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Intermolecular Forces

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Updated 2009-02-04 22:04

Intermolecular Forces

Phase of Matter
Properties
gasassumes volume and shape of container
gascompressible
gasflows readily
gasdiffusion through it is rapid
liquidassumes shape of container but keeps its own volume
liquidvirtually incompressible
liquidflows readily
liquiddiffusion through it is slow
solidkeeps its own shape and volume
solidvirtually incompressible
soliddoes not flow
soliddiffusion through it is extremely slow
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Intermolecular Forces
Properties
Electrostatic Forcesless than about 15% the strength of covalent and ionic bonds
Electrostatic Forcesincludes London dispersion forces, dipole-dipole forces, H-bonds, and ion-dipole forces
Van Der Waals Forcesincludes London dispersion forces, dipole-dipole forces, and H-bonds
Van Der Waals Forcesforces between neutral molecules
London Dispersion Forcesoccur between all molecules
London Dispersion Forcesstronger for larger mass molecules and more polarizable molecules
London Dispersion Forcesgives >80% of HCl's intermolecular interactions
dipole-dipole forcesfor molecules of the same size and mass, these forces are stronger for the molecule with the more polar bonds
dipole-dipole forcesfor molecules of the same polarity, these forces are stronger for the molecule with the smaller volume
H-bondsthese are unusually strong dipole-dipole forces
H-bondsoccur between a polar bond and an unshared electron pair (a lone pair) on a nearby electronegative ion or atom
H-bondsusually occur in molecules containing N-H, O-H, or H-F bonds
H-bonds4 to 25 kJ/mol, much weaker than chemical bonds, but much stronger than dipole-dipole and London dispersion forces
ion-dipole forcesoccur in solutions where ionic compounds are dissolved in polar liquids
ion-dipole forcesoccur between charged particles and the ends of polar molecules
ion-dipole forcesstronger than dipole-dipole forces
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Intermolecular Forces
Systems where they are important
London Dispersion ForcesAr(l), I2(s)
dipole-dipole forcesH2S, CH3Cl
H-bondsH2O, NH3, HF
ion-dipole forcesNaCl or KBr dissolved in water
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When ranking the strength of intermolecular forces in several pure substances each made up of a single molecule (like C2H6 vs. C3H8 vs. C4H10), the following are true:
Fill in the blanks
Answers
If the molecules all have similar masses and shapes, ____.the London Dispersion Forces are all similar
If the molecules all have similar masses and shapes, ____.dipole-dipole forces decide the ranking
If the molecules all have similar masses and shapes, ____.the intermolecular forces rise as the bond polarity rises
If the molecules all have very different molecular weights, ____.the London Dispersion Forces are all different
If the molecules all have very different molecular weights, ____.the London Dispersion Forces decide the ranking
If the molecules all have very different molecular weights, ____.the intermolecular forces rise as the molecular weight rises
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If Intermolecular Forces
are ____
This property is high
strongheat of vaporization
strongspecific heat
strongmelting point
strongboiling point
strongviscosity
strongsurface tension
strongcritical temperature
strongcritical pressure
weakvapor pressure
weakvolatility
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Fill in the blanks
Answers
Viscosity rises as the temperature ____.falls
Vapor Pressure rises as the temperature ____.rises
The critical temperature is the ____.highest temperature for a liquid. Above this temperature, the liquid and gas phases aren't distinct
The critical pressure is the ____.vapor pressure at the critical temperature
Cohesive forces attract molecules of one substance to ____.other molecules of the same substance
Adhesive forces attract molecules of one substance to ____.molecules of a different substance
A concave meniscus occurs when ___.adhesive forces dominate over cohesive ones
A concave meniscus occurs when ___.the level of liquid in a capillary tube is higher near the capillary tube walls than it is near the center of the capillary tube
A concave meniscus occurs when ___.water is in a capillary tube
A convex meniscus occurs when ___.cohesive forces dominate over adhesive ones
A convex meniscus occurs when ___.the level of liquid in a capillary tube is lower near the capillary tube walls than it is near the center of the capillary tube
A convex meniscus occurs when ___.mercury is in a capillary tube
Ionic bonds are important in ____.NaCl and NH4NO3
Metallic bonds are stronger than ____.H-bonds
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Crystalline
Solid
Intermolecular
Forces
Properties
Examples
MolecularLDF,
dpl-dpl,
H-bonds
soft,
low to moderately high melting point,
poor thermal and electrical conductivity
Ar, ice,
methane (CH4),
sucrose,
dry ice (CO2)
Covalent
Network
covalent
bonds
very hard,
high melting point,
poor thermal and electrical conductivity
diamond (C),
graphite (C),
silicon (Si),
quartz (SiO2)
Ionicelectrostatichard, brittle,
high melting point,
poor thermal and electrical conductivity
salts,
NaCl,
Ca(NO3)2,
CaF2
MetallicMetallic
bonds
soft to very hard,
malleable, ductile,
low to very high melting point,
high thermal and electrical conductivity
Cu, Fe, Al, Pt, Ag, Brass
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Unit Cell
Number of Atoms
Per Unit Cell
% of volume
occupied
by atoms
Coordination Number
(Number of Equidistant Nearest Neighbors)
primitive cubic152%6
BCC (body-centered cubic)268%8
FCC (face-centered cubic)
CCP (cubic-closest-packed) ABCA
474%12
HCP (hexagonal-closest-packed) ABAB474%12
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Reference: Chapter 11 in "Chemistry, The Central Science, 10th Edition" by T.L.Brown, H.E.Lemay, Jr., B.E. Bursten. ISBN=0-13-146489-2.