Chemical reactions introduction | Chemistry of life | Biology | Khan Academy
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Chemical reactions introduction | Chemistry of life | Biology | Khan Academy

– [Voiceover] Let’s talk a little bit about chemical reactions. And chemical reactions
are a very big deal. Without chemical reactions,
you or I would not exist. In your body right now,
there are countless chemical reactions going on every second. Without chemical reactions,
we would have no life, we would not even have the
universe as we know it. So what are chemical reactions. Well, they’re any time that
you have bonds being formed or broken between atoms or molecules. So what are we talking about there? Well this is maybe one of the most fundamental chemical reactions. Once again if one never
occurred, we’d be in trouble, we would not have, we
would not have any water. But let’s think about what
it is actually describing. So over here on the left-hand
side we have the reactants. Let me write that down. So here we have the reactants. These are the molecules
that are going to react. And then we have an arrow
that moves us to the product. So let me do that in a different color. So we have an arrow that
moves us to the product, or we could say the products. And so what are the reactants here? Well we have molecular hydrogen and we have molecular oxygen. Now why did I say molecular hydrogen? Because molecular hydrogen,
which is the state that you would typically find hydrogen in if you just have it by itself, it is actually made up
of two hydrogen atoms. You see it right over here,
one, two hydrogen atoms. And what we have in order
to have this reaction, you don’t just need one
molecular hydrogen and one, or one molecule of hydrogen
and one molecule of oxygen. For every, for this reaction to happen, you actually have two molecules
of molecular hydrogen. So this is actually made
up of four hydrogen atoms. So let me make this clear. So this right over here, this is two molecules
of molecular hydrogen. And that’s why we have the two right out front of the H sub-two. This little subscript
two tells us there’s two of the hydrogen atoms in this molecule. And then this big, this big white two that we have right over here,
that tells us that we’re dealing with two of those molecules for this reaction to
happen, that we need two of these molecules for every, for every molecule of molecular oxygen. And molecular oxygen, once again, this is composed of two oxygen atoms. One, two. So under the right conditions,
so you need a little bit of energy to make this happen. If under the right conditions these two things are going to react. And actually it’s very, very reactive, molecular hydrogen and molecular oxygen. So much so that it’s actually
used for rocket fuel. You are going to produce
two molecules of water. We see that right over here. And look, I did not create
or destroy any atoms. I had one, I had one, I had one oxygen atom here. It was part of the oxygen
molecule right here, then I have the second
one right over here now. Now they are part of separate molecules. I had, I had a, I had one two, three, four hydrogens. I now have one, two, three, four hydrogens, just like that. And actually this produces a… So we could say some energy, and I’m being inexact right over here. Some energy and then we
could say a lot of energy. A lot of energy. So this is a reaction that
you just give it a little bit of a kick-start and it
really wants to happen. A lot, a lot of energy. So one thing that you might wonder, and this is something
that I first wondered when I learned about reactions, well how do, how does this happen? You know, is this a very organized thing? You know, do these molecules somehow know to react with each other? And the answer’s no. Chemistry is a incredibly messy thing. You have these things bouncing
around, they have energy. They’re bouncing around all over the place and actually when you provide energy, they’re gonna bounce around
even more rigorously, enough so that they
collide in the right ways so that they break their old bonds and then they form these new bonds. So whenever you see these
reactions in biology or chemistry class, keep that in mind. It looks all neat and
organized but in a real system, these are all of these things just bouncing around in all
different crazy ways. And that’s why energy’s
an important thing here. Because the more energy
you apply to the system, the more that they’re
going to bounce around, the more that they’re going
to interact with each other. The more reactants you put in, the more chance they’re
going to bounce around and be able to react with each other. Now I’m gonna introduce another word that you’re gonna see in chemistry a lot. This water, these two… We see we have two water molecules here. We could call them molecules,
but since they are actually made up of two or more different elements, we could also call this a compound. So water, water is, you
could call it a molecule, or you could call it a compound. So this is a molecule or compound, while this molecular hydrogen, you would not call this a compound. And this molecular oxygen,
of course it’s a molecule, but you would not call
it a compound either. And just to get an appreciation of how much energy this produces, let me just show you this
picture right over here. That’s the space shuttle and this, this big tank right over here, let me… This big tank contains a bunch of liquid oxygen and hydrogen. And to create this
incredible amount of energy, it actually just… You mix the two together
with a little bit of energy and then you produce a ton of energy that makes the rocket, that
makes the space shuttle. Well, space shuttle’s
been discontinued now, but back when they did it, to make it get it’s necessary,
it’s necessary velocity. Now let’s talk about the idea. So, you know, this reaction,
strongly goes in this, in the direction of going to water. But it can actually go the other way, but it’s very, very hard
for it to go the other way. So in general we would consider this to be an irreversible
reaction, even though it is. You know irreversible sounds like, hey you can’t go the other way. It just really means that it’s very unlikely to go the other way. You have to supply a lot of
energy to go the other way. To make this reaction go the other way, you would have to do
something called electrolysis, you provide energy, etcetera, etcetera. But in general, the way
that this is written, because the arrow is only
pointing in one direction, this is implying that it is irreversible. Irreversible. Irreversible. Which probably makes you think, well what about reversible reactions? And I have an example of a reversible reaction, right over here. I have a one bicarbonate ion. And the word ion, that’s
just used to describe any molecule or atom that has either, has an imbalance of electrons or protons that cause it to have a net charge. So this makes this an ion,
and actually right over here, this is a hydrogen, this is a
hydrogen ion right over here. Both of these are charged. One has a positive charge,
one has a negative charge. But they are both, they are both ions. And this reactions right over here, you have the bicarbonate ion
that looks something like this. This is just my hand-drawing of it. Reacting with a hydrogen ion,
it’s really a hydrogen atom that has lost it’s electron,
so some people would even say this is a proton right over here. This is an equilibrium reaction, where it can form carbonic acid. And notice all that’s
happening is this hydrogen is attaching to one of
the oxygens over here. And this is an equilibrium
because if in an actual, in an actual solution,
it’s going back and forth. If you actually provide more reactants, you’re gonna go more in that direction. If you provide more of
the products over here, then you’re gonna go in that direction. And so in an actual, in
an actual environment, in an actual system, it’s constantly going back and forth between these two things. And different reversible reactions might tend to one side or the other. If you provide more of
the stuff on one side, it might go more in the other
directions because these are gonna, they’re gonna be
more likely to interact, Or if you provide more of this, it might go in the other direction because these might more likely react with their surroundings or
disassociate in some way. Now just to get a sense of, you know, it’s nice to kind of, you
know, are these just some random letters that I wrote here? Carbonic acid is actually an
incredibly important molecule, or we could call it a
compound because it’s made up of two or, two or more elements, in living systems and in fact, you know, even in the environment. And even when you go out
to get some fast food. When you have carbonated
drinks, it has carbonic acid in it that disassociates
into carbon dioxide and that carbon dioxide is
what you see bubbling up. Carbonic acid is incredibly
important in how your body deals with excess carbon
dioxide in its bloodstream. Carbonic acid is involved in the oceans taking up carbon dioxide
from the atmosphere. So when you’re studying chemistry, especially in the context of
biology, these aren’t just, you know, interesting things
that seem very academic. These are effecting your
real life and your body and your environment.

