Chapter 12. Intermolecular Forces: Liquids, Solids, and Phase Changes

Transcription

1 Chapter 12 Intermolecular Forces: Liquids, Solids, and Phase Changes There are attractive intermolecular in all solids, liquids (called condensed phases) and gases. Molecules are held together by attractive and intramolecular (bonds within a molecule). gaseous Cl molecules Intramolecular are attractive the bonding that exist within a molecule or ionic compound holding it together (i.e. internal chemical bonds). Ionic Bonded substances have intramolecular bonds resulting from electrons transfer from one atom to another. Intermolecular are attractive between molecules that explain many important physical properties of compounds on the planet. Boiling and Freezing Point Viscosity Covalent molecules have intramolecular bond that result from sharing one or more electrons pairs between atoms usually nonmetals. Surface Tension Wetting or Not Wetting capillary action Intermolecular play a critical role in life as we understand it. Intermolecular hold together the double helix of DNA. Polar heads in aqueous exterior Embedded Proteins Nonpolar tail Aquoues Interior

2 You can think of the different phases as classes of possible molecular motion due to different kinetic energies (caused by temperature differences) and varying degrees of intermolecular. The phase of a substance depends on the interplay between KE(T), which keeps molecules far apart and moving, and IMF which keep molecules close together and condensed. Δ < 0 DEPSITIN Δ > 0 SUBLIMATIN Gas CNDENSATIN EVAPRATIN Δ > 0 Δ < 0 Temperature KE IMF Properties Vs Kinetic molecular theory states that the temperature of collection of particles (system) is proportional to the average kinetic energy of the collection. MELTING Δ > 0 KE = Ek = 3 2 RT Solid FREEZING Δ < 0 Liquid Kinetic Energy Temperature The phase of a substance depends on the interplay between KE(T), which keeps molecules far apart and moving, and IMF which keep molecules close together and condensed. Property Gases Liquid Solids Molecular Mobility Expand to occupy container Assumes shape of container Fixed Shape molecules fixed Density Very low density igh igh Electronegativity is an element s inherent property to draw electrons to itself when chemically bonded to another atom in a molecule. Lookout for bonds having: F,, Cl, N, Br, I F is the most electronegative element Compressibility igh Very Very low Notcompressible Diffusion high molecular speeds Molecule slide past one another flow Atoms and molecules are fixed Kinetic Energy igh Medium Low Intermolecular Forces Few and small Many Many Cs the least Polarcovalent bonds result from differences in electronegativity between any two bonded atoms with geometry taken into consideration. When combined with a molecule s geometry it can produce a net dipole moment (dipole) in molecules. Polar Bond Polar Bond Dipole charge separation We must consider molecular and electron geometry to determine a molecule is polar! 2 EG 3 EG 4 EG Net Dipole Moment Polar Bond Polar Bond No Net Dipole Moment Think of the dipole moment as a molecule with separated charges and that can influence other charged molecules. No dipole (no charge separation) 5 EG 6 EG

3 Comparing the strengths of Intermolecular Forces Force Strength EnergyDistance Interactions London Forces 110 kj/mole 1/r6 Allmolecules DipoleDipole 340 kj/mole 1/r3 Polar molecules ydrogen Bonding 1040 kj/mole 1/r3 N F IonDipole 1050 kj/mole 1/r2 IonsPolar molecules IonIon >>200 kj/mole 1/r CationAnion Connecting Dots: large electronegativity differences in a bond => polar molecules => large dipole moments => gives rise to large IMF => decreased vapor pressure => increased boiling and melting points, higher viscosity, higher surface tension.!!!! Polar with net dipole moment Polar with!! net dipole moment!!!!! Polar with net dipole moment nonpolar with no net dipole moment nonpolar with no net dipole moment Intermolecular are classified into four major types. Summary of 4 types of IMF s Type of Interaction 1. Iondipole: occur between neighboring an ion solution and a polar molecule (dipole) also in solution. Ion Na Dipole 2. Dipoledipole: occurs between neutral polar molecules (they can be different or the same interacting molecules). Dipole 4. London Dispersion Forces are attractive IMF s that occur between all molecules. Spontaneous dipoles are formed randomly or induced by other charged species in neutral polarizable molecules two polar molecules (dipoles) interact electrostatically. 525 ion or a dipole induces a dipole in a polarizable nonpolar molecule. 215 also called London Dispersion Forces occurs between nonpolar molecules via polarizability Induced Dipole Induced Dipole Intermolecular result from electrostatic that occur between ions and dipole moments. 1. IonDipole an ion interacts with a polar molecule (dipole). Dipole Energy (kj/mol) ighest Induced Dipole Interaction Dipole Ion 3. Induceddipoles: occur when a ion or a Ion or Dipole Induced Dipole dipole induces a spontaneous dipole in a neutral polarizable molecule. Strength Lowest Iondipole are attractive between an ion in solution and a neighboring polar molecule. 2. DipoleDipole end of dipole attracted to ions Molecules with a dipole moment in solution Na end of dipole attracted to ions 3. London or Van der Waal Forces An anion in solution A cation in solution

