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Conjugate acid–base pairs

In the Brønsted–Lowry definition of acids and bases, a conjugate acid–base pair consists of two substances that differ only by the presence of a proton (H⁺). A conjugate acid is formed when a proton is added to a base, and a conjugate base is formed when a proton is removed from an acid. Created by Yuki Jung.

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  • mr pink red style avatar for user Karen
    Hi, I have a question. So in the video there is H3O+ -- How does one know if in the products there is going to be a hydronium ion or like water? Thanks!
    (10 votes)
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  • blobby green style avatar for user ngfoongyee92
    NaOH, which is a strong base that dissociates in water, what is its conjugate acid?
    (5 votes)
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  • male robot donald style avatar for user Bayern Munich 0309
    Will the same concept apply to dibasic acids like H2SO4 ?
    (4 votes)
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    • mr pants purple style avatar for user Ryan W
      Yes, the conjugate base of the first reaction can also react with another water molecule, eg:

      H2SO4 + H2O -> HSO4- + H3O+
      HSO4- + H2O -> SO4 2- + H3O+

      H2SO4 and HSO4- are conjugate acid-base pairs, and HSO4- and SO4 2- are also conjugate acid-base pairs
      (7 votes)
  • piceratops ultimate style avatar for user Darmon
    Is there another conjugate pair definition for diprotic or triprotic acids? The one given in the video would not work, as those acids differ from their conjugate bases by 2 and 3 protons, respectively.
    (2 votes)
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  • duskpin ultimate style avatar for user Ansh Mehta
    What is the conjugate acid and base for H2CO3?
    (3 votes)
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  • blobby green style avatar for user gulalai khan
    does the dossociation depend on elactronegativity i mean a highly electronegative element will hardly give away H+ and will be a weak acid?
    (2 votes)
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    • leaf green style avatar for user Sammy Huang
      Yes, that would be true. HF is a weak acid due to strong Coulombic attractions since the small radii of H+ and F- ions augment the difference in electronegativity and prevent the dissociation of protons (although its weak acidic nature is also due to the presence of hydrogen bonding). But for other hydrohalic acids (binary acids with an anion from Group 17), the difference in electronegativity does not cause weak acidity because the anions are larger and the Coulombic attractions are smaller, so protons are more easily dissociated. Thus, HCl, HBr, and HI are strong acids despite their electronegativity difference.

      On the another hand, the strength of oxoacids also depends on electronegativity. H-O-Y acids (Y is usually a Group 17 atom) exhibit greater dissociation if the Y atom is more electronegative. This is because the electronegative Y will pull electrons towards itself and reduce the electron density around the H-O bond, thereby weakening the bond and allowing H+ to dissociate. For H-Y-O(x) oxoacids (where there can be more than one oxygen atom), the dissociation increases as the number of oxygen atoms increases for the same reason: more electronegative oxygen atoms attracts more electrons, electron density around H-O bond reduces, bond weakens, H+ dissociate.

      Overall, dissociation does depend on electronegativity if it's an oxoacid, but in this case, highly electronegative elements facilitate proton dissociation.
      (4 votes)
  • blobby green style avatar for user astunix
    In this video, Dr. Jung references strong and weak acids. What are those, and how are they defined?
    (2 votes)
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    • leaf red style avatar for user Richard
      The strength of an acid describes how much of the acid actually reacts in an acid-base reaction. Another term to describe this is saying how well does an acid dissociate, or break apart. Strong acids dissociate almost completely, and weak acids are any acids who dissociate less than that.

      Usually there's a small list of strong acids which chemistry students memorize. All other acids are assumed to be weak acids of varying degrees. The usual strong acids are: hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid, perchloric acid, and sulfuric acid.