54 thoughts on “Chemical reactions introduction | Chemistry of life | Biology | Khan Academy

  1. One correction to make here: those bright orange thrust plumes are coming from the white solid fuel rocket boosters on either side, not the liquid H2/O2 thrusters on the Space Shuttle itself.  Those are dubbed the SSME's (Space Shuttle Main Engine) and burn with a nice blue flame that isn't very long.

  2. Wait, so the shuttle makes water instead of whatever nasty shit comes out of my car? Does that mean the smoke coming out of it isn't even really smoke, but steam?

  3. Probably a year late, but yeah those large plumes of fire you see are not produced by Liquid Hydrogen, or Liquid Oxygen. That is actually produced by the SRB (Solid Rocket Boosters).

  4. If Lithium rects with water would it's chemical equation be:

    Li(s) + H2O(l) → LiOH(s) + H2(g)


    Li(s) + H2O(l) → LiOH(aq) + H2(g)

    and why is it LiOH and not LiHO?


  5. Strange how chemical reactions are described without regard to energy as if energy is of no significance. Ions they keep popping up as if electrons are freely interacting on their own. Solution somehow means water, aqueous has what role in reactions? Best lectures I have seen yet is Linus Pauling Lecture: Valence and Molecular Structure. (1957). Next lesson.

  6. I am a student typing for a teacher who doesn’t know how to make a screen big and won’t listen to me and will only listen to the man talking HELP!!!

  7. Hydrogen is flammable and oxygen is supporters of combustion they should burn more and should blast but how their reaction form water

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