0:01
You have been through several units in which you have
dealt with equilibria.
First, we introduced the concept of equilibria
and then we applied it in the environment of
acid solutions or base solutions
or salt solutions.
That have acidic and basic properties.
For there we are going to go into this chapter
and this unit where we are going to be looking at
other environments in which aqueous and this unit where we are going to be looking at
other environments in which aqueous
solutions play a big role.
0:29
This unit is divided into two main
categories. We have got
homogeneous equilibrium
and that will be our first homogeneous equilibrium
and that will be our first
8 learning objectives and that will be our first
8 learning objectives
and then we will look at heterogeneous equilibrium.
We will see environments where a solid
will form from solution.
0:59
This is what a buffer is.
It is a solution which resists change to pH. This is what a buffer is.
It is a solution which resists change to pH.
So you create this solution
and if you add an acid or you add
base to the solution, the
pH will not change very much.
It will resist the change.
This is a recipe in a sense.
What is in the solution that makes it a buffer?
It needs to have a week acid
and its salt. It needs to have a week acid
and its salt.
Or a weak base and its salt.
But lets look at what I mean by that.
If it is a weak acid and it is But lets look at what I mean by that.
If it is a weak acid and it is
salt, the salt is
[take a weak acid, and I am going to take a weak acid
and write it in this space right here [take a weak acid, and I am going to take a weak acid
and write it in this space right here
HF for example.] and write it in this space right here
HF for example.]
I take take weak acid
and I were to
think about what its
conjugate base is, that would be F-. think about what its
conjugate base is, that would be F-.
And then if I were to create the salt from that
I would need to add and ion with it.
So a counter ion, like cation
NaF
would be a salt
containing that weak base.
So a buffer consists of a weak acid.
and the salt of its conjugate base.
It could also consist of a weak base
and its salts. So lets think of a weak base.
Ammonia is a weak base.
2:30
Well, I cannot go and obtain some NH4+
off a stockroom shelf I need to
obtain a salt that contains that ion
so I put a counter ion with it.
A negatively charged ion for example
and we would have A negatively charged ion for example
and we would have
2:58
So here is some examples besides the ones I already gave you.
If I took acetic acid
and that is what this substance is acetic acid.
And I were to consider its conjugate base
the conjugate base would be removing that proton right there.
and that would become
anion, which is its conjugate base
and the salt, letsput a counterion with that
we put a sodium for example, so that is a weak acid and the salt, letsput a counterion with that
we put a sodium for example, so that is a weak acid
and its conjugate base salt.
3:28
We often write it with this
slash like this, where you write the base first and the acid second
that is a standard way of writing slash like this, where you write the base first and the acid second
that is a standard way of writing
a buffered solution.
I have already given you that example in the previous slide
and it to, would you write the base first and the acid second
it would be a standard way of writing and it to, would you write the base first and the acid second
it would be a standard way of writing
that buffered solution. it would be a standard way of writing
that buffered solution.
3:52
Here is an example of something that is not a buffer
but often student think that is might be.
If I had a solution containing HCl
and sodium chloride. Well, this is an acid.
This is its conjugate base.
this is its salt.
So it is an acid with a conjugate base salt
but this is not a weak acid.
It is a strong acid.
And as a strong acid
it cannot be used to make a buffer.
Lets have you consider it cannot be used to make a buffer.
Lets have you consider
this solution. We have a solution containing
this substance this solution. We have a solution containing
this substance
and this substance and it is not a buffer.
4:36
It has a weak base in it, it has a salt.
containing an acid. It has a weak base in it, it has a salt.
containing an acid.
Why is that not a buffer?
Think about it and choice your answer.
5:37
It certainly does not constitute
the components of a buffer It certainly does not constitute
the components of a buffer
but acid and bases react with each other
lets consider the reaction that will take place.
The OH- of the base
6:04
we have 10 ml of .1 molar
and 25 ml of .1 molar.
We have a little bit of this comparatively
a lot of this.
So what will happen? Well this
OH- will react with the HF. So what will happen? Well this
OH- will react with the HF.
6:25
And produce some of this.
It will all go away.
And we will have a little less of this
and some of that, and what will we have?
We have in solution and some of that, and what will we have?
We have in solution
a weak acid
and a conjugate base.
Therefore we have a buffer.
So when you have reaction between a strong and a weak
and undergoes a neutralization reaction
if the amounts are just right, it turns into a buffer.
Now we will talk about this a whole lot
more when we get into titrations. Now we will talk about this a whole lot
more when we get into titrations.
7:00
So this is just a reminder, when we get to titrations
we will talk about creating buffer solutions in the So this is just a reminder, when we get to titrations
we will talk about creating buffer solutions in the
course of that, and that won't be
a few learning objectives down the road.
7:16
So buffers are what we call a
common ion situation. So buffers are what we call a
common ion situation.
What does that mean? Well a common ion common ion situation.
What does that mean? Well a common ion
is when you have dissolved two substances
into a solution
and the two
things you are dissolving, both contain
the same ion. That would be a common ion solution.
So if we were to take HF, an acid,
and dissolved in there NaF.
So we have placed into a flask both of these things
what would be the common ion?
What do they have in common? what would be the common ion?
What do they have in common?
Well, they both have F- in them.
7:54
So we know that when we place HF in water
it, to some extend because it is weak,
will produce some of the F- ion,
in solution.
We, know that when you dissolve sodium fluoride
it 100% dissolves
it is a nice soluble salt. 100% break apart
and we end up with some F-. it is a nice soluble salt. 100% break apart
and we end up with some F-.
So this is a common ion situation.
So lets thing about this, this is a question for you to answer.
If you have this reaction happening
8:25
and you add some sodium fluoride,
in which there is some F-, and you add some sodium fluoride,
in which there is some F-,
the question is: What does that added
F-, that we see right here,
what does that added F- do
to this equilibrium?
Does it shift it to the right?
And when you shift this reaction here
to the right, we would say it increases its ionization. And when you shift this reaction here
to the right, we would say it increases its ionization.
Or does it shift it to the left? to the right, we would say it increases its ionization.
Or does it shift it to the left?
And if it shifts it to the left
we would say we are decreasing its ionization.
8:59
Well, if you said decreasing the ionization you are correct.
You are increasing the amount of this
Le Chatelier's principle says that when you add a product You are increasing the amount of this
Le Chatelier's principle says that when you add a product
the reaction will shift left
to try and use up that added product.
9:15
So this shift is known as a common ion effect.
The F- is the common ion So this shift is known as a common ion effect.
The F- is the common ion
shifting it to the left is a common ion effect.
And this is, in a buffer always what happens
when you add
the salt that contains the conjugate base,
for example. You will decrease
the ionization of that acid
and you will have in solution
a fair amount of both
the acid form, and the base form
of that buffer.
9:44
So this ends our first learning objective.
We have completed the idea of what a buffer is,
what it does, what its components are.
And then we are going to start going into
calculating the pH with our next learning objective. And then we are going to start going into
calculating the pH with our next learning objective.
Dr. Allison SoultLecturer
Dr. Kim WoodrumSr. Lecturer
