Hi. It's Mr. Andersen and this is chemistry essentials video 10. It's on
light and matter. Matter is what we're made of. But I've got some matter on the screen
and you can't see it because there's no light. Once I add light we start to see matter. And
so we can see solid, liquid, gas and plasma. And so until we have light we really can't
probe matter and figure out what it's made up of. And so in this video I'm going to talk
about light and matter. Remember light travels in these little packets called photons. And
we can quantify its light using Planck's equation. Where energy is equal to h nu. And h is going
to be Planck's constant. Nu is going to be the frequency of the light. But we can use
these little photons to probe matter and figure out what it is. Because if you have a molecule.
And it absorbs a photon. It absorbs a certain amount of that energy. And when it releases
a photon it releases some of that energy as well. And so we can use different types of
photons to figure out different things about molecules. And so infrared light is incredibly
important at looking at the vibrations within atoms and molecules and figuring out what
bonds they have. If we're using ultraviolet or visible light, we could look at the electronic
structure. In other words where the electrons are found. The outer electrons and what they're
doing. And then finally we can look at solutions using light. And as we shine light through
a solution some of that light is going to be absorbed. And so we can figure out according
to Beer-Lambert law what is the concentration of those absorbing molecules. And so light
remember is a wave and a particle. So if we're looking at it as a wave we have what is called
the electromagnetic spectrum. All the way from radio waves to gamma rays. Radio waves
are going to have a really long wavelength and a low frequency. That's how many waves
we have per second. Gamma rays are going to have really high frequency, small wavelength.
And we're just seeing a small portion of that. When you look at me you're only seeing visible
light. And so light is a wave. Light is also however a particle. And what that means is
that it's quantize or comes in these small little quantities. And so you could think
of it like this. And let me play some music in the background. This is Rachmaninoff for
violin and piano. And so a violin can play any note. You just move your finger out just
an incremental amounts and you can get any note. But a piano is going to be quantized.
You're going to have specific quantities you can only hit a C or a D or an E. And so really
which is light? Light is like a piano. It comes in these small little quantities of
photons. And so we can measure that energy using Planck's equation where h times nu is
going to be Planck's constant times the frequency of the light. And so which of these wavelengths
are going to have the highest amount of energy? Ones with the highest frequency. And those
are going to be the gamma rays right up here. Which ones are going to have really low energy?
Those are going to be the radio waves down at the bottom. And so what we can do is we
can measure photons and how they're picked up by atoms. And it tells us a lot about what
the electrons are doing. And so a spectroscope allows us to see this whole electromagnetic
spectrum. One of the first scientists to do this was Newton. If you wanted to build one
of your own all you would need is a prism and then a small slit. So we run light through
it. And it's going to split that light into the different colors that we could see. So
this would be a compact fluorescent. And you can see the different light, colors of light
that are coming from it. In a typical chemistry class you'll just use a cheap spectroscope
like this. It's going to have a slit on onside and then it's going to have diffraction grading
on the other side. And then an eyepiece down here. And so when you look at a light it's
going to look like this. But if we were to look at an incandescent light, it's going
to look different. And we can look at different atoms and it tells us a lot about their electrons.
And so a spectrometer is simply a sophisticated spectroscope. What you're going to have is
a source. So we're going to have some light on one side. We're then going to have a grating
and the detector now is going to be a camera. It's just like a digital camera that's picking
up more of the light that we can't see. And what you do is you take your sample and then
you slide it in like that. And so different spectrometers are going to allow us to see
different things. And so an IR spectroscopy, what we're looking at is infrared light. And
so what's interesting about atoms is that they're vibrating constantly. There's going
to be vibration between atoms and between molecules as well. And so let me give you
an example. This would be a carbon and 2 hydrogens. And what they have are called degrees of freedom.
And so there's going to be a lot of vibration going on within a molecule. And what you can
have happen is that when you introduce infrared light, it can actually pick-up some of that
energy. The bond is going to pick up some of the energy of that infrared light. And
it's going to make these vibrate even more. And it's kind of like pushing somebody on
a swing. If you keep pushing them at the right rate, eventually they go higher and higher
and higher. And so what happens to that infrared light? It's absorbed. And so what we can do
is we can use an IR spectroscope and we could look at what frequencies of light are being
absorbed. And then we can use a fingerprint or a correlation like this and it's going
to tell us what kind of bonds we're going to find within that molecule. Now if we wanted
to look at more of the electronic structure then we'd be using UV or visible light. And
so what we're doing here, same kind of a thing. Spectroscope where we have light. We have
a detector. And this time we're just using UV light or visible light. And what we'll
find is instead of vibrating those molecules, what we're figuring out is that the photons
are being absorbed by electrons. They're moving up to a higher energy level. And so that's
where the energy is going. and so we can us a spectrum like this. And it's going to tell
us more what those valence electrons or the electrons on the outside are doing. And then
we finally have Beer-Lambert law. And what that is, here's an example right here. We
have a dye and then we're shining a green laser into it. And what we're finding is that
laser light isn't making it all the way through. And that's because we have molecules on the
inside that are absorbing some of that light. And so here's an application of that. It's
just a simulation from phet. We have a light on this side. And then what we're going to
do is we're going to measure the amount of light that is moving through it. So I've turned
the light on. We're going to set that to absorbance. It's the amount of light that gets through.
And we're dealing with cobalt chloride. And so what we've got is a solution. You can see
as I turn down the concentration more light goes through. And as I increase the concentration,
less light is making it's way through. And so this is Beer-Lambert Law. Basically as
I increase the concentration less light makes its way through. And so I can do a quick little
experiment here. I could set it at 0. And now at 50. And now at 100. Look what happens
as we increase it now to 150. Look what's happening to the absorbance. More of it is
being absorbed. Now let's move it to 250. And so here's my data right here. Where on
the left I have the concentration. On the right I have the absorbance. And you can see
it looks like a pretty linear relationship. Let's graph it. So that's perfect. And so
what we're finding is as we increase the concentration of the solution more of that light is being
absorbed. And so what you could now use is you could just take a sample. We could measure
the absorbance of it. And then we could figure out specifically what is the concentration
of that solution. And so what did we learn? That light, and using Planck's equation, travels
in these little photons. And we could quantify that. We can use it to probe matter. And that's
because molecules are picking up or letting go of these photons. And then remember the
three major things that infrared light allows us to look at the vibrations of molecules
and tells us a lot about what their bonds are. Ultraviolet and visible light tells us
more about their electronic structure, where the electrons are. And then in solutions,
those absorbing materials remember will pick up some of that light. And so we can use Beer-Lambert's
law to make predictions on the concentration based on the absorbance levels. So you should
have learned to choose the correct spectroscopy to measure vibrational or electronic motion
of molecules. So again if you're looking at vibrational you better choose infrared. And
if we're looking at electronic structure then you should choose UV or visible light. And
then finally we could use the absorption of light to determine the concentration of absorbing
species in a solution. That's Beer-Lambert's law. As we increase the concentration, we
increase the absorbance and I hope that was helpful.