A conjugate acid is the protonated version of a molecule. In other words, it has an extra hydrogen.
In this post, we're going to cover what a conjugate acid is and then apply that to a very common kind of problem that you might find on your exam.
When talking about conjugate acids and conjugate bases, we’re usually referring to Bronsted-Lowry acids and bases. Those are proton donors and acceptors, respectively.
So, that means that if we want to find a conjugate acid, all we need to do is add a proton! Let’s try that out first with (hydrogen sulfate ion) and then move on to the other two.
Check it out! I’ve drawn this molecule in the three main ways you’ll see it drawn in Orgo. If you need a review of drawing Lewis structures, check out these videos. Notice that on the left is the molecule provided to us and on the right is its conjugate acid: sulfuric acid. All I did is add a hydrogen. Notice that we actually no longer have a negative charge; that’s a consequence of one of the anionic oxygen’s lone pairs being used to form a bond to the hydrogen. Let’s take a quick look at the formal charge of the oxygen on the right.
This equation, Group # – (sticks + dots) = Formal Charge, is very helpful! Sticks = bonds, and dots = lone-pair electrons. Notice how adding the hydrogen to the molecule sets the formal charge on the oxygen to zero. This leads us to a major lesson: If you protonate a molecule, it’ll become more positive; if you deprotonate a molecule, it’ll become less positive (or more negative).
Basically, all you have to do is add a hydrogen!
So now let’s take a look at CO32– (carbonate). Since all we’re doing is adding a hydrogen, we could just write it out as CO3H–. Let’s draw that out in Lewis structure.
Just like the hydrogen sulfate ion above, sulfate becomes less negative when a hydrogen is added to it. This isn’t so bad, right? Let’s go ahead and tackle the last one: NH3 (ammonia). Even though it’s neutral, it can absolutely have another bond to hydrogen. How? It’s got a lone pair that it can use to deprotonate another molecule. Let’s take a look at those Lewis structures.
So, what about other molecules? Let’s take on four others:
S2– Sulfide Lewis structure
So now you know how to give the conjugate acid for each compound! I have awesome videos explaining acid-base reaction mechanisms and how to find the direction of equilibrium that I recommend you check out. Good luck!