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Intermolecular attractions. Thus far…. Bonding covered so far involved intramolecular bonding , i.e. bonding within a molecule or within an ionic crystal lattice. This is the bonding that holds a molecule together.

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slide2

Thus far…

  • Bonding covered so far involved intramolecular bonding, i.e. bonding within a molecule or within an ionic crystal lattice.
  • This is the bonding that holds a molecule together.

Now we’re going to talk about intermolecular forces or attractions between two separate molecules.

This is the “bonding” that holds to TWO molecules together

slide3

Intermolecular Force

  • Intermolecular forces:
    • The attractive forces holding two separate molecules together
    • much weaker than intramolecular forces (i.e. bonds)
  • When a substance melts or boils the intermolecular forces are broken (not the covalent bonds).
  • Melting and boiling point is a relative indicator of the strength of intermolecular force in a substance.
slide4

Intramolecular and Intermolecular Force

  • Bond strength = 431 kJ/mol

The covalent bond is intramolecular.

  • Bond strength16 kJ/mol
slide5

Kinds of Intermolecular Forces

  • 3 types of intermolecular forces:

1. van der Waals forces

2. Dipole-dipole forces

3. Hydrogen bond

slide6

Dipole: seperation of positive and negative charges.

  • In the case of atoms that separation is the negative electrons being separated from the positive nucleus/protons
phet simulation
PhET simulation
  • http://phet.colorado.edu/en/simulation/balloons
slide8

1. van der Waals forces

  • van derwaalsforces:
      • are temporary induced dipoles between compounds and atoms.
      • found in all substances.
      • weakest of all intermolecular forces.
  • In nonpolar compounds, it is the ONLY intermolecular force of attraction.
slide9

Creation of a dipole for van der Waals forces

  • It is possible for two adjacent neutral molecules to affect each other.
slide10

 +

 +







 +





 +

 +





Creation of Instantaneous Dipoles

  • Polarization of molecules could also results from one instance to another simply by chance or collisions between them (in the case of liquids and gases).

Two molecules approaching each other

Collision creates instantaneous dipoles

Instantaneous dipoles induce temporary dipoles in other molecules

induced dipole

induced dipole

slide12

Dependence of van der Waals force on Mass

  • Inert gases and the halogens:

Notice, boiling point increases going down the group.

Similarly molecular weight increases as you go down a group.

Using the information that boiling point is an indicator of intermolecular forces, what can you say about the relationship between molecular weight and intermolecular forces?

The greater the mass the greater the intermolecular forces.

slide13

SiH4 vs. Ar

Mass of SiH4

~32 amu

Mass of Ar

~40 amu

H

Ar

H

Si

H

Actual B.P. of Ar

-186 ° C

Actual B.P. of SiH4

-112 ° C

H

This is opposite of the trend we saw before.

What then can account for the difference in B.P./intermolecular forces?

Answer: Shape, but more specifically surface area of atom/molecule

Based on mass and mass alone which one would you expect to have a higher boiling point and thus higher intermolecular forces?

Answer: Argon

slide14

Van der Waals Forces: A Summary

Van derwaals forces depends on:

  • molar mass
    • The greater the mass the greater the van derwaals forces.
  • Shape
    • The greater the surface area the greater the van derwaals forces.
slide15

2. Dipole-dipole force of attraction

  • Dipole-dipole forces exist between neutral polar molecules
  • Neutral polar molecules have a permanent dipole vs. a temporary one.
  • Since the dipoles are permanent, dipole-dipole forces are stronger than van der Waals forces between substances of comparable molar mass.
slide16

Nature of dipole-dipole force

  • The dipoles align themselves as shown—oppositely charged ends adjacent to one another.

There is a mix of attractive and repulsive dipole-dipole forces as the molecules tumble.

slide17

H2S vs. H2O

Mass = 18 amu

Mass = 34 amu

O

S

B.P. = 100 ° C

B.P. = -60 ° C

Electronegativity values:

H=2.1, S=2.5, O=3.5

H2S and H2O have similar structures.

