# Why do strong acids have weak conjugate bases?

This question is based on the Brønsted-Lowry model. I saw many answers to similar questions online. However none of them have given a detailed answer that is based on molecular structure (the answers all told the OP to rely on a reference table for a way to identify strong and weak acids).

• Think of it this way. A strong acid in the BL model is something that very willingly gives up a proton, while a strong base is one that very willingly accepts a proton. If it really wants to give up the proton, why would the resulting compound want it back? If you look at the resulting structures there is usually some relatively electronegative molecule/atom that is very stable with the negative charge, stable enough that a proton is much less likely to be added back. – Dannnno May 12 '14 at 1:38
• The situation described isn't always the case. Some strong acids do have strong conjugate acids – most venerable sir May 12 '14 at 1:45
• Are you thinking of sulfuric acid and hydrogen sulfate ion? The former is a strong acid by definition; the latter is rather strong as far as weak acids go, but is not considered a strong acid in water solution. – Dissenter May 12 '14 at 2:27
• @user139024 i'm not aware of a situation where that is true in the BL model. If you expand to the Lewis model then are up talking about something like magnesium hydroxide where the dissociated magnesium is a fairly strong Lewis acid? As an earlier post has stated acid salts such as HSO4- don't count because they're orders of magnitude weaker than the original acid and so in acidic environments most will not further dissociate – MY2K May 12 '14 at 6:51
• Nitpick: $HOSO_3^-$ isn't a salt, but something such as $KHOSO_3$ would be a salt (of sulfuric acid). – Dissenter May 12 '14 at 21:45

All chemical processes are reversible at least to some extent.

Keeping this in mind, a strong acid must have a weak conjugate base (I think you mean strong acids have weak conjugate bases).

Why? Let's try to prove by contradiction. If a strong acid had a strong conjugate base, then the base would quickly re-associate itself with a hydrogen proton. This reverses what the acid just did - dissociate itself from a hydrogen proton!

To illustrate this concept, consider hydrochloric acid - the oft-cited strong acid. $\ce{HCl}$ is a strong acid because in water solution it dissociates (nearly) completely into chloride ion and hydrogen protons (which are then solvated by water).

$\ce{HCl + H_2O ->H_3O^+ + Cl^-}$

That is the reaction of hydrochloric acid with water. Note the one-way arrow; the reaction is a one way street. $\ce{HCl}$ is the acid; $\ce{Cl^-}$ is the conjugate base; water is the base and hydronium ion is the conjugate acid. In other words, this is the generalized reaction:

$\ce{(strong)~acid + base -> conjugate ~acid + conjugate ~base}$

In bite-size pieces:

$\ce{(strong)~acid -> conjugate ~base}$

and

$\ce{base -> conjugate ~acid }$

Add the above two "half-reactions" together and you get a complete Brønsted acid/base reaction.

So back to the dissociation of $\ce{HCl}$.

If this reaction (the reverse reaction) happened to a significant extent:

$\ce{HCl + H_2O <- H_3O^+ + Cl^-}$

Then that would be antagonistic to the dissociation of $\ce{HCl}$! $\ce{HCl}$ would no longer be nearly completely deprotonated in water solution since the reverse reaction is significant!

Therefore, we conclude that if one has a strong acid, then its conjugate base must be weak. The converse is also true; if an acid's conjugate base is weak, then the acid must be strong.

• The first equation you wrote is a neutralization reaction, if seen as a reverse reaction. Right? – most venerable sir May 12 '14 at 22:13
• Then can strong bases have weak conjugate acid? In the same way that you have proved about strong acids. – most venerable sir May 12 '14 at 22:15
• The first equation I wrote is not a neutralization reaction no matter how it's looked at - whether forward or reverse. Going forward we make hydronium ion - a strong acid (if weaker than pure, liquid HCl). Going backward (to whatever small extent the reaction does go backward) we make HCl - a strong acid. – Dissenter May 12 '14 at 22:43
• Yes! Strong bases have weak conjugate acids! Take $HO^-$ as an example - hydroxide ion is most definitely a strong base. However, its conjugate acid is $H_2O$ - a very weak acid indeed! You got it! That's conjugate theory! – Dissenter May 12 '14 at 22:44
• Can we also say that a weak acid has a strong conjugate base ? – ado sar May 2 '19 at 12:15

Let us solve the mystery of relative 'strength' and 'weakness' of the acids and bases logically by understanding what makes it strong or weak. It can be said that a weak acid is the molecule which partially dissociate, thus it gives less $\ce{H+}$ ions on dissociation. This is because more energy is needed to break the bond and bring the $\ce{H+}$ ion out of the acid. This energy requirement is large because the molecule attract the hydrogen atom more efficiently. Conversely a strong acid doesn't require more energy more the removal of H+ ion, because it attracts it less efficiently.

Consider the following reaction: $$\ce{HCl + H2O -> H3O+ + Cl-}$$ Here HCl is a strong acid, thus Cl attracts the H atom less efficiently so that it can be easily dissociated. When Cl is a conjugate base in the product's side, the Cl has the same low efficiency of attracting H atom. Consequently it accepts the $\ce{H+}$ ions less efficiently. Thus Cl in the product's side can be called a weak base because according to Bronsted Lowry concept, a base is the one which can accept H+ions and here, the Cl atom is accepting $\ce{H+}$ ions less efficiently.

Conversely if there was a weak acid in the product side, suppose it was RH which is weak because is attracts H atom more, will have the same efficiency in the products side when it has the task of attracting and accepting $\ce{H+}$ ions. Then it will become a strong base because of its efficiency of attracting H atoms.

Strong acids readily give up their protons, because the resulting conjugate base becomes more stable with respect to its new electron distribution. An example of this would be the acid halides with the exception of HF. The conjugate bases of these acids achieve their octets and diffuse polarizability, rendering them electronically stable.