Hi. It's Paul Andersen and this is disciplinary core idea PS1B. It's on chemical
reactions. Lots of times when we hear the word chemical reactions we think of things
like this. This is a thermite reaction. You have metal and a metal oxide. They're combining.
It's giving off a huge amount of heat. A huge amount of light. But also inside me there's
a number of chemical reactions that are going on that allowing me to think and to move and
to speak. And so chemical reactions are incredibly important. What are they? They're essentially
changes. Changes in which atoms are rearranged. The bonds are broken and then reformed again.
And so somethings we clearly think of as a chemical reaction. Like the explosion of a
firework. But somethings are just rearrangement of molecules. And so it's not a chemical reaction
per se. For example, creating a solution or boiling water you can return that to the way
it was very readily. But if we see a lot of gas. If we see heat being produced then we
know that a chemical reaction is occurring. An important concept that you should understand
is the conservation of mass. And so even though the bonds between atoms are being broken and
reformed again, there's a conservation of mass. And so this is the Hindenburg, that
awful disaster. What was going on? We had the hydrogen gas inside the Hindenburg, which
is great. It makes it light and it allowed them to float. But it's combining here with
a spark with oxygen in the air. It's producing water. It's also producing a huge amount of
light and heat. And so we have a chemical reaction now. The hydrogen gas and the oxygen
gas are reforming to make water. But the mass is conserved. In other words, let's balance
this equation for just a second. If we were to take the mass of that hydrogen and oxygen
before and after, it's going to be exactly the same. And I'll show you a quick demonstration
of how you could do that in the classroom. Collision theory is really dictating how chemical
reactions occur. What does that mean? Matter is made up of atoms and those atoms are constantly
in motion. Depending on the state of matter, you're going to have different amounts of
motion. But the more motion we have, the more collisions we have and the more reactions
we're going to have. So how could we increase molecular motion? Just through an increase
in temperature. We also could concentrate the reactants to do that. And then all of
those reactants are going to have a certain amount of potential energy. And depending
on that some reactions are going to move in one direction or another. And so where do
you start as far as in teaching this. Well at the early elementary grades you want to
talk about how changes in temperature can causes changes in molecules. And so for example
boiling water, increasing it is going to increase the molecular motion. What are we going to
create when we do that. We're creating water vapor. Or if we were to freeze some of the
water vapor in the air we could get solids. So like snowflakes. But the kids should realize
that both of these cases are easily reversible. In other words if we were to just add a little
bit of temperature to this, it's going to quickly turn back. It's going to melt, turn
back into water. And then if we heat it up again, it could turn into water vapor like
we would in boiling. But they should know this, that if you change the temperature sometimes
you get reactions that are harder to reverse. And so if you bake bread that looks a lot
different than the dough did originally. Or if we had a flame for example, that's going
to be an increase in temperature to get that, but it's hard to turn it back into what it
was before. So that's a good place to start. That adding temperature can definitely change
molecules. As they move into the upper elementary grades, we should start with this idea of
conservation of mass. And so what's the quintessential chemical reaction that people have been doing
from the beginning of time it seems like is mixing vinegar and baking soda. That's how
you make that volcano kind of overflow. And so what's a great way you could measure this?
Well let's do an experiment where you put a little bit of vinegar in a beaker. We put
some of the baking soda in a balloon and we take the mass of both of those to begin with.
We then empty the baking soda in. We seal the balloon up and we get a lot of bubbles
going on inside there. It's not important that they know what chemical reaction is going
on, but we could then take the mass of that when we're done. And it should be exactly
the same as the sum of those two masses were to begin with. So the chemical bonds were
broken and reformed, but the mass stayed the same. As we move up into the middle school
we should start talking about atoms and how atoms will regroup and form. A great demonstration
I like to do even in my class today is to take a little bit of hydrogen peroxide. Put
it in a zip lock bag with a little bit of yeast. That yeast contains an enzyme that's
going to break the hydrogen peroxide down. What's it going to become? It's going to become
two things. We're going to get water forming. And then we're going to have oxygen forming.
So you get these little bubbles inside the bag. Now it's a great time to talk about the
energy. Because you'll feel that bag get warmer and warmer and warmer. So we know that a chemical
reaction occurred. But we also know that there was more energy or free energy before in the
hydrogen peroxide than there was after. So we release some of that energy. What's going
on here? Well we're taking those atoms, breaking some of those bonds and then we're reforming
them again. As we move up into high school then we really want to talk about what reactions
are occurring. And how that rate can be manipulated. And so a great simulation that allows you
to play around with this is at phet.colorado.edu. This one's chemical reactions. And let me
jump over there and show you what I'm talking about. And so if we look at this reaction
right here, we have these reactants on one side. We'll say A is going to react with B
and C. And that's going to form A, B and C. And so how do we start this reaction? It's
kind of like a pinball machine. Let me launch that molecule. And so you can see that we're
switching forms. And so we're getting chemicals turning into other chemicals and then back
again. And so eventually what will happen is that they'll start to move off of the line
like that. And so if they don't run into each other then we're not going to have that reaction.
How could we speed up or increase the chances of that reaction occurring? We could add a
little bit of temperature to it. And as we add that then we're more likely to bounce
into each other and have a chemical reaction. But what do they have to do? They have to
actually hit each other. Lose that molecule. If I slow it down, likewise, the molecules
are going to be slower so it's less likely that they're going to bounce into each other.
And so you could even play around with this in the simulation. Let's go to here. The same
kind of experiment. What you can do is add some molecules. So add a bunch of A inside
here. Now we're increasing that concentration. Now we could add a lot of B and C in here
as another molecule. And you can see that they're all bouncing around but we're not
having those reactions go off. And so we've got a lot of concentration but we don't have
enough energy. So what can I do if I add the temperature? So if I increase the temperature,
what's going to happen? They're going to bounce in to each other and we're going to get more
of those reactions occurring. And so what we have is A and B, C turning into A, B and
C. But it depends on the rate. If we slow that down, slow that molecular motion down,
then they're not going to hit each. And so we're going to have less of those reactions.
And so that's what chemical reactions are. Again it's simple. It's just breaking of bonds
between atoms and reforming again. But the progression you should teach it is starting
with just this idea of change and then we can eventually get into some chemistry when
we get into high school. And I hope that was helpful.