H2S has more mass then H2O.

Why then does water boil at a higher temperature then hydrogen sulfide?

Lets compare what is different between the two.

H

H

H

H

H – S

EN diff.: 0.4

H – O

EN diff.: 1.4

In general the more polar a bond is the higher

the dipole-dipole intermolecular attraction

and thus a higher M.P./B.P.

Both polar, but one is MORE polar then the other.

What can you say about polarity and intermolecular forces?

slide18

Dipole-dipole force: A Summary

  • To conclude then, polar substances contain dipole-dipole force.

In general, there is a direct relationship between polarity and dipole-dipole force and therefore, boiling point.

slide19

Nature of H-bond

  • Hydrogen bonding:
      • special case of dipole-dipole forces.
      • covalent bond between Hydrogen and either N, O, or F.
      • the more polar the stronger the H-bond.
      • the other molecule must contain a lone pair of electrons.
      • the higher number of hydrogen bonds per molecule, the stronger the intermolecular force
      • H-bonds are stronger then the other two intermolecular forces.
slide20

3. Hydrogen bond

Period 2 hydrides of Groups V, VI and VII:

Except for Carbon, everything

in period 2 has higher B.P.s

then the other hydrides in

their group

slide21

H-bond in ammonia

  • Note there is only one H-bond per molecule
slide22

H

F

H

H

F

F

H-bond in hydrogen fluoride

F

F

H

F

H

H

  • Note there is only one H-bond per molecule

Steric reasons prevent fluorine from forming more than one H-bond.

  • It is too small to accommodate more than two hydrogen atoms around it without the hydrogens themselves repelling each other and destabilizing the bonds causing them to break on their own accord.
slide23

H-bond in Water

  • Note there are two H-bond per molecule on average
slide24

Difference in Boiling Points of NH3, H2O, and HF

  • The order of the strength of individual hydrogen bond is
  • HF > H2O > NH3
  • Because that is the order of the polarity of the three molecules. (HF is the most polar and then water followed by ammonia, because fluorine is most electronegative followed by oxygen and then nitrogen.)
  • The boiling point of HF therefore is higher than that of ammonia, but not than that of water.
  • Even though oxygen is less electronegative than fluorine and has a weaker individual hydrogen bond, the intermolecular force in water is the strongest (and therefore it has the highest boiling point) because water, on average, has 2 hydrogen bonds per molecule while both HF and NH3 have only one per molecule.
slide26

3. Hydrogen bond Summary

    • A bond between a hydrogen bonded to N, O or F and lone pair in the atom hydrogen is bonded to.
    • the more electronegative the element bonded to hydrogen the stronger the H-bond
    • the more the number of hydrogen bonds per molecule, the stronger the intermolecular force
    • strongest of the intermolecular forces
slide27

Wk11Obj1 AND Wk11Obj2

H-C vsCl-ClvsH-F

Electronegativity Values:

2.1 to 2.53.0 to 3.02.1 to 4.0

Difference:

0.4 0.01.9

Polarity of bond:

Polarnon-polar polar

Type of intermolecular force:

Dipole-dipolevan derwaalsH-bonding

Inter forces/B.P./M.P.:

lowvery lowhigh

slide28

Wk11Obj1 AND Wk11Obj2

H – F

H – Cl

What is the strongest intermolecular

force the molecule will allow?

A.) van derwaals

B.) dipole-dipole

C.) hydrogen bonding

What is the strongest intermolecular

force the molecule will allow?

A.) van derwaals

B.) dipole-dipole

C.) hydrogen bonding

Which compound will have the higher boiling point?

A.) H-F

B.) H-Cl

slide29

Wk11Obj1 AND Wk11Obj2

O

S

H

H

H

H

What is the strongest intermolecular

force the molecule will allow?

A.) van derwaals

B.) dipole-dipole

C.) hydrogen bonding

What is the strongest intermolecular

force the molecule will allow?

