DANIEL ROSENBERG: All right,so in our hood we have the hydrogen-chlorine electrolytic
gas generator. Here's the water bath,the trough that we collect the hydrogen and chlorine
gas. And, here we have the actual generator. In this
case, we're getting...almost 4 Volts to drive 3 Amps through a pair of carbon electrodes
that are immersed in 8-molar hydrochloric acid. So that's concentrated hydrochloric
acid, 2 parts, and 1 part water. And at the end of the tube is a glass elbow that allows
me to fill the test tube by water displacement. Now, one very important aspect of this demonstration
is that the...water bath and the gas generator have to run for a number of hours before you
start collecting the gas, because the water that you start out with is saturated with
oxygen, and oxygen is a catalytic poison for this reaction. I'm going to replace the light
screen, because this is a photochemically active reaction, we want to be careful how
much light we expose this to. Right before the demonstration we fill the tube with fresh
hydrogen and chlorine from a water bath that has had hydrogen and chlorine bubbling through
it for about 3 or 4 hours...I generally set it up overnight and put it at a low...a low
current, but that clears...that bubbling of chlorine clears the oxygen out of the water.
It also turns the water into a...the chlorine also...So the chlorine gas bubbling through
the water bath clears the oxygen out of the water, but it saturates the water with chlorine,
and so if you're wearing any jewelry--especially gold jewelry--it's very important to have
glove on or take it off because the chlorine in the water will dissolve the gold right
on your finger! Ok, here we have a quartz test tube full of a stoichiometric mixture
of hydrogen and chlorine, one-to-one! And we understand that hydrogen and chlorine is
a reaction that can be set off with light, so we're going to experiment with different
frequencies of light to see what the threshold of reaction is. So we're going to start with
red light, don't see a reaction--maybe we don't have enough red light? Still no reaction.
We're going to flex our LED collection, we've got orange light...shorter wavelength, higher
energy...yellow light, moving up the energy scale...I've got green light...still no reaction.
We have a flashlight with the green LED, so we're dumping a huge amount of energy in the
form of green photons, and we're still getting no reaction. Now, moving to blue light...no
reaction, and finally, ultraviolet.... [BANG!] And we see that the ultraviolet light causes
an almost immediate reaction! Here we have the product of of the reaction of hydrogen
and chlorine, in the form of hydrogen chloride, which in the humidity of the lecture hall
causes a big ol' cloud to form.