      Hope that helps.
      (3 votes)
  • piceratops ultimate style avatar for user wanglx123456789
    she defines at the end of the video that "Conjugate acid-base pairs forms when you have two species with the same formula except one has an extra proton," Isn't the only difference between NaF and F is the extra proton Na+? So why they are not Conjugate acid-base pairs?
    (1 vote)
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    • leaf red style avatar for user Richard
      When she refers to an extra proton she means H+, or basically a hydrogen atom's nucleus. So sodium fluoride and the fluoride ion are not a conjugate acid/base pair because we can't create either species by adding or removing an H+.

      Hope that clears things up.
      (3 votes)
  • blobby green style avatar for user Yash Patel
    Does atomic number have anything to do with conjugate acid base pairs? I have to create a molecular representation with styrofoam balls and straws of a conjugate acid-conjugate base pair based on an element with an atomic number of 117 (Tennessine)? Everywhere I looked, there was nothing about atomic numbers relating to conjugate acid base pairs. Your help means a lot. Thanks!
    (1 vote)
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    • orange juice squid orange style avatar for user Asher W
      Tennessine is a halogen, so it should form the following acid (probably called hydrotennessic acid): HTs. The conjugate base of HTs is Ts⁻.
      A small Styrofoam ball can represent hydrogen, a large Styrofoam ball can represent tennessine, and a straw can represent the covalent bond between them.
      Hope that helps! (although it's probably too late now)
      (3 votes)
  • hopper cool style avatar for user Iron Programming
    From one source I read that neutralization reactions (acid-base reactions) can only occur in water. But, what does this mean? According to another source, the following chemical reaction qualifies as a neutralization reaction:
    HCl + NH_3 --> NH_4(+) + Cl(-)

    Was it assumed that the chemical reaction was happening in the water, and we just canceled the H2O atoms from both sides? If so, why do we water molecules in order for this to occur? Won't the strong acid (HCl) simply pronate the weak base (NH_3) to form NH_4(+) with leftover chloride anions?

    In short, is the statement "neutralization reactions only occur in water" accurate? I saw a general formula for them as "acid + base --> water + salt", but from what I've seen elsewhere neutralization reactions are broader.

    Thanks for your time.
    (1 vote)
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    • leaf red style avatar for user Richard
      So while it may be more common for acid-base reactions to occur in aqueous solutions (water based), these reactions can occur with different solvents. For example ammonium, which we usually consider a base when in an aqueous solution, can act as a solvent and do similar acid-base reactions that water does. In an aqueous solution, water will self-ionize meaning that two water molecules engage in an acid-base reaction and create a hydronium and hydroxide ion. An ammonia solution can do the same self-ionization yielding an ammonium and amide ion. Many solvents can do this because the acid-base mechanism is quite broad depending on the definition. Using a Brønsted–Lowry definition all we need is a transfer of protons, while a Lewis definition simply requires lone pair acceptors and donors.

      Neutralization reactions are a subset of acid-base reactions in which an acid and base are equimolar (the same amount of both) react completely to form a salt and sometimes water. The result is a neutral, or nearly neutral, solution. Your neutralization reaction would produce only a salt, ammonium chloride, and the ammonium would further react with water in an acid-base reaction to make the water slightly acidic. So like before most common neutralization reactions occur in water, but we can have acid-base reactions occur in nonaqeuous solutions too.

      Hope that helps.
      (2 votes)

Video transcript

- [Voiceover] 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. So some of the HF will dissociate, and you'll get fluoride minus ions. And then that dissociated H plus ion. So this dissociated H plus ion will get donated to our water. So water then becomes H3O plus, or hydronium. And so this process is in dynamic equilibrium, cause 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 gonna 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 minus. 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 it can pick up an H plus, 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 and F minus have this special relationship where you can form one or the other by losing or gaining a proton. And we can see a similar relationship between water and hydronium. So, water here, we said water is accepting a proton from HF, 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 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. 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. 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, chloride. So even though chloride isn't particularly basic, it's still the conjugate base of HCl. And last but not least, we're gonna 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 minus? If we think of our acid up here being H3O plus, if we lose one proton, we saw that its conjugate base is water. If water loses another proton, we get OH minus. 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, 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.