>> Welcome to a screencast on acid-base reactions.
Let's start by considering an aqueous solution of hydrogen chloride.
And when we dissolve HCl in water, we produce an acidic solution and HCl is an acid
because when it dissolves in water, it produces hydrogen ions.
Or more properly, hydrogen ions by themselves don't exist if it's an aqueous solution.
Water molecule, at least one, will combine with it
and make what's called a hydronium ion, H3O plus.
So, H3O plus is more correct for what's in solution.
But we often use H plus as a no quite correct, but shorthand version for this.
So, HCl, when it dissolves in water makes H plus and Cl minus ions.
And we can represent this in the following way, HCl gas dissolves in water
and forms H plus aqueous and Cl minus aqueous or, more precisely,
we really should say HCl gas combines with water
to make hydronium ion H3O plus aqueous and chloride ion Cl minus.
And this is a characteristic, this is what makes something an acid.
It produces H plus or H3O plus when it dissolves in water.
Now, of course, acids have other characteristics as well.
They have a sharp taste.
They turn litmus paper red.
They have low pH. Things like that.
But what makes, makes a thing an acid, in general,
is that it produces hydrogen or hydronium ion in water.
Now, on the other hand, let's consider an aqueous solution of NaOH or sodium hydroxide.
And NaOH is a base.
And it's a base because when it dissolves in water, it produces hydroxide ion OH minus.
And we can represent this in a similar manner
to how we represented the hydrogen chloride solution.
And NaOH solid, when we add water to that, makes Na plus ions and OH minus ions.
And the hydroxide ions, formation of hydroxide ions in aqueous solution,
that's what characterizes sodium hydroxide as a base.
And in general, anything that makes hydroxide ions in water is a base
and it will also have the basic characteristics.
It'll have somewhat of a bitter taste.
It'll have a high pH. It turns litmus paper blue.
Those sorts of things.
Okay. Now, what happens when an acid is combined with a base?
And we'll take an example of a strong acid, HCl, with a strong base, NaOH.
And notice that strong acids are H plus and some other ion.
In this case, Cl minus.
Strong bases are going to be OH minus and some other ion.
In this case, Na plus.
And as we saw with precipitation, when those are combined in one container,
then the possibility exists for the positive and negative ions in solution to swap partners.
And so, the H plus from the acid could combine with the OH minus from the base.
And the Na plus from the base could combine with the Cl minus from the acid
and potentially make NaCl and H2O.
And it turns out that the H2O actually forms and the NaCl, we are writing it here,
them next to each other, but the Na plus and the Cl minus don't actually stick together.
They stay in aqueous solution.
So, we write it as a partner swap, but not all partners actually really recombine.
Also, worth noting, the NaCl that forms, water is, of course, water.
The NaCl is what's called a salt.
And to a chemist, salt doesn't just mean NaCl, it means an ionic compound.
Now, this is a reaction between an acid and a base.
So, wait for it, we call it an acid-base reaction.
Pretty creative.
And in general, acids have H plus, bases have OH minus or produce OH minus.
And when they combine, they make H2O.
And these are essentially, the H plus and OH minus ions essentially neutralize each other.
So, we call this type of reaction also a neutralization reaction.
And as a general rule, when an acid reacts with a base,
it makes a salt, an ionic compound, and water.
Those of you who take the second semester chemistry,
you will see that this quote-unquote neutralization reaction is not quite
as simple as we're saying here.
But that will certainly suffice, for now.
Now, one other thing to note, this reaction can be written out in sort
of expanded ionic equation form as we did with precipitation reactions,
by noting that HCl is a strong acid.
So, it's really H plus ions and Cl minus ions separated from each other.
NaOH, same thing.
But for a base, it's a strong base, so it's really Na plus ions
and OH minus ions separated from each other.
So, I'm writing them as separate rather than combined.
And then, when they potentially swap, swap partners, NaCL is actually an aqueous solution.
The Na plus and the CL minus in, dissolved
in water don't stick together, so I write them separately.
But the H plus and the OH minus do stick together to make H2O.
And what we note is comparing out reactants and our products, we have several substances.
Cl minus and Na plus that are floating around in solution before the reaction.
Floating around in solution after the reaction.
In other words, they're spectator ions and we can cancel them out to ultimately get
to our net ionic equation which is the H plus combines with OH minus to make H2O.
And this is generally what's going to happen when we have a strong acid combining
with a strong base and reacting with each other.
Those are going to reduce to this same net ionic equation, H plus plus OH minus makes H2O.
Also, worth noting, a little more precisely, H plus is really should be written as H3O plus.
So, H3O plus plus OH minus makes 2H2O is a somewhat more precise equation for what happens.
And that's it for acid-base reactions.