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Chemistry Notes. Intermolecular Forces. Intermolecular Forces. What is an Inter molecular Force? Force between molecules (weak force) Differs from an intra molecular force (strong force). Relative Magnitude of Forces. Notice: covalent bonds are almost 40 times the strength. - PowerPoint PPT Presentation
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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