Acid Strength and Molecular Structure

3 List the molecular and ionic species in order of increasing concentration in a solution of ascorbic acid (HjCgHgOe). 

Two factors influence the extent to which the acid undergoes ionization. One is the strength of the H—X bond. The stronger the bond, the more difficult it is for the HX molecule to break up and hence the weaker the acid. The other factor is the polarity of the H—X bond. The difference in the electronegativities between H and X results in a polar bond like 

If the bond is highly polarized (i.e., if there is a large accumulation of positive and negative eharges on the H and X atoms, respectively), HX will tend to break up into and X ions. A high degree 

The strength of an acid depends on how easily the proton, H, is lost or removed from an H—X bond in the acid species. By understanding the factors that determine the ease of proton loss, you will be able to predict the relative strengths of similar acids. 

Electron density plots of acetic acid and sulfuric acid molecules 

High electron densities are shown in red and low densities are shown in blue, with intermediate colors of the spectrum for intermediate densities. Note the hydrogen atoms at the right ends of each molecule (blue colors, indicating low electron densities).These atoms have partial positive charges and thus are acidic. (Electron density figure taken from G.D.Shusterman,A.J. Shusterman,J.Chem.Ed., 74, 771,1997.) 

Two factors are important in determining relative acid strengths. One is the polarity of the bond to which the H atom is attached. The H atom should have a positive partial charge  

The second factor determining acid strength is the strength of the bond—that is, how tightly the proton is held. This, in turn, depends on the size of atom X. The larger atom X, the weaker is Ihe bond and Ihe greate the acid strength. 

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Click Coached Problems for a self-study module on molecular structure and acid strength. 

TABLE 10.6 Correlations of Molecular Structure and Acid Strength... 

Molecular Structure and Acid Strength The strength of a series of structurally similar acids can be compared using parameters such as bond enthalpy, bond polarity, and oxidation number. 

Interactivity Calculating pH of Base Solution (16.6) Interactivity Molecular Structure and Acid Strength (16.8) Interactivity Acid-Base Properties of Salts (16.9)... 

Before we leave the subject of molecular structure and acid strength, let us look at the relative acid strengths of a polyprotic acid and its corresponding acid anions. 

This discussion of the relationship between molecular structure and acid strength concludes with a brief consideration of some organic compounds. Consider first the case of acetic acid and ethanol. Both have an O—H group bonded to a carbon atom, but acetic acid is a much stronger acid than ethanol. 

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