtually incompressible disorder, freedom, Particles are far apart, orderd arrangement, Flows easily total freedom, free to move relative particles are in fixed to each other, Diffusion within a liquid is slow much of empty space, positions, close together total disorder close together Solid Retains its own shape, regardless of container Does not expand to the volume of its container Is virtually incompressible Does not flow Diffusion within a solid is very very slow

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

Slide 13 / 92 Slide 14 / 92 Intermolecular Forces & Boiling Intermolecular Forces Points Boiling represents a transition from a liquid to a gas. To make that transition, molecules in the liquid must H Cl H Cl break free of the intermolecular forces that bind them. Covalent bond Intermolecular The kinetic energy of the molecules is proportional to the (strong) attraction ( week) temperature: as temperature rises, so does kinetic energy. The attractions between molecules, intermolecular forces, are not nearly as strong as the The temperature where the molecules' energy intramolecular attractions that hold compounds overcomes intermolecular forces is called the boiling together. point. They are, however, strong enough to control The boiling point is a measure of the strength of the physical properties such as boiling and melting intermolecular forces: the higher the boiling point - the points, vapor pressures, and viscosities. stronger the intermolecular forces. Slide 15 / 92 Slide 16 / 92 Intermolecular Forces Dipole-Dipole Interactions The interaction between any two like charges is · Molecules that have There are three types of Intermolecular repulsive (black) permanent dipoles are Forces: they are sometimes called van der Waals Forces attracted to each other. + - + - · Dipole-dipole interactions · The positive end of one is - attracted to the negative end · Hydrogen bonding of the other and vice-versa. - + + · 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 Slide 18 / 92 Dipole-Dipole Interactions 6 Which of the below molecules will have the The polarity of a molecule is measured by its dipole highest boiling point? moment, m . The more polar the molecule, the greater its dipole moment. A CH 3 CH 2 CH 3 Substance Molecular Dipole B The more polar the molecule, the higher its boiling point. CH 3 OCH 3 Wt. Moment C CH 3 Cl CH 3 CH 2 CH 3 44 0.1 That's because the attraction between the dipoles holds D CH 3 CHO the molecules together, not letting them boil away. CH 3 OCH 3 46 1.3 E CH 3 CN CH 3 Cl 50 1.9 Substance Molecular Dipole Boiling CH 3 CHO 44 2.7 Weight (amu) Moment u( D) Poi nt ( K) CH 3 CN 41 3.9 Acetonitrile, CH 3 CN 41 3.9 355 Acetaldehyde, CH 3 CHO 44 2.7 294 Methyl chloride, CH 3 Cl 50 1.9 249 Dimethyl ether, CH 3 OCH 3 46 1.3 248 Propane, CH 3 CH 2 CH 3 44 0.1 231

Slide 19 / 92 Slide 20 / 92 London Dispersion Forces 7 Which of the below molecules will have the lowest boiling point? London Dispersion Forces occur between all molecules. A CH 3 CH 2 CH 3 Substance Molecular Dipole They result from the fact that electrons are in constant B CH 3 OCH 3 Wt. Moment motion and sometimes are the same side of the molecule. C CH 3 Cl CH 3 CH 2 CH 3 44 0.1 D CH 3 CHO When they are on one side, the molecule is polarized: one CH 3 OCH 3 46 1.3 E CH 3 CN side is negative and the other is positive; the molecule CH 3 Cl 50 1.9 acts like a dipole. CH 3 CHO 44 2.7 That creates an electric field that oppositely polarizes CH 3 CN 41 3.9 nearby molecules...leading to an attraction. Let's see how that works using Helium as an example. Slide 21 / 92 Slide 22 / 92 London Dispersion Forces London Dispersion Forces e- While the electrons in helium repel each other, they 2+ · occasionally wind up on the same side of the atom. e- These forces are present in all molecules, whether they are polar or nonpolar. Helium atom · At that instant, the helium atom is polar, with an excess The tendency of an electron cloud to distort in this way is called polarizability. of electrons on one side and a shortage on the other. · The larger the molecule, the more polarizable it is...and the stronger the London Dispersion Force. Another helium atom nearby, has a dipole induced in it, δ - δ + as the electrons on the left side of the first atom repel · That means, the higher the molecular weight of a molecule, the more London the electrons in the second. Dispersion Force it experiences. electrostatic attraction London dispersion forces, or dispersion forces, are e- e- 2+ attractions between an instantaneous dipole and an 2+ e- e- induced dipole. Helium atom 1 Helium atom 2 δ - δ + - + δ δ Slide 23 / 92 Slide 24 / 92 London Dispersion Forces 8 Which of the molecules below will have the highest boiling point? · The strength of dispersion forces tends to increase with increased molecular weight. A F 2 · Larger atoms and molecules have larger electron B Cl 2 clouds which are easier to polarize. C Br 2 D I 2 Halogen Molecular Boiling Noble Molecula Boiling Weight Point (K) gas rWeight point (K) ( amu) (amu) F 2 38.0 85.1 He 4.0 4.6 Cl 2 71.0 238.6 Ne 20.2 27.3 Br 2 159.8 332.0 Ar 39.9 87.5 I 2 253.8 457.6 Kr 83.8 120.9 Xe 131.3 166.1