Relating Acid Strength to Structure: Chapter 16.8

Strong acids ionize to a greater extent,

releasing H+ more readily into solution.

Not all H atoms in a compound are ionizable. Ionizable hydrogen atoms are also referred to as

labile hydrogen atoms.

Whether an H atom is ionizable and how easily ionizable it is are both related to the

structure of the molecule.

Generally speaking, bond strength and acid strength are

inversely related.

The stronger the chemical bond holding a hydrogen atom to the molecule, the less likely the

bond will break and release H+ ions.

The relative strength of an acid can often be determined based on

the structure of the molecule, even without knowing bond energies.

There are two main factors affecting bond strength in a binary acid,

• the size of the anion and

• the electronegativity of the environment around the H atom.

Smaller anions have valence orbitals closer in size to the 1s orbital of H and have better overlap. Compare HF and HI:

• In HF, F forms bond to H with a 2p orbital and has good overlap between the orbitals.

• In HI, I forms bond to H with a 5p orbital, which is much larger and has poorer overlap with a 1s orbital.

Ka values can be determined for very strong acids using

specialized instrumental techniques, allowing for comparison.

Acid molecules lose

ionizable hydrogens as H+ ions.

If the bond to the hydrogen atom is highly polar, the hydrogen atom is already

δ+, making it easier to fully ionize.

The more polar the bond between hydrogen and its bonding partner, the more

easily ionizable the hydrogen atom will be.

There are four H atoms in

Acetic Acid

Three of the H’s are bonded to C, which is close to H in electronegativity, making these bonds

nonpolar and nonionizable.

However, the H bonded to the highly electronegative O forms a very polar bond and is

Ionizable

Replacing the nonionizable Hs with electronegative Cl atoms makes the ionizable H

even more ionizable.

A similar trend is seen with acids that contain varying numbers of oxygen atoms, such as the

Oxyacids

More highly electronegative oxygen atoms in the formula generally indicate a

stronger acid

Acid strength steadily increases going from

hypochlorous acid,HClO (weak), to perchloric acid, HClO4(strong).

Similarly, HNO2 is a

weak acid while HNO3 is a strong acid.

Consider the period 2 binary hydrogen compounds,

NH3, H2O, and HF.