This is an introductory course for students with limited background in chemistry; basic concepts such as atomic and molecular structure, solutions, phases of matter, and quantitative problem solving will be emphasized with the goal of preparing students for further study in chemistry.
Solids Part III
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來自 Duke University 的課程
Introduction to Chemistry: Structures and Solutions
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從本節課中
Intermolecular Forces and Phase Changes
<p>Welcome to week 5! This week we will examine the phases of matter more closely, looking at concepts such as intermolecular forces and thermal energy. We will also practice calculations of energetics such as those requiring enthalpies of phase transitions. Finally, we will introduce concepts in metal bonding and solid lattices. We made a few illustrative demonstrations for your viewing pleasure; I hope that these are helpful! </p><p>Remember to continue posting any questions, concerns and suggestions to the discussion forum. Also help your peers out on the forum to build a beneficial learning community together!</p>
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Prof. Dorian A. CanelasAssistant Professor of the Practice
Chemistry
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In this lecture, I will discuss three more types of solids,
network covalent solids, molecular solids, and polymers.
Let's start with network covalent solids.
They are usually formed either from the
semimetals, which remember, are also called the metalloids,
or from other elements that are very close to the metalloids on the periodic table.
In network covalent solids, as implied by the name, there are covalent bonds.
Here are some examples of network covalent solids.
For example, carbon has this different allotropes,
allotropes been different elemental forms of carbon.
You're probably familiar with diamonds and graphite, and you may have
heard of bucky balls and nanotubes, or may be you haven't.
The semi-conductors that are in the device you're using
to watch this video are also network covalent solids.
As are the clay on the ground.
Those are made from aluminosilicates, or perhaps you have sand on
the ground where you are, and there's other special types of structures.
For example, the zinc blende structure.
That has a metal and a non-metal but because of
the small difference in electronegativity, a network covalent solid is formed.
Let's start by looking at the allotropes of carbon.
What's the difference between diamond and graphite?
Well, you probably have some idea
of the qualitative differences of those substances.
Certainly, they have different price points, don't they?
Diamonds are purely made of carbon, but
in diamond, the carbons have sp3 hybridization.
So each of these carbon atoms has a tetrahedral
arrangement of bonds around it to other carbon atoms.
That ends up giving us a three dimensional array of covalent bonds.
And that is one of the things that makes diamonds such a hard substance.
Graphite is also made from pure carbon.
But in this case, the carbons are sp2 hybridized.
Because they're sp2 hybridized, all of the carbons that
are bonded to each other are in the same plane.
And so there are these flat sheets, [SOUND]
of carbons, that are separated by some distance.
Here this particular type of graphite has
a distance of 3.4 angstroms between the sheets.
Each of those sheets, if we were to look at it from the top view, we can
see has carbons arranged in a hexagon shape,
there are other crystal structures of graphite as well.
This is just showing one, but all
of the graphite crystal structure have sp2 hybridization.
The biggest difference between carbon and graphite, of course, besides
the cost, is that graphite is easy to rub off.
It's actually used as a lubricant.
We use graphite in pencils because the sheets of
the carbon can simply through friction be transferred to paper.
Here is an example of some crystals of graphite that were found in nature.
And this particular type of graphite has a hexagon shaped unit cell, which
leads to large hexagon shaped crystals, and you could see that in the sample.
The crystal has grown in this direction to a large enough
dimension that we could pick it up and look at it.
Here's an example of what it looks like from the side.
I think this one's really interesting.
You can see it's wider in the middle, and there there are these sheets.
You can almost see the lines, of the space between the sheets.
Obviously we can't see something that, that's 3.4 angstroms, but you get the idea
that there are these striations in the graphite structure, if you will.
The other forms of a carbon that I
mentioned were the bucky ball and the nanotube.
And these structures are combinations of sp2 and sp3 carbon types.
These substances have been the subjects of intense
research since their discovery a few decades ago.
One of the things that's interesting about
network covalent solids, is that, for example, if
we compare the Lewis dot structures that we
would draw of silicon dioxide versus carbon dioxide.
If we were following the octet rule that we learned
a couple of weeks ago, we would draw these two structures.
The Louis dot structure for the carbon
dioxide is fairly representative of its two true
structure and nature, but the silicon dioxide
structure is not representative of its true structure.
In fact silicon dioxide is sand, and carbon dioxide is a gas.
Obviously these two species are not as closely related in structure as it looks.
