OCR AS Chemistry F332: Hydrogen Bonding

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Hydrogen Bonding
Hydrogen bonding is a type of intermolecular bond.
It is the strongest type of intermolecular bond and can be thought of as a special case of
permanent dipole-permanent dipole bonding.
For hydrogen bonding to occur, the molecules involved must have the following three
A large dipole between a hydrogen atom and a highly electronegative atom such as
oxygen, nitrogen and fluorine
The small hydrogen atom which can get very close to oxygen, nitrogen or fluorine
atoms in nearby molecules
A lone pair of electrons on the oxygen, nitrogen or fluorine atom, with which the
positively charged hydrogen atom can line up
The hydrogen atoms have a strong positive charge because of the highly electronegative
oxygen atom to which they are bonded.
This positive charge lines up with a lone pair on another oxygen atom because it provides a
region of concentrated negative charge.
The hydrogen and oxygen atoms can get very close, and therefore attract very strongly,
because the hydrogen atom is so small.
The lining up of the molecules is important because it means that positive and negative
charges are always lined up with one another, so the forces are always attractive.

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The Effects of Hydrogen Bonding
To boil a liquid, you need to overcome the intermolecular forces, so that the particles can
escape from the liquid surface.
You need more energy to overcome stronger intermolecular forces .
So liquids with stronger intermolecular forces will have higher boiling points.
Hydrogen bonding is quite a strong intermolecular force.
Substances that form hydrogen bonds have higher boiling and melting points than other
similar molecules.
This is because of the extra energy needed to break the hydrogen bonds.…read more

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A substance will dissolve in water if the molecules of the substance are able to form bonds
with the water molecules instead of with each other.
Substances with hydrogen bonding are often soluble in water.
Hydrogen bonds can form between water molecules and molecules of the substance and
this helps the dissolving process.
If the polymer has lots of ­OH groups, the hydrogen bonding between its molecules will be
very strong ­ meaning too much energy is needed to break it down.…read more


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