4 DipoleDipole intermolecular are attractive that occur between polar molecules (same or not the same). Nonpolar Molecule (no dipole moment) Polar Molecule (dipole moment) ydrogen bonding is a special case of dipoledipole intermolecular force that occurs between a hydrogen atom and an unshared pair of electrons in a polar N,, or F bond. VS Notice the very large difference in boiling point caused by IMF s present in acetone (lacking in butane)! Water is a highly polar molecule due to oxygen inherent ability to attract electrons more so than hydrogen. The boiling points of covalent binary hydrides increase with increasing molecular mass down a Group but the hydrides of N3, 2 and F have abnormally high BP because of hydrogen bonding. There is more electron density near the oxygen atom and less around the hydrogen (arrows). This polarity property gives water its dissolving properties of other polar substances. Temperature C ? Increased London Forces Period As dipole moments increase in polar substances of the same mass, IMF s increase as do boiling points (lower vapor pressure) and melting points. London Dispersion Forces (or Van deer waal ) are attractive intermolecular when temporary dipoles are formed due to random electron motions in all polarizable molecules. Two nonpolar polarizable molecules Nonpolar Polar spontaneous movement of electrons forming an instantaneous dipole moment Very polar InducedDipole Moment and IMF between molecules

5 Polarizability is the ease with which an electron distribution (cloud) in the atom or molecule can be distorted by an outside ion or dipole. nonpolar molecule (electron cloud) spontaneous induced dipole that depends on polarizability of substance London Dispersion Force induced dipole dispersion All molecules have at least this IMF Polarizability and London dispersion trends mirror atomic size trends in the periodic table. Size and polarizability increases down a group Polarizability increases right to left (bigger size more distortable. PLARIZABILITY Polarizability increases with molar mass (# e) of a molecule Cations are less polarizable than their parent atom because they are smaller and more compact. Anions are more polarizable than their parent atom because they are larger. London dispersion increase with size and polarizability. Increasing Size/ Polarizability Cations are less polarizable than their parent ground state atoms while anions have a larger polarizability than their ground state atoms. Rank the following in order of increasing polarizability: Increasing Size & Polarizability Na vs Na Br vs Br Mg 2 vs Al Cl vs Cl Rank the following in order of increasing polarizability: Na > Na Br < Br Mg 2 < Al Cl > Cl Larger unbranched molecules are more polarizable than compact branched molecules. neopentane bp=10 o C tetrahedral normal pentane bp=36 o C extended structure Larger polarizability in unbranched molecules explains boiling and melting points trends in isomers. Key Generalizations Good To Know About IMF s 1. IonIon > IonDipole > DipoleDipole > London Forces 2. For polar molecules of approximately the same mass and shape and volume (i.e. polarizability the same), dipoledipole dictate the difference in physical properties. 3. ydrogen bonding (special dipoledipole) occurs with specific polar bonds: F,, N C= and an unshared pair of electrons on a nearby electronegative atom usually F,, or N. 4. For nonpolar molecules of the same molecular mass, longer less compact molecules are more polarizable than compact molecules and show London. 5. For nonpolar molecules of widely varying molecular mass, those with more mass are typically more polarizable and experience greater London dispersion and exhibit higher boiling points and melting points.

