Warmup 11/16/15
Water bends up at the
edges of a graduated
cylinder. What do you
think causes this?
Objective
Tonight’s Homework
To learn about how we define
polar molecules
pp 355: 3, 4, 5
Notes on Polar Molecules
What They Are
Let’s take a closer look at a water molecule.
••
O
••
H
H
Notes on Polar Molecules
What They Are
Let’s take a closer look at a water molecule.
See the unshared electron pairs? These have an
effect on the molecule. Electrons have negative
charge. Since they spend most of their time
around the oxygen, this gives the oxygen a little
bit of overall negative charge. Similarly, the
hydrogens get a little bit of overall positive
charge.
••
••
O
H
Notes on Polar Molecules
What They Are
Let’s take a closer look at a water molecule.
See the unshared electron pairs? These have an
effect on the molecule. Electrons have negative
charge. Since they spend most of their time
around the oxygen, this gives the oxygen a little
bit of negative charge. Similarly, the hydrogens
get a little bit of positive charge.
We show this with a “δ” symbol.
H δ+
••
δ O
••
H δ+
Notes on Polar Molecules
Other Polar Molecules
There are many other molecules that exhibit
this behavior:
NH3
HF
CH3Cl
Notes on Polar Molecules
Other Polar Molecules
There are many other molecules that exhibit
this behavior:
NH3
HF
CH3Cl
The polarity of each of these molecules is
different. Some are stronger than others. Most
aren’t polar at all. Polarity is usually determined
by if the molecule is lopsided or not.
So what does this mean?
Notes on Polar Molecules
Behavior of Polar Molecules
Polar molecules have a
pull on each other. The
partially positive end of
one pulls on the partially
negative end of another.
Notes on Polar Molecules
Behavior of Polar Molecules
Polar molecules have a
pull on each other. The
partially positive end of
one pulls on the partially
negative end of another.
This results in many effects
you’ve seen. The meniscus
is one. This happens because
the molecules pull together a
little. This creates a surface
tension that helps hold the
water together.
Notes on Polar Molecules
This results in many other effects you’ve seen.
Water forming
into droplets:
Notes on Polar Molecules
This results in many other effects you’ve seen.
Water forming
into droplets:
Surface tension
holding up objects:
Notes on Polar Molecules
This results in many other effects you’ve seen.
Water forming
into droplets:
Surface tension
holding up objects:
Water bending
near electric charge:
Notes on Polar Molecules
This results in many other effects you’ve seen.
Water forming
into droplets:
Surface tension
holding up objects:
Water bending
near electric charge:
Water pulling up
through drinking straws:
Notes on Polar Molecules
Interaction Forces
There are a few other forces within and between
molecules. These are usually called weak
forces because they aren’t nearly as strong as
the forces we’ve discussed so far.
Notes on Polar Molecules
Interaction Forces
There are a few other forces within and between
molecules. These are usually called weak
forces because they aren’t nearly as strong as
the forces we’ve discussed so far.
The first type are called dipole-dipole forces.
These are the forces
between polar molecules.
This is a pretty weak
attraction. In water, it can
create surface tension, but
it doesn’t physically bond
molecules.
Notes on Polar Molecules
But we can have one even weaker.
If we bring a polar molecule near a nonpolar
molecule, it can induce a dipole.
Notes on Polar Molecules
But we can have one even weaker.
If we bring a polar molecule near a nonpolar
molecule, it can induce a dipole.
For example, when water gets near chlorine, the
chlorine gains a bit of a dipole. This helps to pull
the 2 molecules together, but it’s a weak force.
δ+
δ+
H
H
•• O• δ
•
δ+
Cl
Cl
δ-
This is an induced dipole force.
Notes on Polar Molecules
There’s one last one, even weaker than these.
If we bring two nonpolar molecules together,
they make each other just a little polar as well.
Notes on Polar Molecules
There’s one last one, even weaker than these.
If we bring two nonpolar molecules together,
they make each other just a little polar as well.
Example: If we bring 2 Br2 molecules together,
they both become a little polar.
δ+ Br
Br
δ-
δ+
Br
Br
δ-
Notes on Polar Molecules
There’s one last one, even weaker than these.
If we bring two nonpolar molecules together,
they make each other just a little polar as well.
Example: If we bring 2 Br2 molecules together,
they both become a little polar.
δ+ Br
Br
δ-
δ+
Br
Br
δ-
This is an extremely weak force and is called a
dispersion force or London force.
Notes on Polar Molecules
All of these weak forces together are called van
der Waals forces and contribute in many small
ways to molecular interactions.
Exit Question
Which end of a water molecule is the partially positive
end?
a) The hydrogen end
b) The oxygen end
c) Halfway between the two
d) None of the above
e) Not enough information