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# Conjugate acid-base pairs

AP.Chem:
TRA‑2 (EU)
,
TRA‑2.A (LO)
,
TRA‑2.A.1 (EK)
,
TRA‑2.B (LO)
,
TRA‑2.B.1 (EK)
,
TRA‑2.B.2 (EK)
,
TRA‑2.B.3 (EK)

## Video transcript

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 in water it will dissociate partially so some of the HF will dissociate and you'll get fluoride minus ions and then that dissociated H+ ion so this dissociated H+ ion will get donated to our water so water then becomes h3o plus or hydronium and so 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 so 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 so 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 so our HF loses a proton that forms our F minus or fluoride ion and then we can look at that same process happening in the backwards reaction so if we look at the backwards reaction which is also happening the fluoride ion can pick up or accept a proton from somewhere so we can pick up an H+ so I'll have a plus h plus here and so when fluoride accepts a proton we reform our HF so we can see that HF nf- have this special relationship where you can form one or the other by losing or gaining a proton and we can see a our relationship between water and hydronium so water here we said water is accepting a proton from each F so we see that water will gain a proton and that will give us hydronium 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+ 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+ in this case we have h F and F minus that are related to each other by that one H+ and so H F and F minus our conjugate acid-base pair we also have water and hydronium which are also related by that one H+ 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 on is the base comes from the bronsted-lowry definition of acids and bases so the bronsted-lowry definition says anything that can donate an H+ so anything that will give away an H+ is an asset 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+ and that's exactly what it does in the reverse reaction your f-minus will pick up an H+ and go back to your acid so we can also get water and h3o plus so here water is gaining a proton or accepting it so water is acting as a base and then 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 H a 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 - 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- so 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 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 HCl is a strong acid so that means it completely dissociates so it gives away all of its protons and when it does that we're left with the conjugate base alright so even though chloride isn't particularly basic it's still the conjugate base of HCl and last 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 so one example is what about the relationship between h3o plus and Oh H - if we think of our acid up here being h3o plus if we lose one proton we saw that it's conjugate bases water if water loses another proton we get Oh H - 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 and then 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 and so these two are also not a conjugate base pair because if we take our fluoride ion and it accepts a proton we don't get sodium fluoride they are related by a sodium ion so by definition that these two are not a conjugate acid-base pair so 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 it can lose to form the base
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