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. 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. Our first learning objective is to learn about buffers. We need to identify what a buffer is and the necessary component that make up a buffer. 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. If we were to think about its conjugate acid that would be NH4+. 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 [and I should not say for example, it has to be negatively charged] chloride for example there would be a salt so we have a weak base. And its conjugate acid salt, that would be a buffer. 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. 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. 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. 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. Well if you said, number 3 then you are correct. There are not conjugates of each other Well if you said, number 3 then you are correct. There are not conjugates of each other while CH_3NH_2 methylamine is a weak base. And while this is a acid, conjugate acid of a weak base. It is not the conjugate acid of this weak base. Therefore, this cannot be a buffer. Now, this is a buffer and this one throws students very often we have sodium hydroxide, well what is sodium hydroxide? It is a strong base. and we have HF it is a weak acid. and we have HF it is a weak acid. 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 will react with the acid. A base reacts with an acid will react with the acid. A base reacts with an acid to form F- and water. So if we look at the amount we have 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. 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. 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. 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. 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 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. 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. 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. 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.
