1. Predict the relative strengths of binary acids.
2. Predict the relative strengths of oxyacids.

Binary Acids. The acid strengths of nonmetal hydrides are related to two features of these molecules. These are the polarity of the X—H bond (X stands for a nonmetal atom), and the strength of the H—X bond. The polarity of a X—H bond increases with the electronegativity of the nonmetal atom. The greater the electronegativity of the nonmetal atom, the more it withdraws electrons away from the hydrogen atom. This facilitates the transfer of H+ as a proton. Thus acid strength increases as the electronegativity of the nonmetal atom increases.

The strength of the X—H bond decreases as the atomic radius of X increases. A hydrogen ion is more easily broken away from a larger atom than a smaller one. This happens because the electron cloud in a large atom is more diffuse. The smaller the nonmetal atom, the more dense the electron cloud. This greater electron density results in a greater attraction for the proton. Therefore, acid strength increases as the atomic radius of the nonmetal atom increases.

When comparing acid strengths of hydrides in a group of the periodic table versus those within a period, one must consider both the electronegativity of the nonmetal atom and the bond energy. Within a series of hydrides of elements in a group the acid strength increases with increasing atomic radius of the nonmetal. For example, note the following hydrides of the Group 6A elements.

H₂S < H₂Se < H₂Te
increasing acid strength
increasing nonmetal radius
increasing electronegativity of X

In this series, the electronegativity of X varies only slightly, and so is outweighed by the more significant increases in atomic radius, as shown below.

	H2S	H2Se	H2Te
Radius of X (pm)	104	117	137
Electronegativity of X	2.5	2.4	2.1

When reading across a row of the periodic table the situation is reversed. The importance of bond polarity in determining acid strength outweighs that of atomic radius. For instance, the hydrides of the elements phosphorus, sulfur, and chlorine show increasing acid strength as shown below. Incxreasing acid strength parallels the increasing electronegativity on the nonmetal atom.

PH₃ < H₂S < HCl
increasing acid strength
increasing electronegativity of X
increasing nonmetal radius

	PH3	H2S	HCl
Radius of X (pm)	110	104	99
Electronegativity of X	2.1	2.5	3.0

The smaller decrease in atomic radius is insignificant in comparison to the larger change in electronegativity. The trend in acid strength results from significant increases in bond polarity when going across the row.

Which member of each of the following pairs is the stronger acid?

1. HCl
   HBr

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2. HCl
   H₂S

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1. These acids are nonmetal hydrides of elements within Group 6A of the periodic table. Variations in atomic radius are more important within a group than electronegativity variations. The stronger acid is the one with the greater radius of its nonmetal atom. HBr is the stronger acid.

2. These acids are nonmetal hydrides of elements within the third row of the period table. The electronegativity changes more significantly going across a row than does the atomic radius. HCl with its more polar bond is the stronger acid.

Oxoacids. For oxoacids with the same structure, but with different central atoms whose elements are in the same group, acid strength increases with increasing electronegativity of the central atom. Thus, acid strength increases in the following series where the central atom is a halogen element:

In each molecule, the O—H bond strength is approximately the same. In this series the ability of the halogen atom to withdraw electron density from the O—H bond increases with increasing electronegativity. As the O—H bond in a series of acids becomes more polar, the acid strength increases.

For oxoacids that have the same central atom, but differing numbers of attached oxygen atoms, the acid strength increases with increasing oxidation number of the central atom. Therefore, HNO₃ is stronger than HNO₂, for example.

The oxygen atoms draw electrons away from the nitrogen atom, making it more positive. The more positive the N atom, the more effective it is in withdrawing electrons from the O—H bond. This increases the polarity of the O—H bond.

Which member of each of the following pairs is the stronger acid?

1. HClO₃
   HBrO₃

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2. H₃PO₃
   H₃PO₄

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1. These two oxoacids have the same structure:

       

   Within this group the strengths of the acids increase with increasing electronegativity of the central atom. The electronegativity of Cl is greater than that of Br. HClO₃ is stronger.

2. H₃PO₃ and H₃PO₄ are oxoacids of the same element. The acid strength increases as the number of O atoms increases. H₃PO₄ is the stronger acid because of the greater number of oxygen atoms bonded to the phosphorus atom. In H₃PO₄, phosphorus has a higher oxidation number than in H₃PO₃.

26. Which member of each pair is the stronger acid?

    1. HNO₂
       HNO₃

    2. CH₄
       SiH₄

    3. HOBr
       HOI

    4. CH₄
       NH₃