Intermolecular forces are classified into four major types.

Transcription

1 Intermolecular forces are classified into four major types. 1. Ion-dipole: IMF s that occur between neighboring an ion solution and a polar molecule (dipole) also in solution. Na+ 2. Dipole-dipole: IMF s that occur between a neutral polar molecules (same or not the same molecules). 3. Induced-dipoles: IMF s that occur when a ion or a dipole induces a spontaneous dipole in a neutral polarizable molecule. Ion or Dipole Induced Dipole London Dispersion Forces are attractive IMF s that occur when spontaneous dipoles are formed randomly or induced by other charged species in neutral polarizable molecules.

2 Intermolecular forces arise from electrostatic forces that depend on distance between molecules. Coulomb s Law: The electric force acting on a point charge q1 as a result of the presence of a second point charge q2 some r meters away is given by: F = k q 1 q2 r 2 k depends on medium (kair = 9.0 x 10 9 N m 2 /C 2 ) units of coulombs + - r units of meters Because Energy = Force x Distance Energy of Attraction is: E = k q 1 q2 r If we can predict charge separation or polarity in molecules we can identify IMF s in play for that molecule. - +

3 Differences in electronegativity between bonding atoms blurs the distinction between covalent, polar covalent and ionic bonding types. δ symbols indicates partial charge on atoms. Covalent Bonding Polar Covalent Bond Ionic Bonding δ+ δ- F2 HF LiF Dipole Moment χ = EN B - EN A

4 Electronegativity is an element s inherent property to draw electrons to itself when chemically bonded to another atom in a molecule. F is the most electronegative element Cs the least

5 Electronegativity values from a table can be used to judge the extent of ionic, polar or covalent bond character in a chemical bond. Mostly Ionic χ χ = EN B - EN A absolute value of electronegativity difference between 2-bond atoms Polar Covalent Mostly Covalent 0.0

6 Polar-covalent bonds combined with a molecule s geometry produce net dipole moments (dipoles) in molecules. Polar Bond Polar Bond Polar Bond Polar Bond No Net Dipole Moment Net Dipole Moment Think of the dipole moment as a molecule with separated charges + and - that can influence other charged molecules. This is the origin of IMF s! Dipole Dipole

7 Recall your molecular and electron geometry from Chem 7. 3 EG 4 EG 2 EG 5 EG 6 EG

8 We identify polar bonds and use the molecular geometry to gauge if there is a net dipole moment (dipole) in a molecule. δ+ Polar with net dipole moment δ- δ- δ+ δ- δ- δ- δ- δ+ δ+ Polar with net dipole moment δ- non-polar with no net dipole moment Polar with net dipole moment non-polar with no net dipole moment

9 We need to connect some learning dots: electronegativity differences => polar molecules => dipole moments => gives rise to IMF => altering vapor pressure => boiling points, freezing points, viscosity, surface tension and more.

10 Summary of Intermolecular Types of Forces and Energies Ion-dipole H-Bond Dipole-Dipole Ion-induced dipole Dipoleinduced dipole Dispersion (London)

11 Summary of Intermolecular Forces (van der waals forces) Type of Interaction Strength Interaction Energy (kj/mol) Ion + - Dipole + Highest --an ion interacts with a polar molecule (dipole) Dipole + - Dipole + --two polar molecules (dipoles) interact electrostatically Ion + Dipole + - Induced Dipole - + Induced Dipole ion or a dipole induces a dipole in a polarizable non-polar molecule. --occurs between non-polar Induced Dipole molecules via polarizability --also called London Dispersion - + forces Lowest

12 Ion-dipole intermolecular forces are attractive forces between an ion in solution and a neighboring polar molecule. - end of dipole attracted to + ions Molecule with a dipole moment + end of dipole attracted to -ions The greater the dipole moment or ion charge the stronger the IMF. The closer the ion is to the dipole the stronger the interaction: Coloumb s Law F= q1 q2/r 2 An anion in solution A cation in solution

13 Dipole-Dipole intermolecular forces are attractive forces that occur between polar molecules (same or not the same). Polar molecules give rise to dipole-dipole IMF s Boiling point differences are explained by dipole-dipole interactions More polar molecule => stronger dipoledipole IMF s => higher boiling points => lower vapor pressure.

14 Dipole-induced dipole are attractive intermolecular forces that occur when a polar molecule (dipole) induces a dipole by distorting an electron cloud in a polarizable molecule. polar molecule non-polar molecule induced dipole

15 As dipole moments increase in polar substances of the same mass, IMF s increase as do boiling points (lower vapor pressure) and melting points. Non-polar Polar Very polar

16 Hydrogen bonding is a special case of dipole-dipole intermolecular force that occurs between a hydrogen atom and an unshared pair of electrons in a polar N-H, O-H, or F-H bond. Double H-bonds in acetic acid CH3COOH Boiling points of two isomers: EtOH and DME 78.3 C C

17 Water is a highly polar molecule due to oxygen inherent ability to attract electrons more so than hydrogen. 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.

