Double replacement reactions | Chemistry | Khan Academy

- [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.