Intermolecular Forces Chemistry. The forces of attraction between molecules The forces of attraction between molecules Vary in strength but are generally

Intermolecular ForcesIntermolecular Forces

ChemistryChemistry

Intermolecular ForcesIntermolecular Forces

The forces of attraction between The forces of attraction between moleculesmolecules

Vary in strength but are generally Vary in strength but are generally weaker than bonds that join atoms in weaker than bonds that join atoms in molecules, ions in ionic compounds, molecules, ions in ionic compounds, or metal atoms in solid metalsor metal atoms in solid metals

Let’s talk about what happens when Let’s talk about what happens when we boil waterwe boil water

Boiling Points: Nonpolar Boiling Points: Nonpolar Covalent MoleculesCovalent Molecules

Substance Boiling point (1atm, °C)

H2 -253

O2 -183

Cl2 -34

Br2 59

CH4 -164

CCl4 77

C6H6 80

Boiling Points: Polar Boiling Points: Polar Covalent MoleculesCovalent Molecules


PH3 -88

NH3 -33

H2S -61

H2O 100

HF 20

HCl -85

ICl 97

Boiling Points: Ionic Boiling Points: Ionic


NaCl 1413

MgF2 2239

Boiling Points: Metallic Boiling Points: Metallic


Cu 2567

Fe 2750

W 5660

Molecular Polarity and Molecular Polarity and Dipole-Dipole ForcesDipole-Dipole Forces

The strongest intermolecular forces The strongest intermolecular forces exist between polar moleculesexist between polar molecules

Dipole – created by equal but Dipole – created by equal but opposite charges that are separated opposite charges that are separated by a short distanceby a short distance

Polar molecules act as tiny dipoles Polar molecules act as tiny dipoles because of their uneven charge because of their uneven charge distributiondistribution


The direction of a dipole is from the The direction of a dipole is from the dipole’s positive pole to its negative dipole’s positive pole to its negative pole pole

Represented by an arrow with a head Represented by an arrow with a head pointing toward the negative pole pointing toward the negative pole and a crossed tail situated at the and a crossed tail situated at the positive polepositive pole

Example – HClExample – HCl


The negative region in one polar The negative region in one polar molecule attracts the positive region molecule attracts the positive region in adjacent moleculesin adjacent molecules

Dipole-dipole forces – the forces of Dipole-dipole forces – the forces of attraction between polar molecules attraction between polar molecules Short-range forces, acting only between Short-range forces, acting only between

nearby moleculesnearby moleculesBoiling Points are affected by this forceBoiling Points are affected by this force

Polar BrF – -20°CPolar BrF – -20°CNonpolar FNonpolar F22 – -188°C – -188°C


The polarity of diatomic molecules The polarity of diatomic molecules such as BrF is determined by just one such as BrF is determined by just one bondbond

For molecules containing more than For molecules containing more than two atoms, molecular polarity two atoms, molecular polarity depends on both the polarity and the depends on both the polarity and the orientation of each bondorientation of each bond

Example – WaterExample – WaterExample – Ammonia Example – Ammonia


In some molecules, individual bond In some molecules, individual bond dipoles cancel one another, causing dipoles cancel one another, causing the resulting molecular polarity to be the resulting molecular polarity to be zerozero

Example – carbon tetrachlorideExample – carbon tetrachlorideExample – carbon dioxideExample – carbon dioxide


A polar molecule can induce a dipole A polar molecule can induce a dipole in a nonpolar molecule by temporarily in a nonpolar molecule by temporarily attracting its electronsattracting its electronsResults in a short-range intermolecular Results in a short-range intermolecular

force that is somewhat weaker than the force that is somewhat weaker than the dipole-dipole forcedipole-dipole force