So to reconcile that confusing aspect,
we need to look more closely at the structure of silicon dioxide.
Here's a piece of silicon dioxide, which forms Quartz, and
you can see that it forms a network covalent solid.
The silicon is in the center, and then surrounding it are four oxygens.
Again this is an sp3 hybridization, or you could look
at an oxygen and see that each oxygen is attached to two silicons.
If you're really interested in geology and mineralogy, there's
lots of museums that have excellent exhibits where they show
the crystal structure of each type of mineral that
they have on a little card next to the mineral.
And that's pretty interesting to look at once you know something about chemistry.
So that was network covalent solids.
Network covalent solids have a three dimensional arrangement of the nonmetals
or the metalloids that grows in three dimensions and there's covalent bonds.
How about molecular solids?
Molecular solids are formed when nonmetals bond with nonmetals,
and the electronegativity difference between the species is relatively low.
Could be polar but it's certainly not ionic.
And so, the forces that hold something into the solid state, if
it's a molecular solid, are those
intermolecular forces we learned about last week.
London dispersion forces, dipole-dipole forces, and hydrogen bonding.
Let's look at an example of a molecular solid.
Nitrogen.
Here's the solid form of nitrogen.
Each nitrogen molecule is shown as two nitrogens
with a bond between them, but of course
we know that really there's a triple bond, and there's a lone pair on either side.
But that's, this is showing eight of those nitrogen
molecules, if it was frozen, if this was solid nitrogen.
What's holding the nitrogens together in this lattice, you can see
that this is arranged in a cubic type of lattice, right?
You see the cube that's traced underneath the nitrogens?
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Well, of course, there's this strong intramolecular force or
covalent bond holding each pair of nitrogens together, isn't there?
So there's a bond holding two nitrogens together,
but between the molecules there's only the intermolecular force.
The intermolecular force here is relatively weak.
There's only dispersion forces present for this particular example.
Some other molecular substances might have
hydrogen bonding or dipole-dipole interactions, but
generally we think of these intermolecular
forces as being weaker than covalent bonds.
And therefore, the melting points of these types of substances, would you
expect that to be a high melting point or a low melting point?
Let's say, relative to the ionic solids.
Thank you for submitting your answer.
The melting points for molecular substances are generally
lower than the melting points of ionic solids.
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To review, the molecular solids were nonmetals bonded to
nonmetals, and they were held together by intermolecular forces.
There's a special case called the polymer.
Polymer can be made from all nonmetals, or it can incorporate semimetals or metals.
But polymers are different from molecular solids
because in polymers the molecules are very gigantic.
And because they're so large, you can think of them as
being very long chains, they can tangle up with each other.
And once two chains have become entangled like
this, it's now really difficult to pull them apart.
If you ever had a bunch of necklaces in a jewelry box, just the
minor vibration of the Earth over time causes them to tangle up, doesn't it?
And so because you have long chains it can be difficult to get them apart.
Here's an atomic force microscope image of some single polymer chains.
Now in this case, they've pulled a single chain out of a sample.
But if you imagine a whole bunch of these chains together, like a bowl
of spaghetti, and you try to grab one of them and pull it out.
Well what would happens is, if you put your fork in there
try to pull out one, you'd probably pull out a big clump.
And that's what happens with polymers.
I could talk for polymers for an entire course, but I won't.
I've only got one slide, this is great restraint for me.
Here are some examples of materials that are polymers.
Anything that you think of as a plastic.
It could be a thin baggie or it could
be a really sturdy plastic like bullet proof glass.
Anything that you think of as being a
rubber, like a rubber tire or a rubber band.
Anything that is a fiber, and here are
some examples of fibers, from nature's made fibers, like
cotton and silk, through some very fancy man-made
fibers, such as nylon, or bulletproof vest material, kevlar.
Your body is full of polymers as well.
Your proteins are polymers.
You DNA are polymers.
There are long chains of repeat, of the
same small type of structure repeating over and
over and over, and they have a three
dimensional or tertiary structure that's associated with that.
So there's such a huge range of polymers,
I don't really want to make any gross generalizations
about things like melting points or properties, because
there are just too many different types of polymers.
But I really wanted you to know that that's
another type of solid, that you encounter all around you.
So this concludes the introductions to solids.
We've talked about five types of solids.
Metals, ionic compounds, covalent network
solids, molecular solids, and finally, polymers.
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