Chemistry Notes

Chemistry NotesIntermolecular Forces

Intermolecular Forces

What is an Intermolecular Force?• Force between molecules (weak force)• Differs from an intramolecular force

(strong force)

Relative Magnitude of ForcesIntermolecular

ForcesIntermolecular Forces

Covalent Bonds H-Bonds Dipole-

dipoleLondon

Dispersion

400 kcal 12-16 kcal 2-0.5 kcal Less than 1 kcal

Notice: covalent bonds are almost 40 times the strength

Intermolecular Forces

What creates an Intermolecular force?• Unequal distribution of electrons

– Created as a result of differences in:• Electronegativity

Hydrogen and bromide bond with an unequal charge distribution. Bonded hydrogen atoms showing

equal charge distribution

What creates an Intermolecular force?

• The unequal distribution of electrons causes HBr to be attracted to another HBr

Br

Br

Intermolecular Forces

Main types of IMF we will discuss:• London Dispersion Forces• Dipole-Dipole• Hydrogen Bonds

London Dispersion Forces• Is the result of temporary charge imbalance

– Exists due to the random movement of electrons

• When imbalance is present, nucleus of another atom is attracted to partially charged part of the atom or molecule

• Found in ALL molecules – polar and nonpolar

• Weakest IMF

London Dispersion Forces

Dipole-Dipole• Exists between polar molecules• Polar molecules have partial positive

charges at the end of the molecules• The stronger the polarity the stronger the

dipole force

Dipole-Dipole

Dipole-Dipole

Dipole-Dipole forces exist between neutral polar molecules.

Hydrogen Bonds• Exist between a hydrogen that is bonded to

a highly electronegative atom and unshared pair of electrons on another highly electronegative atom

• H-bonds are possible only with N, O, and F.– Seen in Water,– Ammonia, and DNA

• Strongest IMF

Hydrogen BondsThe molecules which have this extra bonding are N,O, and F

NOTICE where the H-bond is occurring. See previous slide.

Hydrogen Bonds

Oxygen

Notice: H’s electrons are virtually pulled out of it’s 1s sublevel

Hydrogen

Hydrogen Bonds

Water

Notice: the virtually empty 1s orbitals on H are attracted to the unshared pair of electrons on the O from another H2O.

The evidence for hydrogen bondingMany elements form compounds with hydrogen - referred to as "hydrides". If you plot the boiling points of the hydrides of the Group 4 elements, you find that the boiling points increase as you go down the group.                    

The increase in boiling point happens because the molecules are getting larger with more electrons, and so dispersion forces become greater.

The evidence for hydrogen bondingIf you repeat this exercise with the hydrides of elements in Groups 5, 6 and 7, something odd happens.

• Although for the most part the trend is exactly the same as in group 4 (for the same reason), the boiling point of the hydride of the first element in each group is abnormally high.

• In the cases of NH3, H2O and HF there must be some additional IM forces of attraction, requiring significantly more heat energy to break.

• These relatively powerful IM forces are described as hydrogen bonds.

Hydrogen Bonds

Overview of IMF

Other IMF Types

Summary

Be able to…• Explain what an IMF is and talk about their

relative strengths• Explain what causes IMF• Describe the different types of IMF • Go in depth about H-bonds• Draw diagrams that represent the IMF

types