Hi. It's Mr. Andersen and this is chemistry essentials video 29. It's on synthesis and decomposition
reactions. If I were to ask you which of the two reactions you see on the left is a decomposition
reaction, the tendency is to choose the rusting railroad car. And the reason why is it looks
like it's breaking down. But if you think about it, what is rust? Rust is a combination
of iron and oxygen and so this is actually a synthesis reaction. And so if we ever take
atoms and molecules and combine them together to make a new compound, then we've got a synthesis
reaction. So what's an example of that a decomposition? The breakdown of water into oxygen and hydrogen
gas is going to be an example of that. And so if we take atoms or molecules and we combine
them together to form a new compound, we call that a synthesis reaction. We're making something.
Likewise, if we take a compound and break it down into atoms or molecules we call that
a decomposition reaction. Lot's of times that will generate a little bit of heat. We can
use both of these reactions in the chemistry lab to do simple stoichiometry and practice
our lab techniques. And so synthesis reactions are when we are taking atoms and molecules
and combining them together. So let me give you a couple of examples. Let's say we take
magnesium. Get it a little bit hot. And then allow it to combine with oxygen. It's going
to make magnesium oxide. And so when it's done you're going to have a new solid that's
created. We're taking a solid and a gas. And we're making a new solid out of it. Another
example of a synthesis reaction could be the carbonation of water. And so this is me adding
a little bit of carbon dioxide to my soda stream at home. And what's going to happen
inside there is that the carbon dioxide is going to combine with the water. And it's
going to make carbonic acid. So that carbonic acid is going to lower the pH to around 4
and it's going to change the taste of that carbonated beverage. What's decomposition
then? It's when we're taking a compound and breaking it down. So here's an example. If
we take sodium azide and break it down into sodium metal and nitrogen gas, that would
be a decomposition reaction. So if we have one reactant and two products it's a good
idea that this is probably decomposition reaction. Where is an example of this actual reaction?
Well this was used in early airbags. And so what you can do is you can put some sodium
azide inside the bag right here. It's totally stable. But if you add a little bit of heat
to it, so there's an ignitor inside there, it's going to quickly break into sodium and
nitrogen gas. And that nitrogen gas is going to fill up the bag really quickly. Now you
could imagine a bunch of nitrogen in there is not a good thing. That's highly reactive
as well. And so they also put chemicals in here that react with the sodium. Another
example could be right here. I'm taking hydrogen peroxide and then I'm simply adding a catalase
to it. I'm adding an enzyme that's found in yeast. And so the yeast isn't part of the
chemical reaction. It just serves as a catalase. And so what I can do is I can add hydrogen
peroxide to it. Seal it up. And I can give it a little bit of a shake. And what it's
going to do is it's going to break down into water. And then it's going to breakdown into
oxygen. And that oxygen is going to fill up the bag. Now if you feel the bottom of the
bag it's going to get warmer and warmer. And the reason why is that we're releasing energy
as we break those bonds and form new bonds. To the point where it can eventually explode.
And so sometimes you have to let a little bit of that gas go. That scared me. So did
you learn the difference between a synthesis and a decomposition reaction? Hopefully. In
this one when we're burning magnesium or combining it with oxygen, that's going to be a synthesis
reaction. And could you do simple stoichiometry on that? Well one way to do it would be to
weigh the magnesium to start. We could put it in a crucible. Heat it up and then we could
weigh it when it's done. We could figure out the mass of the products and we could work
backwards to figure out how much oxygen is actually being added. And I hope that was
helpful.