Acids and Bases: Molecular Structure and Behavior

Molecular Structure of Brønsted Acids

What is it about the molecular structure of acids that allow them to donate a proton? There are two important features:

All acids must have a proton that can be donated.
This means compounds that do not contain hydrogen (such as N₂O) can not act as acids.

In most cases, the proton is bonded to a highly electronegative atom (such as oxygen or a halogen).
In a bond between an electronegative atom and hydrogen, the electrons will be more attracted to the electronegative atom, making it easy for the proton to be donated to a base. The figures below compare the electron densities of a carbon hydrogen bond (where both atoms have approximately the same electronegativity) and a chlorine hydrogen bond (where chlorine is much more electronegative).

What is electronegativity?

An electronegative atom anywhere in the molecule will pull electrons towards itself; an electronegative atom not directly bonded to the hydrogen will still affect the acidity.

Which of the following compounds will be a stronger acid?

What is meant by "stronger acid"?

Good! Chlorine is more electronegative than bromine. Even though the electronegative atom is not directly bonded to the hydrogen, its identity still affects the acidity of the hydrogen. The acidity of the hydrogen in HOCl is ten times greater than the hydrogen in HOBr.

Hypochlorous (HOCl) and hypobromous (HOBr) acids are examples of oxoacids , acids in which the hydrogen is bonded directly to an oxygen atom. Other examples include phosphoric (H₃PO₄) and sulfuric (H₂SO₄) acids.

Is chlorine or bromine more electronegative? What effect will this have on the acidity of the hydrogen?

Another class of acids are hydrated metal ions. Metal ions become hydrated in aqueous solutions, often by having six water molecules around it, [M(H₂O)₆]ⁿ⁺. These hydrated cations are acidic because the positive central ion attracts the electrons of the water molecule. This increases the polarity of the O-H bond in water, allowing the proton to be acidic.

The ability of an ion to polarize the O-H bond increases with the intensity of the charge on the ion. Also, since the positive charge is centered in the nucleus of the ion, the smaller the ion, the closer the water molecules are to the positive charge. A water molecule that is very close to a positive charge will be more polarized than a water molecule that is further away from the charge. Hydrated cations that form from small, highly charged positive ions tend to be more acidic than those from larger, less highly charged ions.

Which of the following hydrated metal ions will be the strongest acid?

Ni(H2O)62+	Cr(H2O)63+	Fe(H2O)62+