Chapter 1. Dipole Moments, Molecular Polarity and Intermolecular Forces

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1 Chapter 1 Dipole Moments, Molecular Polarity and Intermolecular Forces

2 Polarity of Molecules For a molecule to be polar, it must have polar bonds, and have an unsymmetrical shape Polarity affects the intermolecular forces of attraction and therefore affects boiling points and solubilities Nonbonding pairs affect molecular polarity.

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4 CH4 NH3 H2O

5 Predicting Polarity of Molecules 1. Draw the Lewis structure and determine the molecular geometry. 2. Determine whether the bonds in the molecule are polar. 3. Determine whether the polar bonds add together to give a net dipole moment.

6 Molecular Polarity : F N F F μ = D

7 Attractive Forces Particles are attracted to each other by electrostatic forces. The strength of the attractive forces depends on the kind(s) of particles. The stronger the attractive forces between the particles, the more they resist moving. The strength of the attractions between particles of a substance determines its physical state.

8 Kinds of Attractive Forces Dispersion Forces between Molecules Temporary polarity in molecules due to unequal electron distribution Dipole Dipole Attractions between Molecules Permanent polarity in molecules due to their structure Hydrogen Bonds between Molecules An especially strong dipole dipole attraction resulting from the attachment of H to an extremely electronegative atom Ion Dipole Attractions - Not Intermolecular Between mixtures of ionic compounds and polar compounds (esp. aqueous solutions)

9 Some molecules are considered nonpolar because of the atoms which they contain and the arrangement of these atoms in space. CH4 BH3 C2H2 CO2 Nonpolarized electron clouds But these molecules can all be condensed.

10 Origin of Instantaneous Dipoles δδδδ+ The δδ+ charge attracts electrons. The δδ- charge repels electrons.

11 Size of the Induced Dipole The magnitude of the induced dipole depends on several factors: Polarizability of the electrons Volume of the electron cloud larger molar mass more electrons larger electron cloud increased polarizability stronger attractions Larger molecules have more electrons, leading to increased polarizability.

12 Size of the Induced Dipole Shape of the molecule more surface-to-surface contact larger induced dipole stronger attraction Molecules that are flat have more surface interaction than spherical ones.

13 Effect of Molecular Size on Magnitude of Dispersion Force As the molar mass increases, the number of electrons increases. Therefore, the strength of the dispersion forces increases. The stronger the attractive forces between the molecules, the higher the boiling point. Gas Radius Molar Mass B.P.(K) He Ne Ar Kr Xe Rn

14 Boiling Points of Straight Chain Alkanes NonPolar Molecules

15 Effect of Molecular Shape on Size of Dispersion Force n-pentane molar mass=72.15 b.p = 36.1 ºC 2-methylbutane molar mass=72.15 b.p = 27.9 ºC 2,2-dimethylpropane molar mass=72.15 b.p = 9.5 ºC A larger surface-to-surface contact between molecules results in stronger dispersion force attractions and a higher boiling point.

16 Practice Choose the Substance in Each Pair with the Higher Boiling Point a) CH4 C4H10 b) C6H12 C6H12

17 Dipole Dipole Attractions Some molecules are inherently polar because of the atoms which they contain and the arrangement of these atoms in space. H2O NH3 CH2O HCl δ δ+ A crude representation of a polar molecule

18 Dipole Dipole Attractions Polar molecules have a permanent dipole because of bond polarity and shape 1) dipole moment 2) as well as the always present induced dipole The permanent dipole adds to the attractive forces between the molecules

19 Effect of Dipole Dipole Attraction on Boiling and Melting Points Name Formula Molar mass Structure Structure b.p. m.p. formaldehyde CH2O º -92º H H C H ethane C2H º -172º H C H H

20 Determine if dipole dipole attractions occur between CH2Cl2 molecules Formula Lewis Structure Bond Polarity Molecule Polarity Cl C = 0.5 polar 4 bonding areas no lone pairs tetrahedral shape C H = 0.4 nonpolar polar molecule; therefore dipole dipole attractions do exist

21 Hydrogen Bonding When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it: O H, N H, F H Because hydrogen has no other electrons, when its electron is pulled away, the nucleus becomes deshielded, exposing the H proton. The exposed proton acts as a very strong center of positive charge.

