Chap 11A - Chem 210 / WANG / Chapter 11A Intermolecular...

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Unformatted text preview: Chem 210 / WANG / Chapter 11A Intermolecular Forces Chapter 11.1 through 11.3 Intermolecular Intermolecular Forces • Gaseous state: – Kinetic energy completely overcomes intermolecular forces; particles move freely & freely unrelated unrelated to each other. • Liquid state: – Intermolecular forces keep particles close to close each other; kinetic energy keeps them kinetic moving moving around each other. • Solid state: – Intermolecular forces hold particles in fixed fixed positions; kinetic energy keeps them kinetic vibrating vibrating in place. 11.1 A Molecular Comparison of Molecular Gases, Liquids, Solids. 11.2 Intermolecular Forces 11.3 Some Properties of Liquids Some 11.4 11.4 Phase Changes 11.5 11.5 Vapor Pressure 11.6 11.6 Phase Diagrams “ “ “ “ “ “ “ “ • Intermolecular: Forces between molecules between – Determines the state of matter (g, l or s). state or • Intramolecular: Forces within molecules within – Determines bond type (ionic or covalent). bond covalent). Inter” versus “Intra” Inter” versus “Intra” “ “ “ “ “ “ “ “ Inter” versus “Intra” Inter” versus “Intra” Intra – molecular O-H covalent bond Two Two per H2O molecule, definite. Involves chemical Involves chemical changes, such as: H2 + ½O2 ½O electrolysis: H2O Intermolecular force • Boiling is a “phase” change, no change no occurs with the “molecules” themselves. – Intermolecular forces are broken. are – (Molecules fly apart.) – Intramolecular bonds (covalent bonds) are NOT broken (molecule remains the same.) remains Major Major Types of Intermolecular forces (Also called van der Waals forces) van Dipole-dipole attraction DipoleBetween polar Between polar molecules 2. Dispersion forces Between non-polar molecules non3. Hydrogen bonding Between molecules containing O-H, N-H and/or F-H bonds. NFAs the chemical bonds, each of the above are also electrostatic electrostatic in nature they involve attractions between electrically charged species (i.e. δ+ or δ– electrically charges on polar molecules, etc.) These are weaker weaker forces (typically <50 kJ/mole). Numerous Numerous & fluctuates. Involves physical Involves physical changes, such as: viscosity, surface tension, vapor pressure, boiling & melting points, etc. 1. dipole-dipole attraction dipoleBetween polar molecules polar 1. Fall 2009 UM-SJTU JI 3 states of matter are closely related to Intermolecular forces Intermolecular Gas, Gas, Liquid and Solid on the Molecular Level • Polar molecules have charge separation. charge • One end is δ+, the other is δdipole! dipole •Slight positive side •Smaller electronegativity •Slight negative •Larger electronegativity Dipole – dipole attraction Major Major Types of Chemical Bonds (“Intra”) 1. 2. 3. Ionic Bond Between metallic and nonmetallic elements metallic Covalent Bond Between nonmetallic elements nonmetallic Metallic bond Between metals Each of the above are electrostatic in nature electrostatic they involve attractions between electrically electrically charged species (electron ↔ proton; cation ↔ anion, etc.) These bonds require 102 to103 kJ 10 /mole to break apart. 1. Dipole-dipole forces DipoleExist among polar molecules causing them to have “preferred” orientations in the liquid / solid states. • Opposite-poles attract • Like-poles repel Page 1 Chem 210 / WANG / Chapter 11A Intermolecular Forces 2. Dispersion forces 2. Dispersion forces Between nonpolar molecules nonpolar • Nonpolar molecules have no charge separation. • This force disappears when electrons move randomly around and redistribute. Dispersion forces among nonpolar molecules. separated Cl2 molecules instantaneous dipoles They attract each other through temporary dipoles. temporary • Dispersion force is a relatively weak force relatively because of the momentary nature of the momentary dipole. Dispersion Dispersion force δ+ δ- Greater Polarizability Greater Polarizability ↔ Greater Dispersion forces Dispersion Polarizability refers to the ease with which ease charge the charge distribution in a molecule can be distorted distorted by an external electric field. larger atoms with more electrons are easily distorted Note the difference between Polarizability versus Polarity versus Polarity Polarity “permanent” shifting of electron cloud due to difference in electronegativity inside the molecule. Polarizability “ease” of shifting / distortion of electron cloud due to external influence. Br H F Shape which allows greater contact surface greater easier easier to induce electron cloud distortion