18 The boiling points of covalent binary hydrides increase with increasing molecular mass down a Group but the hydrides of NH3, H2O and HF have abnormally high BP because of hydrogen bonding.? Increased London Forces

19 London Dispersion Forces are attractive intermolecular forces that occur when temporary dipoles are formed due to random electron motions in all polarizable molecules. 2-neutral non-polar polarizable molecules spontaneous movement of electrons forming an instantaneous dipole moment Induced-Dipole Moment and IMF between molecules

20 Polarizability is the ease with which an electron distribution (cloud) in the atom or molecule can be distorted by an outside ion or dipole. London Dispersion non-polar molecule (electron cloud) induced dipole dispersion ion-induced dipole interaction dipole-induced dipole interaction

21 Polarizability and London dispersion trends in the mirrors atomic size trends in the periodic table. Polarizability increases right to left across a period (bigger size more distortable. Polarizability increases down a group POLARIZABILITY 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.

22 Examples of increasing London dispersion forces with increase in size and polarizability. Increasing Size/ Polarizability Increasing Size/ Polarizability

23 Larger unbranched molecules are more polarizable than more compact branched molecules. More polarizable = higher London forces, and explains trends in boiling and melting points in non-polar molecules. neopentane bp=10 o C tetrahedral normal pentane bp=36 o C extended structure

24 Alkanes: Molecular Weight vs Boiling Point Alkane (CnH2n+2) Melting Point C Boiling Point ( C) CH4 Methane CH3CH3 Ethane CH3CH2CH3 Propane C4H10 Butane C5H12 Pentane C6H14 Hexane C7H16 Heptane C8H18 Octane C9H20 Nonane C10H22 Decane

25 Some Generalizations About IMF s 1. Ion-Ion > Ion-Dipole > Dipole-Dipole > Dispersion 2. For polar molecules of approximately the same mass and shape and volume (i.e. polarizability the same), dipole-dipole forces dictate the difference in physical properties. 3. Hydrogen bonding occurs with polar bonds in particular H- F, H-O, H-N and an unshared pair of electrons on a nearby electrognegative atom usually F, O, or N. 4. For non-polar molecules of the same molecular mass, longer less compact molecules are generally more polarizable and have greater dispersion forces and show higher boiling and melting points. 5. For non-polar molecules of widely varying molecular mass, those with more mass are typically more polarizable and experience greater London dispersion forces and exhibit higher boiling points and melting points.

26 Comparing the strengths of Intermolecular Forces Force Strength Energy- Distance Interactions London Forces Dipole- Dipole Hydrogen Bonding 1-10 kj/mole 1/r 6 All-molecules 3-40 kj/mole 1/r 3 Polar molecules kj/mole 1/r 3 O-H N-H H-F Ion-Dipole kj/mole 1/r 2 Ions-Polar molecules Ion-Ion >>200 kj/mole 1/r Cation-Anion

27 Flowchart for classifying intermolecular forces

28 In general, the bulk physical properties of liquids correlate the intermolecular forces between molecules. and their correlation to IMF s

29 Three common physical properties of liquids that depend on the magnitude of IMF s include surface tension, capillarity and viscosity. capillarity surface tension viscosity The surface tension forms a skin like surface that can support mass. Viscosity is a measure of resistance to flow of a fluid.

30 Surface tension is the amount of energy required to stretch or increase the surface area of a liquid. Units of energy per unit length (J/m). molecules at the surface feel a net force downward We say that the leave has a low surface energy as water will not spread out (it takes energy to spread the water out) molecules in the interior experience equal force in 3-D

31 Surface Tension of Some Liquids Substance Formula Surface Tension (J/m 2 ) at 20 0 C Major IMF s diethyl ether CH 3 CH 2 OCH 2 CH 3 1.7x10-2 dipole-dipole; dispersion ethanol CH 3 CH 2 OH 2.3x10-2 H bonding butanol CH 3 CH 2 CH 2 CH 2 OH 2.5x10-2 H bonding; dispersion water H 2 O 7.3x10-2 H bonding mercury Hg 48x10-2 metallic bonding

32 The surface tension of a liquid is a function of temperature. Why might this be? (what forces compete here?) Hot water works better than cold water in cleaning your clothes or hands as it can more effectively wet dirt (water gets into pores and not get stuck ).

33 Capillary effect occurs when the adhesive IMF s between a liquid and a substance are stronger than the cohesive IMF s inside the liquid. Cohesion is the intermolecular attraction between like molecules. Adhesion is an attraction between unlike molecules Cohesion Adhesion Capillary rise implies that the: Adhesive forces > cohesive forces Capillary fall implies that the: Cohesive forces > adhesive forces

34 Water will rise to different heights in tubes depending on the diameter of the capillary. Can you surmise why and how?

35 Water will rise to different heights depending on the diameter of the capillary. Why?

36 Viscosity is the measure of a liquid s resistance to flow relative to one another, and is thus related to the intermolecular forces. Oil for your car is bought based on this property: 10W30 or 5W30 describes the viscosity of the oil at high and low temperatures (W means winter dudes higher the number the more viscous). In general, viscosity decreases as temperature increases and visa versa.