Intermolecular Forces

Transcription

1 Intermolecular Forces H covalent bond (stronger) Cl H Cl intermolecular attraction (weaker) The attractions between molecules are not nearly as strong as the covalent bonds that hold atoms together. They are, however, strong enough to impact physical properties such as boiling and melting points, vapor pressures, and viscosities. These intermolecular forces as a group are sometimes referred to as van der Waals forces. 1

2 van der Waals Forces London dispersion forces Dipole-dipole interactions Hydrogen bonding Johannes van der Waals Fritz London 2

3 Three Types of Dipoles 1. Permanent Dipoles: Polar molecules which are the result of electronegativity differences between atoms and/or molecular geometry. H Cl Examples: HCl, H2O, NH3 δ+ δ- 2. Instantaneous Dipoles: A temporary dipole which results from a fluctuation in the electron cloud. May also occur in noble gases. N N δ+ δ- 3. Induced Dipoles: A temporary dipole forms due to the influence of a nearby dipole. δ+ δ- H Cl N N N δ+ δ- N 3

4 Dipole-Dipole Interactions Polar molecules (permanent dipoles) are attracted to nearby polar molecules due to electrostatic forces and dipoles present. (stronger than London dispersion forces for molecules with similar sized electron clouds) The molecules must be close together for dipoledipole forces to take effect. Which of these molecules would have dipole-dipole interactions? CO2, CO, ClF, N2 CO, ClF 4

5 Dipole-Dipole Interactions dimethyl ether C 3 H 8 ethanal C2H4O propane C3H8 acetonitrile C 2 H 3 N chloromethane CH 3 Cl All of these molecules have the same size electron cloud. Rank them according to increasing boiling point. Explain. 1. Propane (BP: 231 K; Dipole Moment: 0.1 D) 2. Dimethyl ether (BP: 248 K; Dipole Moment: 1.3 D) 3. Ethanal (BP: 249 K; Dipole Moment: 1.9 D) 4. Chloromethane (BP: 294 K; Dipole Moment: 2.7 D) 5. Acetonitrile (BP: 355 K; Dipole Moment: 3.9 D) 5

6 Instantaneous Dipoles Due to fluctuations in the electron clouds of atoms and molecules, partial positive and partial negative regions of atoms and molecules form. The tendency of an electron cloud to distort is called polarizability. Larger electron clouds are described as being more polarizable. δ+ δ- δ+ δ- 6

7 Instantaneous Dipoles An instantaneous dipole can induce a charge in a nearby non-polar molecule. This results in a type of London dispersion force called an instantaneous dipole-induced dipole attraction. δ+ δ- δ+ δ- This is the type of attraction that holds liquid bromine molecules or solid iodine molecules together. 7

8 Study this table: Halogen Electrons Boiling Point (K) F Cl Br I Observations? Relationship? Explanation? Direct relationship (increase in electron cloud size results in a higher boiling point) due to greater polarizability of the electron cloud which results in stronger dispersion forces and therefore more energy is needed to separate the molecules. 8

9 Same pattern for the noble gases: Noble Gas Electrons Boiling Point (K) He 2 5 Ne Ar Kr Xe Larger electron clouds are more polarizable which results in stronger intermolecular forces. 9

10 Which Have a Greater Effect: Dipole-Dipole Interactions or Dispersion Forces? If two molecules are of comparable size and shape, dipole-dipole interactions will likely be the dominating force. If one molecule is much larger than another, dispersion forces will likely determine its physical properties. 10

11 Describe the relationship between the strength of the existing intermolecular forces and the following properties: Melting Point: Boiling Point: Vapor Pressure: Viscosity: Enthalpy of Fusion: Enthalpy of Vaporization: Surface Tension: Direct Direct Inverse Direct Direct Direct Direct 11

12 Observations? H2O, HF, & NH3 are out of place. 12

13 Hydrogen Bonding The dipole-dipole interactions experienced in molecules where H is bonded to N, O, or F are unusually strong. We call these interactions hydrogen bonds. The hydrogen bond is the attraction of the hydrogen of one molecule to a lone pair of electrons on a nearby molecule. Which of these molecules would hydrogen bond to molecules identical to itself? HF, CH4, NH3, HCl HF, NH3 13

14 Two essential requirements for the formation of a hydrogen bond: One molecule must contain at least one H atom attached to a highly electronegative atom (F, O or N). The other molecule must contain an F, O, or N atom that provides the lone pair of electrons. 14

15 Hydrogen Bonding H F H F H N H H H N H H 15

16 Hydrogen Bonding H2O Water undergoes extensive hydrogen bonding. 16

17 Each H 2 O molecule can be bonded tetrahedrally to four H 2 O molecules

18 Hydrogen Bonding Hydrogen bonding can take place between dissimilar molecules: H O H H N H H H O H H N H H O O O H H 18

19 Hydrogen Bonding Make a model of methanol (CH3OH) and water (H2O). Arrange the models to show two possible hydrogen bonding configurations. Show them to the instructor. 19 Why is this arrangement NOT a depiction of hydrogen bonding? Hydrogens bonded to carbon lack the ability to hydrogen bond.

