-
- [Instructor] Check this out.
-
I have two clear colorless
solutions over here.
-
Let's pour them into each other.
-
We pour the first one and
we pour the second one.
-
And boom, we now get a
white colored solution.
-
Here's another example.
-
Again, two colorless solutions.
-
We pour one into another, and boom!
-
We again get a beautiful
yellow colored solution.
-
What's going on over here?
-
To find out, let's dig a little deeper.
-
Here's a more curious
question for us, okay?
-
so in the first case, what did we do?
-
We poured sodium chloride
and silver nitrate together,
-
and that gave us a white
colored solution, right?
-
But now if I were to
change just one element,
-
instead of silver, if I
had potassium over here,
-
everything else is the same,
-
so if I had poured potassium nitrate
-
and sodium chloride into each other,
-
I wouldn't have gotten anything.
-
I would've just gotten
a colorless solution.
-
It's not that interesting.
-
And therefore, I couldn't
find any footage online.
-
So this is just an edited
image, but you get the point.
-
We wouldn't get anything
interesting over here.
-
But the big question is why?
-
Why silver nitrate and sodium chloride
-
gives us a white colored solution,
-
whereas potassium
nitrate, just one change,
-
and sodium chloride does
not give us anything?
-
Let's look at it. Let's
look at it one by one.
-
So in the first case,
-
we are reacting sodium
chloride aqueous solution
-
with silver nitrate aqueous solution.
-
What do we get?
-
Well, remember that in aqueous solutions,
-
ions usually dissociate.
-
So over here, we'll get basically NA+ ions
-
and CL- ions.
-
And over here, we have
Ag+ ions and NO3- ions.
-
So when I pour them together,
-
we just get all those ions together, okay?
-
Now, because these are together,
-
they can form new combination.
-
Of course, cations should always
combine with anions, okay?
-
So NA can now combine with NO3,
-
but there's nothing special because again,
-
they will dissociate.
-
But Ag can also combine with Cl.
-
When Ag combines with Cl,
something interesting happens.
-
What? Well, guess what?
-
AgCl is insoluble,
-
and therefore, it will precipitate out.
-
And that's the reason why
this whole thing looks white
-
because of the AgCl precipitation.
-
So what do we end up with?
-
We'll end up with AgCl,
which is insoluble.
-
So that's why it's written
as solid over here.
-
It precipitates out.
-
So let me just share it over here
-
to show that it's precipitating, okay?
-
And what remains in the aqueous solution?
-
Well, sodium ions and nitrate ions,
-
so we get sodium nitrate aqueous solution.
-
The white color is due to
the AgCl precipitating out.
-
Now, if you zoom out and
look at the reaction,
-
see what has happened,
-
sodium and silver cations
have switched places.
-
Sodium has replaced silver over here
-
to get sodium nitrate,
-
and silver has replaced sodium over here
-
to give us silver chloride.
-
So since there are two
cations replacing each other,
-
there's a double replacement happening.
-
This is called, no surprise,
a double replacement reaction.
-
We can also call this double
displacement reaction.
-
So what we witnessed was a
double replacement reaction
-
and one of the products precipitated
-
giving us that white color.
-
Now, before we look at the other one,
-
a quick question for you is,
-
can you identify which of the
elements underwent oxidation
-
and which ones underwent reduction?
-
Pause and think about this.
-
Okay, whenever I want to think about that,
-
I just look at the
charges on the elements.
-
Well, over here, sodium
has a positive charge.
-
It's a cation.
-
On the other side, well,
it's still a positive cation.
-
So no change happened to
the charge on the sodium.
-
Nothing happened to it, okay?
-
What about Ag?
-
No change.
-
The same is the case
with the anions as well.
-
No change, no change,
-
which means look, nothing
is undergoing an oxidation,
-
nothing is undergoing a reduction,
-
so double replacement reactions
are not redox reactions.
-
And you may be wondering,
-
why are you excited about the fact
-
that it's not a redox reaction?