6 ! Arrange the following substances in order of increasing boiling points.! Arrange the following substances in order of increasing boiling points. C 2 6, N 3, Ar, NaCl, As 3 C 2 6, N 3, Ar, NaCl Ar < C 2 6 < N 3 < NaCl nonpolar nonpolar polar ionic London London dipoledipole bonding ionion Which of the following substances exhibits bonding? For those that do, draw two molecules of the substance with the bonds between them. Which of the following substances exhibits bonding? For those that do, draw two molecules of the substance with the bonds between them. (a) C 2 6 (b) C 3 (c) C 3 C N 2 (a) C 2 6 (b) C 3 (c) C 3 C N 2 PLAN: Find molecules in which is bonded to N, or F. Draw bonds in the format B: A. PLAN: Find molecules in which is bonded to N, or F. Draw bonds in the format B: A. SLUTIN: (b) (a) C 2 6 has no bonding sites. C (c) C 3 C N C N C 3 C C 3 C N N C 3 C Which substances experience dipoledipole intermolecular? SiF4,CBr3, C2, S2 Which substances experience dipoledipole intermolecular? SiF4,CBr3, C2, S2 SiF4, geometry tetrahedral, SiF bonds are polar, but no molecular dipole; bond dipoles cancel. No dipoledipole interactions. C2, linear geometry, C bonds are polar but symmetry cancels net diple, but no molecular dipole; bond dipoles cancel and no dipole IMF s S2, bent geometry, S bonds are polar and do not cancel. Sulfur lone pair dipole only partially offsets net bond dipole. as dipoledipole CBr3, tetrahedral geometry, C and CCl bonds are polar and do not cancel S2 and CCl3 experience dipoledipole intermolecular.

7 Arrange the following nonpolar molecules in order of increasing melting point. SiF4, CS2, CI4, GeCl4 Arrange the following nonpolar molecules in order of increasing melting point. SiF4, CS2, CI4, GeCl4 Solution. None of these molecules poses a net dipole moment. nly dispersion exist and these are expected to increase with increasing molecular mass (more polarizable as a molecule gets larger). The molar masses of these substances follow: Substance Molar Mass CS SiF CI GeCl The intermolecular, and the melting points, should increase in the following order: CS2 < SiF4 < GeCl4 < CI4 The experimentally determined melting points are 110.8, 90, 49.5, and 171 o C, respectively. For each pair of substances, identify the dominant intermolecular in each substance, and select the substance with the higher boiling point. (a) MgCl 2 or PCl 3 (b) C 3 N 2 or C 3 F (c) C 3 or C 3 C 2 (d) exane (C 3 C 2 C 2 C 2 C 2 C 3 ) or 2,2dimethylbutane PLAN: C 3 C 3 CC 2 C 3 C 3 Bonding are stronger than nonbonding(intermolecular). ydrogen bonding is a strong type of dipoledipole force. Dispersion are decisive when the difference is molar mass or molecular shape. For each pair of substances, identify the dominant intermolecular in each substance, and select the substance with the higher boiling point. SLUTIN: (a) Mg 2 and Cl are held together by ionic bonds while PCl 3 is covalently bonded and the molecules are held together by dipoledipole interactions. Ionic bonds are stronger than dipole interactions and so MgCl 2 has the higher boiling point. (b) C 3 N 2 and C 3 F are both covalent compounds and have bonds which are polar. The dipole in C 3 N 2 can bond while that in C 3 F cannot. Therefore C 3 N 2 has the stronger interactions and the higher boiling point. (c) Both C 3 and C 3 C 2 can bond but C 3 C 2 has more C for more dispersion force interaction. Therefore C 3 C 2 has the higher boiling point. (d) exane and 2,2dimethylbutane are both nonpolar with only dispersion to hold the molecules together. exane has the larger surface area, thereby the greater dispersion and the higher boiling point. What type Describe of intermolecular the intermolecular can you that recognize exist between in the following each of ionic the following and covalent molecules? compounds. Br(g) What type Describe of intermolecular the intermolecular can you that recognize exist between in the following each of ionic the following and covalent molecules? compounds. Br(g) Br is a polar molecule: dipoledipole. There are also dispersion between Br molecules. C 4 C 4 C 4 is nonpolar: London dispersion. S 2 S 2 S S 2 is a polar molecule: dipoledipole. There are also dispersion between S 2 molecules.

8 Any Name questions the types of on intermolecular homework from in Chapter the following 11? compounds. In which substances would hydrogen bonding occur between molecules? C3C2C2C3 C3 C26, CCl3, C3C2, N3, P3 C3CN2 C3C C3C2C2C2 C3C2CC3 In which substances would hydrogen bonding occur between molecules? C26, CCl3, C3C2, N3, P3 Solution. ydrogen is bonded to one of the very electronegative atoms in C3C2 and N3. ydrogen bonding should occur in both of these substances. Water Is Wild and Unique and Taken For Granted igh bonding = high cohesive responsible for the transport of water in roots and xylem of trees and plants. Basepairing in doublestranded DNA eat capacity, high heat of vaporization, inverted density of water and ice. Water Is A Unique Substance Ice is less dense than water Density of Water Maximum Density 4 0 C great solvent due to high polarity and hydrogen bonding ability exceptional high specific heat capacity and!vaporization Density (g/cm 3 ) high surface tension and capillarity large density differences of liquid and solid states Temperature C