Conjugate acid–base pairs | Chemical reactions | AP Chemistry | Khan Academy

In this video, we're going to be talking about conjugate acid-base pairs. We're going to introduce the idea of a conjugate acid-base pair using an example reaction. The example reaction is between hydrogen fluoride, or HF, and water.

So, hydrogen fluoride is a weak acid, and when you put it in water, it will dissociate partially. Some of the HF will dissociate, and you'll get fluoride ions and then that dissociated H+ ion. So, this dissociated H+ ion will get donated to our water. Water then becomes H3O+, or hydronium. This process is in dynamic equilibrium because it can go forward and it can go backward, and eventually, those two rates are equal, and they're both happening at the same time.

In this reaction, we have a couple things going on, and we're going to think about it in terms of hydrogen ions being exchanged. If we just look at the hydrofluoric acid and we look in the forward direction, our HF is becoming F- and it's doing that by donating or losing. So, I'll put a minus for losing a proton. Our HF loses a proton which forms our F- or fluoride ion.

Then we can look at that same process happening in the backward reaction. If we look at the backward reaction, which is also happening, the fluoride ion can pick up or accept a proton from somewhere. So, it can pick up an H+. I'll have a plus H+ here. When fluoride accepts a proton, we reform our HF.

So, we can see that HF and F- have this special relationship where you can form one or the other by losing or gaining a proton. We can see a similar relationship between water and hydronium. Here, we said water is accepting a proton from HF. So, we see that water will gain a proton, and that will give us H3O+. In the reverse reaction, hydronium can lose a proton to reform water, so minus H+.

Again, we have these two species, water and hydronium, that are related to each other by having or not having one H+. In chemistry, we call these species that are related in this way conjugate acid-base pairs. The official definition, or my official definition, of a conjugate acid-base pair is when you have two species that are related to each other.

Let's see, two species that are related to each other by one H+. In this case, we have HF and F- that are related to each other by that one H+. So, HF and F- are conjugate acid-base pairs. We also have water and hydronium, which are also related by that one H+. So, water and H3O+ are also a conjugate acid-base pair.

You can probably tell from the name, but whenever you have a conjugate acid-base pair, one thing in the pair will be an acid and the other thing will always be a base. The definition of which one is the acid and which one is the base comes from the Brønsted-Lowry definition of acids and bases.

The Brønsted-Lowry definition says anything that can donate an H+ is an acid. So, we can see that in this case, our hydrofluoric acid is acting as the acid in the conjugate acid-base pair, and that means that fluoride has to be acting as a base. That makes sense because the Brønsted definition of a base is something that will accept an H+, and that's exactly what it does in the reverse reaction.

Your F- will pick up an H+ and go back to your acid. We can also look at water and H3O+. Here, water is gaining a proton or accepting it, so water is acting as a base. In the reverse reaction, H3O+ is donating a proton, so H3O+ is acting as an acid.

The relationship between conjugate acid-base pairs can be written a little bit more generally. If we represent any generic acid as HA, so this is our acid, we said that an acid is something that donates a proton. It will lose the proton, and when it does that, it will form the conjugate base, which is represented by A-.

In the reverse reaction, our base A- can gain a proton and remake our acid or conjugate acid. So, whenever you have two species that have basically the same formula, which we abbreviated here as A-, except for one has an H+ and one doesn't, then you know you have a conjugate acid-base pair.

So, let's look at some more examples of conjugate acid-base pairs. We saw above HF, or hydrofluoric acid; its conjugate base is F-. Here, HF is our acid, and when it loses that proton, we are left with F-. We saw in the same reaction that water can act as a base.

So, if water is our A-, if that water accepts a proton, it forms the conjugate acid H3O+. The example we've gone through so far, HF, is for a weak acid, but we can also talk about the conjugate base of a strong acid like hydrochloric acid. HCl is a strong acid, so that means it completely dissociates.

It gives away all of its protons, and when it does that, we're left with the conjugate base chloride. So even though chloride isn't particularly basic, it's still the conjugate base of HCl. Lastly, but not least, we're going to go through two examples where it looks like we might have a conjugate acid-base pair, but we actually don't.

One example is what about the relationship between H3O+ and OH-. If we think of our acid up here being H3O+, if we lose one proton, we saw that its conjugate base is water. If water loses another proton, we get OH-. So the difference between these two species here is two protons instead of one proton.

So, these two, hydronium and hydroxide, are not a conjugate acid-base pair because they differ by two protons instead of one. The last example we'll look at is we said that fluoride is a conjugate base of HF. So what about the relationship between sodium fluoride and fluoride?

These two are also not a conjugate acid-base pair because if we take our fluoride ion and it accepts a proton, we don't get sodium chloride. They are related by a sodium ion, so by definition, these two are not a conjugate acid-base pair.

In this video, we learned that a conjugate acid-base pair is when you have two species, and they have the same formula, except one has an extra proton. So the acid has an extra proton which you can lose to form the base.