Hi. It's Mr. Andersen and this is chemistry essentials video 62. It's on reversible reactions.
Most of the reactions we're familiar with, like the explosion of fireworks goes from
reactants to products. In other words that gun powder and the binder and the metals are
converted into products but it doesn't go back to being fireworks again. Or if we burn
wood we're converting that into products, carbon dioxide, water. We're releasing a lot
of that energy as light and heat, but it doesn't go back to being reactants again. And so it
took chemists a long time to identify the reversible reaction. It was first identified
by Claude Bethollet. And what he was doing was looking at some lakes in Egypt and he
found something that normally is a reactant. He found some sodium carbonate. He knew that
sodium carbonate and calcium chloride would combine to form salt and calcium carbonate
which is essentially limestone. But he didn't know that the reverse occurred. And so he
found different levels of this and identified it as a reversible reaction. And we use an
arrow that goes both directions to show that the reactants are being converted to products
and the products back to reactants again. And so reactions can either be irreversible
or reversible. And an irreversible reaction the arrow goes from left to right. Reactants
become products. But in a reversible one reactants become products, products become reactants
and it eventually hits something called an equilibrium state, where even though reactants
are becoming products and products reactants, the sum amount is not changing. And so some
examples I'll talk about are going to be physical, for example when you evaporate water and then
it condenses again, that's a reversible reaction. Or when you dissolve salt in water, remove
the water and it's salt again, that would be a chemical reversible reaction. They're
incredibly important in biology, so smell for example is a reversible reaction where
molecules in the air bind to olfactory receptors in your nose and then they'll let go again.
Or environmentally as carbon moves from life, to soil, to atmosphere, that's going to be
reversible reaction as well. And so I want to start with a model. This is a PHET model.
It's not a reversible reaction per se, but it shows how they work. And so on the left
side we've got some molecules, we'll call those A and we'll color them green. And they're
just going to randomly bounce around. When they move to the right side they're going
to become red and we'll call those products. And you can see as long as the level is the
same we have about the same amount. But as I move it down on the right side, now there's
more potential energy over there. And so it's moving from the left side to the right side.
Now we have more products then we do reactants. But it goes both ways. Or if we move it down
on the left side, now we're starting to get more reactants than products. The molecules
are moving back and forth. It eventually stabilizes or it reaches an equilibrium state. And so
a way to think about this, another analogy, is to think about it as a dance. Imagine that
we have a dance. And the females and males, before the dance, are going to be like reactants.
And as they choose partners they're becoming products. But each time the song changes they
may switch to a different partner. And so as they do that we're mixing. Reactants are
becoming products but sometimes those products are becoming reactants. And so if we label
them, everybody before they started coupling up is a reactant. Everybody after is going
to be a product. But it doesn't just stay that way. It can reverse again. And so if
I were to write this as an equation we would have our reactants on the left side and then
this double arrow means they can become products. But those products, just as likely can become
reactants again. Now again, let's add a real chemical reaction here. Now we've got nitrogen
dioxide which is forming this kind of a dance where it's forming together to make a product
called dinitrogen tetraoxide. And so we can get the movement of this nitrogen dioxide
into this product. The product can go back to becoming reactants again. So we would write
the equation like this. We have simply two moles of this gas on the left side, one mole
on the right side. And so let me give you some examples of this. A physical example
of a reversible reaction could be evaporation of water. When we heat it up we're breaking
those hydrogen bonds and it's moving from a liquid to a gas. But if we cool it down
then we're simply going to have condensation again. So we've reversed that physical reaction.
Or chemically. If we take salt and dissolve it in water, so if we add sodium chloride
to water what's going to happen to it? It's going to dissolve. And that's because the
water molecules are ushering these ions of sodium and chloride away from each other.
But if we remove that water, what's going to happen? We're going to precipitate that
salt and it's going to come back again. In biology it's incredibly important, so how
do you smell? There are molecules out in the air and as you smell those move up into your
nose. And at the top of your nose, up here, you have this olfactory bulb. If we were to
zoom into that a little bit what happens is you have all of these modified neurons, we
call those olfactory neurons, that are just waiting to receive a molecule. And as those
molecules chemically bind to that olfactory sensor you sense smell. That neural impulse
is going to go to your brain and you can smell it. But if it didn't unbind you would just
have that same smell forever. So it's important that this is a reversible reaction. Environmentally
as well. Where is most of the carbon on our planet? It's actually locked in the soil.
But we can release that into the atmosphere through respiration. And we could move it
back into living material through photosynthesis. And so carbon is incredibly important, levels
of carbon in the atmosphere are really important. But it's a reversible environmental reaction.
And so did you learn to construct an explanation based on observations of different reversible
processes? I hope so. And I hope that was helpful.