Chem143 Gas Producing Chemical Reactions

Chemical reactions involving gas evolution.

>> In this video, we're talking

about some special chemical
reactions that produce gas

when they occur that gas
is one of the products.

So first thing is here they are,

and this you just
have to memorize.

This is not something that
you're going to figure

out from stuff we've
talked about so far.

You just had to remember

that when you're doing a double
replacement reaction, so,

you know, doing it on
paper and you're figuring

out what the products would be,

if you do double
replacement reaction

and you see carbonic acid as
one of the products H2CO3,

you just have to know that it
will spontaneously decompose

to make carbon dioxide
gas and with water.

Likewise, sulfurous acid
will spontaneously decompose

to make sulfur dioxide
gas in water.

And ammonium hydroxide
will decompose

to make ammonia gas and water.

And you have to recognize
this, so memorize this.

And we should illustrate why
that's going to be important.

So let's say you're
reacting sodium carbonate

with hydrochloric acid
and you want to figure

out what the overall
place is going to be.

Well, you recognize this as a
double replacement reaction,

sodium carbonate and
hydrochloric acid,

so you switch the
cations, right?

Hydrogen ion replace this
sodium to make a carbonic acid.

Excuse me, yeah, carbonic acid.

And the sodium ion
replaces the hydrogen ion

to make sodium chloride where
have balance the charges

to get subscripts and
balance the equation

with the coefficients,
the 2 and the 2 here.

But-- And you just
have to recognize this

by memorizing the pervious slide

that when you see
this carbonic acid,

you have to remember
that it decomposes.

And when it does, it makes
carbon dioxide gas and water.

So when we-- the overall
reaction that we see is going

to be the net result
of these two.

And what you do, OK, to get the
overall reaction is you just add

these two equations.

Now when you add
chemical equations,

it's just like adding
algebraic equations.

If we have the x on the
left side of an equal sign

in one equation, and an x on
the right side of the equal side

in another equation and
you're adding them together,

the x's cancel, right?

Well, here, we have carbonic
acid on the right hand side

of the arrow in one
equation that we're adding

and carbonic acid on the
left hand side of the arrow

on the other equation that
we're adding and they cancel.

And so everything else that's
left is our overall equation.

So on the left side
of the arrow,

we have sodium carbonate
reacting with hydrochloric acid

to make sodium chloride
from here,

carbon dioxide gas
and liquid water.

And so what you see when
you add hydrochloric acid

to sodium carbonate
is you see bubbles,

the bubbles are the carbon
dioxide gas evolving.

Let's look at another example.

Let's say you're reacting
solid strontium hydroxide

and solid ammonium chloride,

this is really interesting
reaction

because two solid ionic
compounds react with each other.

And it's a double
replacement reaction.

And so if you take the ammonium
ion which is the cation here,

right, NH4 plus and
replace strontium over here,

you get ammonium hydroxide
whereas we balance the charges.

Each hydroxide has a negative 1,

the ammonium has a
plus 1 which is NH4OH.

And when the strontium
replaces the ammonium,

we get strontium chloride,

2 chlorines to balance
the positive 2 towards

the strontium.

But, again, yeah, just
has to recognize this.

Ammonium hydroxide is
one of those compounds,

the special ones that will
decompose spontaneously

like this, the ammonium
hydroxide makes ammonium

and water.

Now, I've put the two in front
here because we had the two here

and I want to add
these equations again.

And so when we add them, again,

the two ammonium hydroxides
cancel on each side.

And our overall equation,
what we observe is this,

the strontium hydroxide
racks with ammonium chloride

to make strontium chloride
and ammonia gas and water.

So when you do this reaction,

what you see is you mix
two white powders basically

together, swirl them together.

And if you smell it,

you'll smell it's really
kind of a harsh odor.

That's the ammonia
gas being given off.

It also become a slurry that's

because the water
is being formed.