electron greater greater polarizability stronger dispersion stronger dispersion forces Higher Higher boiling point Same molar mass, different shape. Boiling point drops ~30oC due to less surface contact. Fall 2009 UM-SJTU JI 2. Molar mass is a Molar major major factor influencing influencing Dispersion Dispersion forces Higher Higher molar mass Larger Larger electron cloud to shift around Greater polarizability Stronger dispersion forces Higher Higher Boiling point. BP increases rapidly down increases each group as mass↑. Smaller atom has few electrons hard to distort 2. Molecular Shape is a minor factor influencing Dispersion forces Instantaneous dipoles affecting neighboring molecules when they are close to each other in the liquid state. 3. 3. Hydrogen bonding An intermolecular attraction between intermolecular molecules with a highly electronegative atom electronegative (such as O, N or F) bonded directly to a hydrogen hydrogen (H) atom. Examples: H2O NH3 (contains H - O bond) (contains H - N bond) 4. Ion-dipole attractions Ionbetween ions and molecules Attractive forces between: an ion (cation or anion) & polar molecules. Comparative strength: Higher than most intermolecular forces Example: KBr in H2O or KBr(aq) K+ and Br- ions stabilized by surrounding dipoles (H2O). Page 2 Chem 210 / WANG / Chapter 11A Intermolecular Forces Relative comparison of forces between molecules or ions. Relative comparison of chemical bonds. Note the “kJ/mol” is much higher than those of the much than previous slide. Summary diagram for analyzing the intermolecular forces in a sample. INTERACTING PARTICLES (atoms, molecules, ions) ions present ions only IONIC BONDING (Section 8.2) ions not present nonpolar molecules only DISPERSION FORCES only polar molecules only DIPOLE-DIPOLE FORCES ion + polar molecule ION-DIPOLE FORCES H bonded to N, O, or F HYDROGEN BONDING polar + nonpolar molecules DIPOLEINDUCED DIPOLE FORCES DISPERSION FORCES ALSO PRESENT Name the type of intermolecular forces existing in this compound. H2CO First consider: Is it a polar molecule? has Polar bond: C-O shape: Name the type of intermolecular forces existing in this compound. First consider: Is it a polar molecule? All bonds (C-H, C-C) are nonpolar. First consider: Is it a polar molecule? Has polar bond: C-Cl H H H C H C Planar Triangular; polar molecule Second consider: Is there O-H bond in it? No. non polar molecule The force is “Dipole-dipole interaction.” “Dipole- H The force is “London Dispersion.” Name the type of intermolecular forces existing in this compound. H Second consider: Is there O-H bond in it? yes. H C H H O C H C6H6 CH3NH2 H H H COH H It has “hydrogen bonding” Fall 2009 UM-SJTU JI H HOC H H H Question: Name the intermolecular force in each compound. First consider: Is it a polar molecule? polar bonds: C-O, O-H Shape: Tetrahedral bent H polar Name the type of intermolecular forces existing in this compound. H H N CH3 Cl C H C Cl HH tetrahedral; slightly polar Second consider: Is there O-H bond in it? No. The force is “Dipole-dipole interaction.” “Dipole- Hydrogen bonding in water Hydrogen bonding in water causes it to have • High surface tension – Surface molecules have strong attractions with each other • Low vapor pressure & High boiling point • Molecules do not escape from liquid state due to strong attractions with each other • Ice floats on water • In ice, molecules orient in positions most favorable for H-bonding, leaving large “holes”, lowering its density. Page 3 Chem 210 / WANG / Chapter 11A Intermolecular Forces The H-bonding ability of the water molecule. Ice crystal maintained by hydrogen bonding maintained hydrogen Figure 11.10 Question: Describe how intermolecular forces relate to the boiling points of hydrides. hydrogen bond donor hydrogen bond acceptor Looking at the above structural depiction of ice, can you explain why ice floats on water? Question: Question: Describe how intermolecular forces relate to the boiling points of hydrides. For group IVA hydrides (XH4) • Boiling point ↑ from C Sn. • Due to ↑ dispersion forces. Fall 2009 UM-SJTU JI Question: Describe how intermolecular forces relate to the boiling points of hydrides. For group VIA VIA hydrides (XH2): • B.P.↑ from S Te. Te. • But H2O is higher higher than all due to Hydrogen Hydrogen Bonding being the much stronger type of intermolecular force. Page 4 ...
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This note was uploaded on 07/30/2011 for the course CHEM 210 taught by Professor Zhang during the Spring '09 term at Shanghai Jiao Tong University.

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