OCR AS Chemistry F332: Dipoles and Intermolecular Bonds

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Preview of OCR AS Chemistry F332: Dipoles and Intermolecular Bonds

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Dipoles and Intermolecular Bonds
If a molecular substance contains dipoles, they can attract each other.
All intermolecular bonds arise from the attractive forces between dipoles.
There are three kinds of bond:
Bond
Instantaneous-Induced Where two or more Occurs between the
permanent dipoles attract permanent dipoles in HCl
one another
Permanent-Induced A permanent dipole induces Occurs between HCl and
a dipole in another Cl2 molecules
molecule, then the two
attract one another
Instantaneous-induced An instantaneous dipole Occurs between Cl2
induces a dipole in another molecules and between
molecule, then attracts it the atoms in a noble gas

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Instantaneous dipole ­ Induced dipole bonds
These are the weakest type of intermolecular bonding.
They act between all molecules.
This is because instantaneous dipoles can arise in molecules that already have a permanent
dipole.
You notice them more clearly in substances such as the noble gases and the alkanes,
because there are no other intermolecular bonds present.
This shows how an instantaneous dipole in a xenon atom induces polarisation in a
neighbouring atom.…read more

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Permanent dipole ­ Permanent dipole bonds
An example of a polar molecule would be H2O.
Oxygen is much more electronegative than hydrogen, so the O-H bond is polar.
One side of the molecule is positive, the other is negative.
The water molecule has a dipole with the positive charge centred between the two
hydrogen atoms.
An example of a more complicated polar molecule is 1,1,1-trichloroethane.…read more

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Dipole ­ Dipole bonds
Molecular dipoles are liquid at room temperature, and in the liquid phase the molecules
are constantly moving and tumbling around.
Sometimes the negative end of one molecule will be lined up with the positive end of
another, making them attract, but often there will be two negative ends or two positive
ends together, in which case they will repel.…read more

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