How to qualitatively determine the relative
strength of two acids

When one asks “Which is the stronger acid?”Asking this question is the same as asking which
acid has the most stable conjugate base. So one needs to consider the relative stability of the
conjugate base. The first step is therefore write out the equation for the disassociation of the acid.
Then consider the stability of the conjugate base.

HA+H2O
acid

A-+H3O+
conjugate
base

Note that all of the following rules depend on one general principle: spreading out charge stabilizes
a molecule.

The stability of an anion depends on:

1. Size of the atom with charge.The larger the atom with the charge the more stable. Note that
size changes substantially as you move down a group in the periodic table. Therefore apply this
rule only when comparing conjugate bases which have charge on atoms that are in differentrows.

2. Electronegativity of the atom with a charge.More electronegative atoms stabilize charge
better.

3. Proximity of atom with charge to other electronegative atoms. The closer the atom
carrying the charge is to an electronegative atom the more stable the anion.

4. The number of electronegative atoms located near the atom with charge.The more
electronegative atoms there are in a molecule the more stable.

5. The relative electronegativity of atoms located near the atom with charge.The more
electronegative an atom near the charge the more stable.

Examples:

Which is the stronger acid, HCl or HI?
To answer this first write out the dissociation reaction of each acid in water and label the conjugate
base.

HCl+ H2O

Cl-+H3O+
conj base

Theconjugate bases are Cl-and I-

HI+ H2O

I-+H3O+

conj base

these are the atoms carrying the charge. They are in different
rows so rule number 1 applies. The iodide ion is large therefore it can better handle the negative
charge and it is the more stable conjugate base, this means that HI is a stronger acid than HCl.

Which is the stronger acid?

F3C

C

O

OH

Cl3C

C

O

OH

or