20 Identify the hydrogen atoms of the following species that are capable of forming hydrogen bonding with water molecules. Soluble in water adenine glucose 20

21 Hydrogen Bonding The unusually strong dipole interactions found in hydrogen bonding arises from: 1. The high electronegativity of nitrogen, oxygen, and fluorine. 2. The small size of nitrogen, oxygen, and fluorine. 3. When hydrogen is bonded to one of those small and very electronegative elements, the hydrogen nucleus is exposed. 21

22 Effect of hydrogen bonding on DNA The presence of intermolecular H-bonds helps maintain the double helical shape of DNA molecules. hydrogen bonds 22

23 Hydrogen bonding in DNA Cytosine Guanine (3 hydrogen bonds) 23 Thymine Adenine (2 hydrogen bonds)

24 Strength of van der Waals forces Depends on three factors (in decreasing order of importance) : 1. Size of molecule 2. Shape (Surface area) of molecule 3. Polarity of molecule 24

25 1. Size of Molecule Usually! Molecule Helium Neon Argon Fluorine Chlorine Bromine Methane Ethane Propane 25 Boiling point ( o C) Rel. Size molecular of molecule mass Size of electron cloud Polarizability Dispersion forces

26 2. Surface area of molecule The van der Waals forces also increase with the surface area of the molecule. van der Waals' forces are short-ranged forces Atoms or molecules must come close together for significant induction of dipoles. 26

27 Pentane (C 5 H 12 ) Dimethylpropane (C 5 H 12 ) Boiling point: 36.1 C Boiling point: 9.5 C Observations: Same # of electrons; both are non-polar; pentane has a higher boiling point Explanation: The shape of pentane (rod or plate) has more surface area (and contact area) than the shape (ball) of dimethylpropane. Pentane is more polarizable. 27

28 H 3 C δ+ C O δ H 3 C H 2 C C H 2 CH 3 H 3 C 32 electrons boiling point = 50 C 34 electrons boiling point = 0 C Observations? Explain: Dispersion and Dipole-dipole forces Dispersion forces 28

29 CH 3 Cl CH 3 Br CH 3 I Boiling Point ( C ) Dipole Moment (D) Observations? Explain: Chloromethane is more polar; the effect of dispersion forces outweighs that of dipole-dipole forces. 29

30 H 3 C H 2 C CH CH 3 CH 3 H 3 C CH 3 C CH 3 CH 3 H 3 C H 2 C CH2 H 2 C CH3 Match the boiling point to the molecule: Explain: 9.5 C Rod or plate shaped 27.7 C molecules are more polarizable than 36.1 C ball shaped molecules. 30

31 F 2 Cl 2 ClF CH 2 Cl 2 Match the boiling point to the molecule: Explain: Cl 2 > ClF because: 39.6 C C 1. Cl 2 has more electrons and is more than polarizable than ClF C -188 C 2. Although ClF is polar, the effect of dispersion forces outweighs that of dipole-dipole forces. 31

32 Substance electron cloud size Boiling point ( C) NH HF H 2 O Explain: HF is more polar than NH 3 32 H 2 O is able to form two hydrogen bonds per molecule (HF and NH 3 only form one).

33 hydrogen bond Each NH 3 molecule has only ONE lone pair. On the average, each NH 3 molecule can form 33 only ONE hydrogen bond

34 Each HF molecule has only ONE hydrogen atom. On the average, each HF molecule can form only ONE hydrogen bond 34

35 hydrogen bond Each H 2 O molecule has TWO hydrogen atoms and TWO lone pairs. On the average, each H 2 O molecule can form 35 TWO hydrogen bonds

36 Compound Propanal CH 3 CH 2 CHO Ethanol C 2 H 5 OH Methanol CH 3 OH # of electrons Boiling point ( o C) Explain the boiling point differences. For molecules with similar structures, their boiling points depend on the polarizability of their electron clouds. As the relative size of the electron cloud of ethanol is greater than that of methanol, the boiling point of ethanol is higher. Methanol and Ethanol both have hydrogen bonding, propanal does not have hydrogen bonding. 36

37 Vapor Pressure C4H10O Observations: Diethyl ether (42 e - ) has a lower vp and nbp than ethanol (27 e - ), water (10 e - ), or ethylene glycol (36 e-). H2O C2H6O C2H6O2 37 Explanation: Ethanol, water, and ethylene glycol have hydrogen bonding.

38 Name the types of bonds or intermolecular forces that are broken and formed in the following processes. H 2 O(s) H 2 O(g) 2Mg(s) + O 2 (g) 2MgO(s) H 2 (g) + Br 2 (l) 2HBr(g) HBr(l) HBr(g) 2Na(s) + 2H 2 O(l) 2NaOH(aq) + H 2 (g) CH 3 CH 2 OH(l) + 3O 2 (g) 2CO 2 (g) + 3H 2 O(l) 38 broken: H bond; formed: none broken: metallic bond, covalent bond; formed: ionic bonds broken: covalent bond, dispersion forces; formed: covalent bond broken: dispersion forces; formed: none broken: covalent, metallic & H bonds; formed: ion-dipole & covalent bonds broken: covalent & H bonds; formed: covalent & H bonds

39 A separation technique. Chromatography Paper Gel Many forms: paper, gel, gas, liquid. Two phases: stationary & mobile Based on molecule interactions with the phases. Size, shape, nature of molecules (polar, non-polar) determine how quickly they move. Applications: Forensics, DNA, Medicine, Analytical Chemistry. 39

40 Relative strength of London dispersion forces, hydrogen bond, and covalent bond Phenomenon Energy absorbed (kj mol -1 ) Forces overcome O 2 (s) O 2 (g) 3.63 dispersion forces H 2 O(s) H 2 O(g) hydrogen bonds O 2 (g) 2O(g) covalent bonds 40