Strength of Acids and Bases

How to Determine the Stronger Acid by Looking at Conjugate Bases

We just learned that the stronger an acid, the (stronger/weaker) ________ its conjugate base.

For a strong acid, the conjugate base is so weak that it has no basic properties!

Weak conjugate bases (want to/don't want to) ________ react.

Therefore, it makes sense that weak conjugate bases are (more/less) ________ stable.

Example #1: If we consider HF vs HI, which of these is the stronger acid?

We can compare their conjugate bases: ________

Now which of the conjugate bases is more stable?

Both conjugate bases have a negative charge, so do you think the more stable conjugate base would be the one with the negative charge spread over a smaller or larger distance? ________

Therefore, the more stable conjugate base is ________ (________ Atomic Radius) and the stronger acid is ________!

Example #2: Order the following acids in order of increasing acidity:

HI, PhOH, and H2SO4

So first, let's consider the conjugate bases: ________

Again all of the conjugate bases have a negative charge, so the most stable conjugate base will be the one that stabilizes that charge the most

• If we want our negative charge to be stabilized, do we want the charge to be spread over a large distance, or concentrated over a small distance? ________
• If we want our negative charge to be stabilized, would we want a more EN atom to have the charge or a less EN atom to have the charge? ________

Now let's draw out the conjugate bases to see what their structures look like:

I-, PhO-, and HSO4-

Do any of the structures have resonance? ________

If they do, which ones have resonance? (draw out resonance if they have it!)

Would resonance make the conjugate more or less stable? ________

Therefore, our least stable conjugate base is: ________. This means that ________ is the ________ acid.

Now, out of the conjugate bases with resonance, what element has the charge initially? ________

Do you think this is more or less stable than having the negative charge on C? ________

If you consider the resonance structures, does the charge always stay on O? Yes or no and for which conjugate base?

Therefore, in order of increasing acidity (in order of least stable conjugate base to most stable conjugate base):

Do you think if we had more resonance that would make the conjugate base more or less stable? ________

Summary

1) Resonance is the most important factor in determining conjugate base stability

More resonance present=more stable conjugate base

2) Having the negative charge on the most electronegative element is next most important.

Also pay attention to which atoms carry the negative charge in resonance structures.

The more stable conjugate base will have the negative charge on more electronegative elements.

3) Having other electronegative atoms present also help to stabilize the charge in the conjugate base (inductive effect)

4) When comparing atoms with negative charges, the larger the atomic radius, the more stable the conjugate base.

Acidity of Oxyacids

Oxyacid: an acid made up of a non-metal element and an acidic -OH group

Examples:

The strength of an oxyacid depends on two things:

1) The Number of O atoms

Think about the conjugate base of the acid.

• If we deprotonate the acid and there are many Os, those Os are quite electronegative and can help spread the negative charge so it is not concentrated in one area, thus stabilizing the conjugate base.
• An oxyacid with more Os is a stronger acid than an oxoacid with less Os

Example: Which one is more acidic? Carboxylic acid or sulphuric acid?

2) The EN of the central non-metal atom

If we are trying to determine the stronger oxyacid, consider their conjugate bases.

• The conjugate base that has a central non-metal atom that is more electronegative will be able to spread the charge on the conjugate base, stabilizing it.
• Therefore, the oxoacid with the more electronegative central atom will be the stronger acid (if we are dealing with 2 oxoacids with the same # of Os)

Example: Which is the stronger acid? Sulphuric acid or phosphoric acid?

Summary

In general, for oxyacids with the same # of O atoms, we have increasing acid strength with (increasing/decreasing) ________ EN of central non-metal atom

And for oxyacids with a different # of O atoms, the stronger oxyacid will be the one with (more/less) ________ O atoms