Brønsted-Lowry acid base theory (article) | Khan Academy
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In the reverse reaction, hydronium can lose a proton to reform water. So, minus H plus. So again we have these two species, water and hydronium, that are related to each other by having, or not having, one H plus. So in chemistry, we call these species that are related in this way conjugate acid-base pairs. So 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, related by one H plus. In this case, we have HF and F minus that are related to each other by that one H plus. And so HF and F minus are a conjugate acid-base pair. We also have water and hydronium, which are also related by that one H plus. So water and H3O plus 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 Bronsted-Lowry definition of acids and bases.
Conjugate acid-base pairs
So the Brondsted-Lowry definition says anything that can donate an H plus, so anything that will give away an H plus 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 the base. And that makes sense, because the Bronsted-Lowry definition of a base is something that will accept an H plus. And that's exactly what it does in the reverse reaction.
Your F minus will pick up an H plus and go back to your acid. So we can also look at water and H3O plus. So here, water is gaining a proton, or accepting it, so water is acting as a base. And in the reverse reaction, H3O plus is donating a proton, so H3O plus is acting as an acid. The relationship between conjugate acid-base pairs we can write a little bit more generally. So, if we represent any generic acid as HA. So this is our acid. We said that a acid is something that donates a proton.
So it'll lose the proton, and when it does that, it will form the conjugate base, which is represented by A minus.
In the reverse reaction, our base, A minus, 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 minus, except for one has an H plus 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, it's conjugate base is F minus. So here HF is our acid, and when it loses that proton, we are left with F minus. We saw in the same reaction that water can act as a base.
So if water is our A minus, if that water accepts a proton, it forms the conjugate acid H3O plus. So 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. Water has lone-pair electrons and is an anion, thus it is a Lewis Base. Aluminum ion acts as a Lewis acid and accepts the electrons from water, which is acting as a Lewis base.
This helps explain the resulting hexaaquaaluminum III ion. The Lewis Acid accepts the electrons from the Lewis Base which donates the electrons.
Thus, Lewis Acid and Base Theory allows us to explain the formation of other species and complex ions which do not ordinarily contain hydronium or hydroxide ions. One is able to expand the definition of an acid and a base via the Lewis Acid and Base Theory. Therefore, by defining a species that donates an electron pair and a species that accepts an electron pair, the definition of a acid and base is expanded.
Amphoterism As of now you should know that acids and bases are distinguished as two separate things however some substances can be both an acid and a base.
Acids and Bases | Chemistry | Visionlearning
You may have noticed this with water, which can act as both an acid or a base. This ability of water to do this makes it an amphoteric molecule. Water can act as an acid by donating its proton to the base and thus becoming its conjugate acid, OH. Water acting as an Acid: Water does not act as an acid in an acid medium and does not act as a base in a basic medium. Thus, the medium which a molecule is placed in has an effect on the properties of that molecule.
Other molecules can also act as either an acid or a base.