A.) van derwaals

B.) dipole-dipole

C.) hydrogen bonding

Which compound will have the higher boiling point?

A.) H2O

B.) H2S

slide30

Wk11Obj1 AND Wk11Obj2

H

H

H

H

N

P

H

H

What is the strongest intermolecular

force the molecule will allow?

A.) van derwaals

B.) dipole-dipole

C.) hydrogen bonding

What is the strongest intermolecular

force the molecule will allow?

A.) van derwaals

B.) dipole-dipole

C.) hydrogen bonding

Which compound will have the higher boiling point?

A.) NH3

B.) PH3

slide31

Wk11Obj1 AND Wk11Obj2

H

H

H

O

H

C

C

H

H

H

H

What is the strongest intermolecular

force the molecule will allow?

A.) van derwaals

B.) dipole-dipole

C.) hydrogen bonding

What is the strongest intermolecular

force the molecule will allow?

A.) van derwaals

B.) dipole-dipole

C.) hydrogen bonding

H

C

H

O

C

Which compound will have the higher boiling point?

A.) CH3OCH3

B.) CH3CH2OH

H

H

slide32

Wk11Obj1 AND Wk11Obj2

H

H

H

H

H

H

H

O

H

C

H

C

C

C

C

H

H

H

H

H

H

H

H

O

C

C

Which compound will have the higher boiling point?

A.) CH3CHO

B.) CH3CH2CH3

C.) CH3CH2OH

H

slide33

Summary

  • Intermolecular forces arise from compositional (identities and properties of the atoms themselves) and structural features of the molecules of the substance.

1. Van der Waals forces

    • Forces of attraction between temporary dipoles
    • found in all substances; the bigger the mass stronger the van der Waals forces (everything else being the same)
    • in nonpolar compounds the only intermolecular force found
    • between molecules of comparable mass, long, non-spherical molecules have stronger van der Waals forces than spherical molecules
      • Long molecules are more easily polarized and the degree of polarization is also greater leading to a stronger van der Waals forces.
      • The difference in boiling point brought about by this structural difference between molecules becomes relevant when considering the boiling points of different isomers of organic compounds, especially different structural isomers (See Core Organic Chemistry: Isomerism).
slide34

Summary

  • 2. Dipole-dipole forces
    • Forces of attraction between permanent dipoles
    • found in polar molecules
    • the bigger the dipole moment stronger the dipole-dipole forces (everything else being equal, of course)
  • 3. Hydrogen bond
    • A bond between a hydrogen bonded to N, O or F and lone pair in the atom hydrogen is bonded to.
    • the more electronegative the element bonded to hydrogen the stronger the H-bond
    • the more the number of hydrogen bonds per molecule, the stronger the intermolecular force
    • strongest of the intermolecular forces
slide35

Are the molecules polar?

No

Yes

Everything else being equal,

higher boiling point

Do they have different

structural features?

No

Yes

Bigger mass has stronger van

One with more atoms or long molecules

der Waals forces and therefore

has stronger van der Waals forces

higher boiling point

& therefore higher boiling point

Examples:

Examples:

_____________________________

_____________________________

_____________________________

_____________________________

_____________________________

_____________________________

Dichotomous key for determining the difference in boiling points

(van der Waals forces)

(Dipole-dipole forces or H-bond)

(See next slide)

slide36

Dipole-dipole forces or H-bond

Do the molecules have hydrogen

covalently bonded to N or O or F?

No

Yes

Everything else being

equal,higher boiling point.

Difference in polarities?

No

Yes

Is there a difference in mass?

Is polarities the only difference?

No

Yes

No

Yes

Similar boiling

One with bigger

One with the larger

points

mass has higher

dipole moment will have

boiling point

higher boiling point

Examples:

Examples:

Examples:

Examples:

____________________

_______________________

_______________________

______________________

____________________

_______________________

_______________________

______________________

____________________

_______________________

_______________________

______________________

(Dipole-dipole forces)

(Hydrogen bond)

(See next slide)

Depends!