Intermolecular Forces Irresistible attraction…

DO NOW: Rank the following phases of matter in order from least to greatest for… A) Energy B) Intermolecular Forces High Energy Medium Energy Low Energy Weak Intermolecular Forces Medium Intermolecular Forces Strong Intermolecular Forces Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 441

ATTRACTIVE FORCES Always electrostatic in nature Intramolecular forces bonding forces These forces exist within each molecule. They influence the chemical properties of the substance. Intermolecular forces nonbonding forces These forces exist between molecules. They influence the physical properties of the substance.

Why Are Molecules Attracted to Each Other? Intermolecular attractions are due to attractive forces between opposite charges + ion to − ion + end of polar molecule to − end of polar molecule H-bonding especially strong even nonpolar molecules will have temporary charges generally over much larger distances

Why Are Molecules Attracted to Each Other? Larger charge = stronger attraction Longer distance = weaker attraction However, these attractive forces are small relative to the bonding forces between atoms generally smaller charges generally over much larger distances

Polarity Draw the dipole diagram for each of the following. Then, predict whether the compounds are polar or nonpolar. HI N2

Intermolecular forces (IMF) London dispersion forces: The forces that exist among noble gas atoms and non-polar molecules Dipole-dipole attraction: Molecules with dipole moments (polar molecules) can attract each other by lining up so that the positive and negative ends are close to each other. Hydrogen bonding: when hydrogen is bonded to a highly electronegative -FON Increasing Strength of IMF

Dispersion Force

Dispersion forces among nonpolar molecules. separated Cl2 molecules instantaneous dipoles

Increase in MM (molar mass) = Increase in London Dispersion Forces

Dipole-Dipole Bonds Dipole-Dipole Attraction: the attractive force resulting when polar molecules line up such that the positive and negative ends are close to each other Usually weak bonds (1% as strong as covalent or ionic bonds) Become weaker as the distance between the dipoles increases Strong forces occur between molecules in which hydrogen is bound to highly electronegative atoms (ex: nitrogen, oxygen, or fluorine) Two Factors Account for Strength: The great polarity of the bond The small size of hydrogen allowing the dipole moments to be close together

Dipole–Dipole Attractions Polar molecules have a permanent dipole because of bond polarity and shape dipole moment as well as the always present induced dipole The permanent dipole adds to the attractive forces between the molecules raising the boiling and melting points relative to nonpolar molecules of similar size and shape

Polar molecules and dipole-dipole forces. solid liquid

Hydrogen Bonding When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it O─H, N─H, or 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, attracting all the electron clouds from neighboring molecules

H-Bonding HF

Drawing Hydrogen Bonds Between Molecules of a Substance PROBLEM: Which of the following substances exhibits H bonding? For those that do, draw two molecules of the substance with the H bonds between them. (c) (a) (b) Find molecules in which H is bonded to N, O or F. Draw H bonds in the format -B: H-A-.

(a) C2H6 has no H bonding sites. SOLUTION: (b) (c)

Predicting the Type and Relative Strength of Intermolecular Forces PROBLEM: For each of the following pairs of substances, identify the dominant intermolecular forces in each substance, and select the substance with the higher boiling point. Use the formula, structure Bonding forces are stronger than nonbonding (intermolecular) forces. Hydrogen bonding is a strong type of dipole-dipole force. Dispersion forces are decisive when the difference is molar mass or molecular shape.

(a) MgCl2 or PCl3 SOLUTION: (a) Mg2+ and Cl- are held together by ionic bonds while PCl3 is covalently bonded and the molecules are held together by dipole-dipole interactions. Ionic bonds are stronger than dipole interactions and so MgCl2 has the higher boiling point.

(b) CH3NH2 or CH3F SOLUTION: (b) CH3NH2 and CH3F are both covalent compounds and have bonds which are polar. The dipole in CH3NH2 can H bond while that in CH3F cannot. Therefore CH3NH2 has the stronger interactions and the higher boiling point.

(c) CH3OH or CH3CH2OH SOLUTION: (c) Both CH3OH and CH3CH2OH can H bond but CH3CH2OH has more CH for more dispersion force interaction. Therefore CH3CH2OH has the higher boiling point.

(d) Hexane(CH3CH2CH2CH2CH2CH3) or 2,2-dimethylbutane SOLUTION: (d) Hexane and 2,2-dimethylbutane are both nonpolar with only dispersion forces to hold the molecules together. Hexane has the larger surface area, thereby the greater dispersion forces and the higher boiling point.

staircase memory tool! handy moving up this while of IMFs increase Intermolecular Forces (IMFs) Forces (not bonds!)BETWEEN molecules IMFs are much weaker than real bonds Three important types to know Have important effects on boiling points What to Know: definition of forces which substances have which forces how these forces effect boiling points *** m’c = molecules *** HYDROGEN “BONDING” staircase memory tool! Attraction of m’c to other m’c due to dipole moments of polar m’c that include an H bonded to either an N, O or F. The positive H side of one m’c attracted to negative N, O or F side of a different m’c & vice versa (even stronger than dipole-dipole) Polar m’c (m’c with dipole moments) that have an H bonded to either an N, O or F have them These m’c “stick together” even more strongly, and so are harder to boil; this leads to higher boiling points than m’c without hydrogen bonds. **Please forgive the terrible job we chemists did naming these forces & remember that hydrogen bonds are not really bonds, they’re IMFs! handy DIPOLE – DIPOLE FORCES Types of IMFs London Dispersion Dipole-Dipole Hydrogen Bonding moving up this Attraction of m’c to other m’c due to dipole moments of polar m’c. Positive side of one m’c attracted to negative side of a different m’c & vice versa (strong) Polar m’c (m’c with dipole moments) have them These m’c “stick together” more, and so are harder to boil; this leads to higher boiling points than m’c without dipole-dipole IMFs while LONDON DISPERSION FORCES of IMFs increase Attraction of m’c to other m’c due to gravity and instantaneous or temporary dipoles (very weak) All m’c have them Higher molar mass, higher boiling point Strength

Summary diagram for analyzing the intermolecular forces in a sample. INTERACTING PARTICLES (atoms, molecules, ions) ions present ions not present ions only IONIC BONDING (Section 9.2) nonpolar molecules only DISPERSION FORCES only polar molecules only DIPOLE-DIPOLE FORCES H bonded to N, O, or F ion + polar molecule ION-DIPOLE FORCES polar + nonpolar molecules DIPOLE- INDUCED DIPOLE FORCES HYDROGEN BONDING DISPERSION FORCES ALSO PRESENT