-
I'm excited because I used to think
-
that all chemical reactions
-
must involve electron
transfers, and therefore,
-
all chemical reactions should
have something oxidizing
-
and something else reducing.
-
But I was wrong. Look,
right in front of our eyes.
-
We can see examples of chemical reactions
-
where there are no electron transfers,
-
where there is no oxidation or reduction,
-
so that's pretty cool.
-
But anyways, now let's
look at the other one.
-
What happens when I
pour these two together?
-
Well, let's look at the reactants.
-
This time, the reactants
are NaCl and KNO3.
-
Both are aqueous solutions.
-
I pour them together.
-
So just like before, I will now have
-
all the four different
kinds of ions over here.
-
Na can combine with NO3.
-
Remember, cations can only
combine with anions, okay?
-
Those are the only new
combinations you can form.
-
So Na can combine with NO3-,
-
but again, it'll dissociate.
-
K+ can also combine with Cl-.
-
But what's important over here
-
is that KCl, potassium
chloride, is soluble.
-
Therefore, when K and Cl combine,
-
again, they will dissociate.
-
So nothing happens over here.
There's no precipitation.
-
I'll just end up with a solution
-
where all the four different kinds of ions
-
are just floating around together.
-
So no chemical changes happened.
-
And that's the reason why I
don't get any colorations.
-
I don't get anything over here.
-
So over here, I get
essentially no reaction.
-
So you notice the key difference?
-
The key difference was AgCl was insoluble.
-
That's why it precipitated out.
-
And that's why in order for us
-
to get a double replacement reaction,
-
we need one of the
products to precipitate.
-
If both are soluble and
they form aqueous solution,
-
then nothing will happen.
-
We'll just get a solution
-
with all the four different kinds of ions.
-
No chemical change at all.
-
So in general, we can now write down
-
what a displacement reaction looks like.
-
We can say that if you have
an aqueous solution of AB
-
reacting with an aqueous solution of CD,
-
then a double replacement reaction,
-
the two cations replace each other.
-
So A will now combine with
D and C will combine with B.
-
But that'll only be the case
if one of them is insoluble
-
and precipitates out.
-
Precipitation, sorry, is the key
-
to having double replacement reaction.
-
So look, if you pour any
two aqueous ionic solutions,
-
do not expect to get a
double replacement reaction.
-
You'll only get them if one
of the products is insoluble.
-
But now we'll be wondering,
-
how do we know whether a particular salt
-
is soluble or insoluble?
-
I'm glad you asked that question
-
because that brings us
to the solubility chart.
-
A solubility chart is basically that,
-
it tells us whether a
salt is soluble or not.
-
So here's how we can read it.
-
If you wanna look at potassium chloride,
-
here's potassium cation,
-
here's the chloride anion, sorry.
-
And now we can just say,
hey, this is where they meet
-
and so this is the solubility
of potassium chloride
-
and you can see it is soluble.
-
But what about silver chloride?
-
Silver is here.
-
Chloride is here.
-
Again, try to make them
meet. And what do you notice?
-
Silver chloride is insoluble.
-
And what about this
yellow slightly soluble?
-
Well, don't worry too much about that.
-
We'll only work with the soluble
-
and the insoluble ones, okay?
-
And just by looking at this chart,
-
you can see some trends.
-
For example, you can see salts of lithium,
-
sodium, potassium, and even ammonium.
-
Almost all are soluble.
-
Of course, there are some exceptions,
-
but they're all soluble.
-
In contrast, salts of
lead are almost insoluble.
-
You can also see salts
which have nitrate ions
-
and acetate ions, pretty much soluble.
-
Anyways, now equipped with
this solubility chart,
-
we can predict whether certain
double replacement reactions
-
are gonna happen or not, okay?
-
So let's check that. Here's the first one.
-
We're gonna pour lead two
nitrate aqueous solution
-
and potassium iodide
aqueous solution together.
-
What will we get?
-
Pause the video and try
to do this yourself.
-
First, think about what
the potential products are
-
by swapping the cations
-
and then check whether
one of them is insoluble.