22 H-Bonding in Water

23 Hydrogen Bonding and Boiling Points 100 H2O Group IVA Group VA Group VIA Group VIIA HF Boiling Point (ºC) NH3 H2S PH3 HCl H2Se AsH3 HBr GeH4 H2Te SbH3 HI SnH4 SiH4 CH Period

24 Effect of Hydrogen-Bonding on Boiling and Melting Points Name Formula Molar mass Structure Structure b.p. m.p. ethanol C2H6O º º dimethyl ether C2H6O º º

25 One of these compounds is a liquid at room temperature (the others are gases). Which one and why? MM = Polar No H-Bonds MM = Polar No H-Bonds MM = Polar H-Bonds b.p. -19ºC -78ºC +150ºC Because only hydrogen peroxide has the additional very strong H-bond additional attractions, its intermolecular attractions will be the strongest. We therefore expect hydrogen peroxide to be the liquid.

26 Hierarchy of Intermolecular Forces Molecules containing O-H, N-H, or F-H Bonds Polar Molecules H-bonding All Molecules Dipole forces Dispersion forces

27 Melting Points of n-alkanes

28 Boiling Points of Other Organic Families Alkanes Ethers Alcohols Amines CH3CH2CH3-42.1º CH3OCH3-23.7º CH3CH2OH +78º CH3CH2NH º CH3CH2CH2CH3-0.5º CH3OCH2CH º CH3CH2CH2OH +97.4º CH3CH2CH2NH º CH3CH2CH2CH2CH º CH3CH2OCH2CH º CH3CH2CH2CH2OH CH3CH2CH2CH2NH º

29 Solubility

30 Solubility When one substance (solute) dissolves in another (solvent) it is said to be When one substance does not dissolve in another it is said to be The solubility of one substance in another depends on two factors - nature s tendency toward mixing, and the solute and solvent.

31 Attractive Forces Acting Between Ions & Molecules Pure Electrostatic Attractions - chiefly ionic compounds Dipole-dipole attractions - chiefly between molecules δ+ δ δ+ δ δ+ δ δ+ δ δ+ δ δ+ δ δ+ δ δ+ δ Dispersion Forces Very weak, due to a temporary shift in electron distribution dependent on the size of the molecule

32 What happens when you dissolve an ionic compound in water?? What happens when you dissolve a polar molecule in water??

33 What Happens When an Ionic Compound Dissolves in Water?

34 What Happens When a Polar Covalent Compound Dissolves in Water? dipole-dipole attractions

35 What happens when you try to dissolve a nonpolar molecule in water??

36 What happens when you try to dissolve a nonpolar molecule in water?? Non polar solvents, such as ethanol, carbon tetrachloride, ether, and hexane, are also commonly used to dissolve nonpolar solutes, such as grease and oils.

37 General Solubility Rule: Like Dissolves Like Polar solutes form solutions with polar solvents. Nonpolar solutes form solutions with nonpolar solvents.

38 Selected Polar and Nonpolar Solvents POLAR SOLVENTS water, H2O methanol, CH3OH ethanol, C2H5OH acetone, C3H6O methyl ethyl ketone, CH3CH2C(O)CH3 formic acid, HCOOH acetic acid, CH3COOH NONPOLAR SOLVENTS hexane, C6H14 heptane, C7H16 toluene, C7H8 carbon tetrachloride, CCl4 chloroform, CHCl3 methylene chloride, CH2Cl2 ethyl ether, CH3CH2OCH2CH3

39 Solubility* of a Series of Alcohols in Water and Hexane