Intermolecular Actions IMI s. IMI Summary Sheet. Trends in the Strength of Intermolecular Attraction? 7/8/2013

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1 Intermolecular Actions IMI s More Accurately: Interspecies Interactions What holds SPECIES together in liquids and solids? Species may be atoms or molecules or ions. Must distinguish between IMI s and bonds. Why are molecules attracted to each other? intermolecular attractions are due to attractive forces between opposite charges +i ion to - ion + end of polar molecule to - end of polar molecule H-bonding especially strong even nonpolar molecules will have temporary charges larger the charge = stronger attraction longer the distance = weaker attraction however, these attractive forces are small relative to the bonding forces between atoms generally smaller charges generally over much larger distances Tro, Chemistry: A Molecular Approach 2 Trends in the Strength of Intermolecular Attraction? the stronger the attractions between the atoms or molecules, the more energy it will take to separate them boiling a liquid requires we add enough energy to overcome the attractions between the molecules or atoms the higher the normal boiling point of the liquid, the stronger the intermolecular attractive forces Type of Intermolecular Interaction London Dispersion (LD) IMI Summary Sheet Present when... All free atoms, ions and molecules Stronger Interaction indicated by Higher MM If MM approximately the same, then must consider geometry Dipole-Dipole (DD) Polar molecules Greater polarity as determined by greater EN and geometry Hydrogen Bonding (HB) Typical Energy in KJ/mole 1 s H bonded to N, O or H F>O>N 10 s and lone pair on N, O or H Ionic Bonding (IB) Ionic compound charges and sizes of ions 100 s to 1000 s 1 s Tro, Chemistry: A Molecular Approach 3 1

2 London Dispersion Forces Definition: instantaneous (spontaneous) dipole-induced dipole All molecules exhibit London forces; Only intermolecular force in nonpolar molecules. What molecular properties affect London Forces? a. Polarizability of functional groups b. Number of functional groups c. Accessibility of functional groups Note: London forces and London dispersion interactions, etc. describe the same class of IMI s. London Dispersion Animation A 1 st Order Approximation for Small or Simple Molecules What is the best first order approximation used to compare London forces between two small or simple molecules? MM ( total number of electrons) How is the number of London forces present between two molecules (and thus the intensity of the overall London interaction) affected by molecular shape? More compact shape (less surface area) results in fewer interactions and weaker London forces. Some Definitions Small molecule a molecule that has no more than a few central atoms. Simple molecule a molecule that is made up of only one type of functional unit. Functional group a portion of a molecule that contains a particular flavor of the property being considered. If one is considering chemical reactions, the part of a molecule that can be involved in a particular type of reaction (in and of itself) is a functional group. If one is considering IMI s, then a functional unit is one that displays a certain type of IMI and is usually going to be limited to the atoms attached to a particular central atom or a bond. Dipole-Dipole Interactions 1. What molecules exhibit dipole-dipole interactions? Another way to ask this question: what molecules have dipoles? 2. What factors affect the strength of the dipole-dipole interaction? Another way to answer this question: what factors affect dipole moment intensity in a molecule? a. b. c. Hint: the measurement of polarity is the dipole moment (μ) which for a single dipole (one polar bond) is the partial charge (δ) multiplied by the bond length (l): μ = δ.l. μ is measured in debye. 1 debye = C.m (C represents coulombs, the standard unit of charge). 2

3 Figure Dipole moment and boiling point Dipole-Dipole Interactions ICl IBr Boiling Point (K) HF H 2S PH 3 SiH 4 HCl O 3 ClF BrF BrCl Group II hydrides and HF Heteronuclear diatomics O 2 N Other gasses (MM) 2 Noble gases (atm wgt) Halogens (MM) Molar Mass (g/mol) Hydrogen Bonding A moderately strong attraction of a H which is bonded to N,O or F to a lone pair of electrons on a N, O or F atom on another molecule. Evidence for H-bonding What two conditions must be present for a hydrogen bond to occur?

4 Hydrogen Bonding Examples: Which of the following molecules exhibit hydrogen bonding? H 2 O CH 3 F NH 3 Factors Effecting Strength of H-Bonds CH 4 HF HC1 pyridine N dd Ion-Ion Interactions What compounds exhibit ion-ion interactions? What is the magnitude of ion-ion interactions? We will learn how to compare relative strengths of ionion interactions when we look at ionic solids. 4

5 Type of Intermolecular Interaction London Dispersion (LD) IMI Summary Sheet Present when... All free atoms, ions and molecules Stronger Interaction indicated by Higher MM If MM approximately the same, then must consider geometry Dipole-Dipole (DD) Polar molecules Greater polarity as determined by greater EN and geometry Hydrogen Bonding (HB) Typical Energy in KJ/mole 1 s H bonded to N, O or H F>O>N 10 s and lone pair on N, O or H Ionic Bonding (IB) Ionic compound charges and sizes of ions 100 s to 1000 s 1 s Comparing IMI s Example 1 HCl vs. HBr 5

6 Comparing IMI s Example 2 HCl vs. HF Comparing IMI s Example 3 CCl 4 vs. CF 4 Comparing IMI s Example 4 NaCl vs. H 2 O 6

7 Figure Molecular shape and boiling point Comparing IMI s Example 5 Hydrocarbons and Alcohols Alcohols Comparing IMI s Example 5 Hydrocarbons and Alcohols perature (deg C) Tem Alkane MP's Alkane BP's Alcohol MP's Alcohol BP's isoalkane BP's Molar Mass (g/mol) 7

8 Comparing IMI s Example 6 What is wrong with this figure from a Gen Chem book? H 2 S vs. H 2 O Boiling Points of Methyl Halogen Compounds F Cl Br CH CH 3 X CH 2 X CHX CX Comparing IMI s Example 6 CH 3 Cl vs. CH 3 F 8

9 dichlorobenzenes Cl Cl Cl Cl Cl Cl 1,2-dichlorobenzene 1,3-dichlorobenzene 1,4-dichlorobenzene (o-dichlorobenzene) (m-dichlorobenzene) (p-dichlorobenzene) m.p. = , b.p. = m.p. = , b.p. = 173 sublimes, b.p. =