Last time I started writing about acid-base reactions. We looked at this list of stabilities of anions going across the topmost row of the periodic table.
Because of this, we were able to say that H-F was the most acidic, because it had the most stable conjugate base.
And H-CH3 (methane)was the least acidic, because it had the least stable conjugate base.
Let’s look at the flip side of this reaction. Instead of starting with HF, H2O, H3N, and CH4 and asking how likely they are to donate a proton to a common base (water in our example) , imagine we start with the anions [ F-, HO-, H2N- and H3C- ] and have them take a proton away from a common acid (such as water).
Which reactions would be most favorable? Which would be least favorable?
The same principle applies. The less stable the anion, the more likely the reaction will be to proceed to completion.
So in this case, the reaction of F- with H2O would be the least favored, because F- is the most stable. And the reaction of H3C- with H2O would be the most favored, becuse H3C- is the least stable.
[A clarification: these are equilibrium reactions. So what I mean by favored here is the extent to which the equilibrium would favor the products on the right]
Notice the role that each of these anions plays in these reactions: it is accepting a proton from water, so in other words it is acting as a base.
Therefore, our whole discussion of the “stability” of anions, for lack of a better term, goes by another name you’re familiar with: basicity.
In other words:
- the more stable a lone pair of electrons is, the less basic it will be.
- the less stable a lone pair of electrons is, the more basic it will be.
Let’s tie these two posts together with a common thread:
- For any group of acids, H-X (where X can literally be anything), the strongest acid will have the most stable conjugate base. Since stability is inversely correlated with basicity, another way of putting it is:
- The stronger the acid, the weaker the conjugate base.
- Today’s post is about how the opposite is also true: The weaker the acid, the stronger the conjugate base.
Next time, we’ll apply this framework to other stability trends we’ve discussed previously.
P.S. One last note: a common misconception students have is that “weak acids are strong bases”. Not true! Methane (CH4) is a weak acid, but it can’t act as a base – it doesn’t have a lone pair.
The proper way to say it is that “weak acids have strong conjugate bases”. So the conjugate base of CH4, CH3(-) is an extremely strong base.