Molecular Polarity & Intermolecular Forces. How to Predict Whether a Molecule is Polar or Nonpolar.
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Transcript of Molecular Polarity & Intermolecular Forces. How to Predict Whether a Molecule is Polar or Nonpolar.
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Molecular Polarity & Intermolecular Forces
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How to Predict Whether a Molecule is Polar or Nonpolar
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– You already learned how to determine whether a BOND is polar or nonpolar using a Table of Electronegativity.
– You learned to determine and draw the correct molecular geometry of a molecule.
– You can put these two pieces together and determine whether the overall molecule is polar.
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– Although you might think that a molecule which has at least one polar bond would automatically be a polar molecule, this would be an incorrect assumption.
– There are lots of nonpolar molecules which contain polar bonds. However, it is true that a molecule with no polar bonds and no lone pairs must be nonpolar.
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– How is this possible? Because of the overall 3-D shape!
– If the molecule and the polar bonds are symmetrically arranged, they may cancel out.
– Thus, the molecule would be nonpolar with polar bonds.
– Let’s take a look:CCl4 CH3Cl NH3 PH3
H2S SiH4
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– So not only are the individual bond dipoles important, but lone pairs of electrons affect the molecular polarity as well.
– Lone pairs of electrons pull electron density away from the central atom, so they have a dipole as well.
– We can draw an overall dipole for the entire molecule, called the dipole moment.
– This dipole moment is the vector addition of the individual bond dipoles as well as the pull of lone pairs on electron density.
– If you look at a table of molecular geometries, there are some generalizations about molecular polarity which can be made:
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• Your Turn: Determine whether CO2, CF4 , CH2Cl2, and H2O are polar or nonpolar
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Partial Ionic Character
• We often say that a molecule like HF has “partial” ionic character.
• Or we can say that an “ionic” compound like AlN has “partial” covalent character.
• This can actually be calculated.
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– If a molecule is polar, it has a nonzero dipole moment. If a molecule is nonpolar, it has a dipole moment of zero.
– The dipole moment is defined as: μ= Qr• where μ is the Dipole moment in debye units (D),
Q is the charge in coulombs (C), and r is the distance between the charges.
– The higher the dipole moment, the more polar the molecule is or you could say that the bonds have more ionic character.
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– We can use this equation to calculate a dipole moment, but as it is actually easy to experimentally measure a molecule’s dipole moment, we more often use the dipole moment to calculate the partial charge or %-ionic character of a molecule.
– HBr has a bond length of 141 pm and a dipole moment of 0.79 D. Given that the full charge of an electron is 1.60x10-19C and 1 D = 3.336x10-30Cm, what is the %-ionic character of the H-Br bond?
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Intermolecular Forces
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Intermolecular Forces– You already know that CO2 is a gas at room
temperature, while water is a liquid, and sucrose is a solid.
– Why? They are all molecular species.– What holds water molecules together in the
liquid phase at room temperature?– Or what determines what phase or state a
compound will exist in at room temperature?
– Let’s review the 3 states of matter:
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Gas Liquid Solid
no fixed volume fixed volume fixed volume
no fixed shape no fixed shape fixed shape, so rigid
very low densities high densities high densities
density varies with T and P
not compressible not compressible
rapid, random motion
fluid, motion little motion
high kinetic motion
some kinetic energy
little kinetic energy
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Intermolecular Forces– So what determines the state of a
compound at room temp?– The strength of the attractions
between separate molecules: the stronger the attraction, the more likel the compound will be a solid, the weaker the attractions, the more likely it will be a gas.
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Intermolecular Forces– These attractive forces “glue”
solids or liquids together.– What are these forces?
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Intermolecular Forces
– The forces that hold individual molecules together in the solid or liquid phases are called intermolecular forces.
– They are also called van der Waal forces (although there is also a repulsive van der Waal force)
– They are responsible for many physical properties, including MPt and BPt
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Intermolecular Forces
– There are several types DEPENDING on the MOLECULAR POLARITY!
– Here are the 3 Main Types• Ion-Dipole (Ch 12)•Dipole-Dipole (and H-Bonding)•London Dispersion
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Dipole Dipole Forces• Attractive force between POLAR molecules (have a
dipole moment)
• Electrostatic attraction of partial positive end of
molecule to partial negative end of another molecule
• In liquid or solid, molecules align themselves so are
attracted to several other molecules
• These dipole-dipole forces are much weaker than a
real covalent bond, about 3-4kJ/mol.
• So they may be broken with a low amount of energy,
so the solid melts, and the liquid evaporates!
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431 kJ/mol 16 kJ/mol
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Strength of Dipole-Dipole Forces
• There are 2 main factors in the strength
of dipole-dipole forces:
– Distance between molecules (the
closer they are, the stronger the
dipole-dipole forces)
– Molecular Polarity (the more polar, the
stronger the dipole-dipole force)
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Hydrogen Bonding: A special case of dipole-dipole forces
• Some molecules have such a strong
dipole-dipole force, that this extra-strong
dipole force was given its own name:
Hydrogen Bonding
• It is NOT really a bond, it is just an extra
strong dipole-dipole force.
• It occurs under certain circumstances.
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H-Bonding
• Occurs when a molecule is small and very
polar
• Occurs when have a N-H, O-H, or H-F bond
• The molecules have a very large dipole
moment, and they can get very close to one
another due to the small size of H, N, O, F
• H-Bonding is responsible for the very high
melting and boiling point of water
• H-Bonding is responsible for shape of
proteins and DNA
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London Dispersion Forces
• But CO2 is a nonpolar molecule,
so how can it ever be a solid?
• There is another force, called
the London Dispersion Force.
• The London Force acts upon
ALL molecules, polar or not!
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Loondon Dispersion Forces
• But London Forces are the ONLY
intermolecular force that operates
in nonpolar molecules.
• So how does it work when there
are no dipoles to create an
attraction?
• Instantaneous, induced dipoles!
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London Dispersion Forces
• London Forces seem weak, but
they can be very important,
and many nonpolar molecules
are solids or liquids.
• Waxes, oil, gasoline are
examples.
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London Dispersion Forces
• There are 2 factors in the
strength of London Force:
– Size and mass of molecule or
atom
– Shape of molecule
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Effect of Size/Mass on London Dispersion Forces• The larger the size or mass of a molecule,
the more electrons it has.
• These electrons are also generally
further from the nucleus.
• So the electrons can be distorted or
pushed and pulled from one end of the
molecule more readily, creating a larger
temporary dipole.
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Effect of Shape on London Dispersion Forces
• The closer molecules can align, the
stronger the London Force, and the
higher the melting and boiling points.
• Bulky molecules with “branching” can’t
get as close, and so have lower London
Forces and will have lower MPt and BPt
than an “unbranched” molecule of the
same mass.
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Intermolecular Force Summary
• London Forces act upon all molecules, but are most
important for larger, heavier molecules
• Dipole-dipole forces are found in polar molecules and
depend on the size and polarity of the molecules
• H-bonding occurs when very small and very electronegative
N, O, or F atoms are present along with N-H, O-H, or H-F
bonds
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Intermolecular Force Summary
• Although they are weak compared to covalent
bonds, these intermolecular forces greatly affect
the boiling and melting points of a compound
• The stronger the forces, the higher the melting and
boiling points
• The stronger the forces, the more likely a
compound will be a solid or liquid
• They also mean more deviations of a gas from the
Ideal Gas Law!
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