-
If it is, then it'll precipitate it out.
-
We'll get the reaction.
-
If both are soluble, we'll get nothing.
-
So pause and try.
-
All right, here it goes.
-
So one of the potential products is
-
lead cation combines with iodide ion.
-
So before writing, let
me just check over here.
-
Where is lead?
-
Lead is over here
-
and iodide lead cation, okay.
-
Iodide is over here.
-
So if you look at that,
there you go. It's insoluble.
-
So I know immediately,
-
lead
-
iodide,
-
and I need to be careful,
-
lead has a +2 charge and iodine
over here has a -1 charge.
-
So to compensate, I have
to put two over here.
-
So I get lead two iodide.
That is insoluble.
-
So that will precipitate out.
-
And what else will I get?
-
Well, potassium can combine with nitrate.
-
And again, we can check
for it. Where is potassium?
-
Potassium is here.
-
Nitrate is over here.
-
So if I go down, go over
here, look, it's soluble.
-
So I'll get potassium
nitrate, which is soluble,
-
charge is +1, -1, okay,
so I'll just get this.
-
So I'll get an aqueous solution.
-
And, of course, I'll
have to balance it out.
-
Let's quickly do that.
-
So I have two iodine over here,
-
so I'll put a two here.
-
So two potassium, so I'll put a two here.
-
And that balances everything out.
-
And this is the experiment
that we saw earlier.
-
We are pouring potassium
iodide into lead two nitrate.
-
What is that yellow color?
-
That's basically the lead two
iodide being precipitated.
-
And now the aqueous solution
-
contains potassium and nitrate ions.
-
All right, why don't we try another one?
-
This one looks a little bit intimidating,
-
but the idea is the same.
-
So why don't you pause the
video and try this again.
-
All right, we start by thinking about
-
what the potential products are.
-
How do we do that? We
swap the cations, okay?
-
So ammonium cation, let's combine
them with the acetate ion.
-
Again, before writing it,
-
let's just look over here.
-
So where is ammonium?
-
Here's ammonium, and acetate is over here.
-
So let's look at that.
Oh yeah, that is soluble.
-
So this one is soluble.
-
The other one would be
sodium and sulfate ions.
-
So sodium is here, sulfate is here.
-
What do we get? Oh, that's also soluble.
-
Nothing is insoluble over here.
-
What we'll get is soluble,
so nothing precipitates out,
-
which means we'll just
end up with a solution
-
where you have all these
four kinds of ions.
-
So that means we will get no reaction.
-
All right, so the final thing
-
is that there's a special kind
-
of double replacement reaction,
-
which we call acid-base neutralization.
-
Now, we'll talk about
what acids and bases are
-
in detail in future videos,
-
we'll look at all the cool
properties and everything,
-
but for now, think about acid
-
as basically an ionic solution,
-
which has hydrogen cation
and some other anion,
-
and base as an ionic solution,
-
which contains a hydroxide anion.
-
And, of course, some metal cation.
-
For example, consider HCl,
-
which is an acid because
it has a hydrogen cation,
-
reacting with sodium hydroxide,
-
which is a base because
it has a hydroxide anion.
-
What will happen?
-
Well, we just swap the cations.
-
So sodium will combine with chlorine
-
to give me sodium chloride,
and that is soluble,
-
so I'll get an aqueous solution.
-
But the interesting part over here is
-
what happens when hydrogen
combines with OH-?
-
What do we get? This is
no longer an ionic salt.
-
This is H2O. This is water.
-
Water is covalently bonded.
-
So we now end up with a
covalently bonded molecule.
-
So we will get water, H2O.
-
And since it's no longer
an ionic solution,
-
we just write as liquid.
-
So look what we get in general.
-
When you combine acid with a base,
-
they neutralize each other
to give us a salt and water.
-
So this is a special kind of
double replacement reaction
-
because there are no
precipitates over here,
-
but the reaction happens
-
because we get a covalently
bonded liquid water.