Intermolecular ForcesIrresistible attraction…
ATTRACTIVE FORCES
Always electrostatic in nature
Intramolecular forces
bonding forces
These forces exist withinwithin each molecule.They influence the chemicalchemical properties of the substance.
Intermolecular forces nonbonding forces
These forces exist betweenbetween molecules.They influence the physicalphysical 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
• 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
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
SOLUTION:
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.
C2H6(a) CH3OH(b) CH3C NH2
O
(c)
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.
(c)(b)C O H
H
H
H
COH
H
H
H
CH3C N
O
H
H
CH3CN
O
H
H
CH3CN
O
H
H
CH3CN
O
H
H
Dispersion Force
Dispersion forces among nonpolar molecules.
separated Cl2
molecules
instantaneous dipoles
Predicting the Type and Relative Strength of Intermolecular Forces
PROBLEM: For each pair of substances, identify the dominant intermolecular forces in each substance, and select the substance with the higher boiling point.
(a) MgCl2 or PCl3
(b) CH3NH2 or CH3F
(c) CH3OH or CH3CH2OH
(d) Hexane (CH3CH2CH2CH2CH2CH3)
or 2,2-dimethylbutaneCH3CCH2CH3
CH3
CH3Use 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.
SOLUTION:
Predicting the Type and Relative Strength of Intermolecular Forces
(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 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) 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 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.
Summary diagram for analyzing the intermolecular forces in a sample.
INTERACTING PARTICLES(atoms, molecules, ions)INTERACTING PARTICLES(atoms, molecules, ions)
ions onlyIONIC BONDING(Section 9.2)
ions onlyIONIC BONDING(Section 9.2)
ion + polar moleculeION-DIPOLE FORCESion + polar moleculeION-DIPOLE FORCES
ions present
ions not present
polar molecules only
DIPOLE-DIPOLE FORCES
polar molecules only
DIPOLE-DIPOLE FORCES
HYDROGENBONDING
HYDROGENBONDING
polar + nonpolar moleculesDIPOLE-INDUCED DIPOLE FORCES
polar + nonpolar moleculesDIPOLE-INDUCED DIPOLE FORCES
nonpolar molecules onlyDISPERSIONFORCES only
nonpolar molecules onlyDISPERSIONFORCES only
DISPERSION FORCES ALSO PRESENT
H bonded to
N, O, or F