Solubility of nonpolar OSolubility of nonpolar O22 in water, the in water, the positive pole of a water molecule attracts positive pole of a water molecule attracts the outer electrons of an adjacent oxygen the outer electrons of an adjacent oxygen moleculemoleculeThe oxygen molecule, then, has an induced The oxygen molecule, then, has an induced

negative pole on the side toward the water negative pole on the side toward the water molecule and an induced positive pole on the molecule and an induced positive pole on the opposite sideopposite side

Hydrogen BondingHydrogen Bonding

Strong type of dipole-dipole forceStrong type of dipole-dipole forceExplains the high boiling points of Explains the high boiling points of

some hydrogen-containing some hydrogen-containing compoundscompoundsHFHFHH22OO

NHNH33


In such compounds, the large In such compounds, the large electronegativity differences between electronegativity differences between hydrogen atoms and fluorine, oxygen, or hydrogen atoms and fluorine, oxygen, or nitrogen atoms make the bonds nitrogen atoms make the bonds connecting them highly polarconnecting them highly polar

This gives the hydrogen atom a positive This gives the hydrogen atom a positive chargecharge

Since hydrogen is so small it can come in Since hydrogen is so small it can come in close to an adjacent molecule’s unshared close to an adjacent molecule’s unshared pair of electronspair of electrons


Hydrogen bonding – intermolecular Hydrogen bonding – intermolecular force in which a hydrogen atom that force in which a hydrogen atom that is bonded to a highly electronegative is bonded to a highly electronegative atom is attracted to an unshared pair atom is attracted to an unshared pair of electrons of an electronegative of electrons of an electronegative atom in a nearby moleculeatom in a nearby molecule

Hydrogen bonds are usually Hydrogen bonds are usually represented by dotted lines represented by dotted lines

London Dispersion ForcesLondon Dispersion Forces

Even noble-gas atoms and molecules Even noble-gas atoms and molecules that are nonpolar experience a weak that are nonpolar experience a weak intermolecular attractionintermolecular attraction

Remember electrons are in Remember electrons are in continuous motioncontinuous motion

At any instant the electron At any instant the electron distribution may be slightly unevendistribution may be slightly uneven


This momentary uneven charge This momentary uneven charge creates a positive pole in one part of creates a positive pole in one part of the atom or molecule and a negative the atom or molecule and a negative pole in anotherpole in anotherTemporary dipoleTemporary dipoleCan then induce a dipole in an adjacent Can then induce a dipole in an adjacent

atom or moleculeatom or moleculeThe two are held together for an instant The two are held together for an instant

by the weak attraction between the by the weak attraction between the temporary dipolestemporary dipoles


London Dispersion Forces – the London Dispersion Forces – the intermolecular attractions resulting from intermolecular attractions resulting from the constant motion of electrons and the the constant motion of electrons and the creation of instantaneous dipoles (named creation of instantaneous dipoles (named after Fritz London, who first proposed their after Fritz London, who first proposed their existence in 1930) existence in 1930)

These forces act between all atoms and These forces act between all atoms and moleculesmolecules The only forces acting among noble-gas atoms, The only forces acting among noble-gas atoms,

nonpolar molecules, and slightly polar nonpolar molecules, and slightly polar molecules (these atoms/molecules have low molecules (these atoms/molecules have low boiling points – look back at the chart)boiling points – look back at the chart)


Since London forces are dependent on Since London forces are dependent on the motion of electrons, their strength the motion of electrons, their strength increases with the number of electrons increases with the number of electrons in the interacting atoms or moleculesin the interacting atoms or moleculesLondon forces increase with increasing London forces increase with increasing

atomic or molar massatomic or molar massCompare boiling points of helium (He) and Compare boiling points of helium (He) and

argon (AR) to hydrogen (Hargon (AR) to hydrogen (H22), oxygen (O), oxygen (O22), ), chlorine (Clchlorine (Cl22), and bromine (Br), and bromine (Br22))

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Intermolecular Forces Chemistry. The forces of attraction between molecules The forces of attraction between molecules Vary in strength but are generally