M15Q3: Molecular Structure and Acid Strength

Learning Objectives

  • Describe the relationships between acid strength and molecular structure, and use them to predict the strength of an acid based on its molecular structure.
  • Identify carboxylic acids and amines and their conjugate bases/acids.

| Key Concepts and SummaryGlossary | End of Section Exercises |

Effect of Molecular Structure on Acid-Base Strength

For most binary compounds of hydrogen with nonmetals, acidity increases as the H-A bond strength decreases (moving down in a group in the periodic table). For group 17 (or 7A depending on your periodic table), the order of increasing acidity is HF < HCl < HBr < HI. Likewise, for group 16 (or 6A), the order of increasing acid strength is H2O < H2S < H2Se < H2Te.

Across a row in the periodic table, the acidity of binary hydrogen compounds increases with increasing electronegativity of the nonmetal atom because the polarity of the H-A bond increases. Thus, the order of increasing acidity (for removal of one proton) across the second row is CH4 < NH3 < H2O < HF; across the third row, it is SiH4 < PH3 < H2S < HCl (see Figure 1).

This diagram has two rows and four columns. Red arrows point left across the bottom of the figure and down at the right side and are labeled “Increasing acid strength.” Blue arrows point left across the bottom and up at the right side of the figure and are labeled “Increasing base strength.” The first column is labeled 14 at the top and two white squares are beneath it. The first has the number 6 in the upper left corner and the formula C H subscript 4 in the center along with designation Neither acid nor base. The second square contains the number 14 in the upper left corner, the formula C H subscript 4 at the center and the designation Neither acid nor base. The second column is labeled 15 at the top and two blue squares are beneath it. The first has the number 7 in the upper left corner and the formula N H subscript 3 in the center along with the designation Weak base and K subscript b equals 1.8 times 10 superscript negative 5. The second square contains the number 15 in the upper left corner, the formula P H subscript 3 at the center and the designation Very weak base and K subscript b equals 4 times 10 superscript negative 28. The third column is labeled 16 at the top and two squares are beneath it. The first is shaded tan and has the number 8 in the upper left corner and the formula H subscript 2 O in the center along with the designation neutral. The second square is shaded pink, contains the number 16 in the upper left corner, the formula H subscript 2 S at the center and the designation Weak acid and K subscript a equals 9.5 times 10 superscript negative 8. The fourth column is labeled 17 at the top and two squares are beneath it. The first is shaded pink, has the number 9 in the upper left corner and the formula H F in the center along with the designation Weak acid and K subscript a equals 6.8 times 10 superscript negative 4. The second square is shaded a deeper pink, contains the number 17 in the upper left corner, the formula H C l at the center, and the designation Strong acid.
Figure 1. As you move from left to right and down the periodic table, the acid strength increases. As you move from right to left and up, the base strength increases.

Inorganic compounds containing oxygen and one or more hydroxyl (OH) groups tend to be acidic, depending on the position in the periodic table of the central atom E, the atom bonded to the hydroxyl group. Such compounds have the general formula OnE(OH)m, and include sulfuric acid, O2S(OH)2, sulfurous acid, OS(OH)2, nitric acid, O2NOH, perchloric acid, O3ClOH:

This figure contains two structures. The left structure has a central atom labeled E with a single bond to the left to nothing, a single bond down to nothing, a single bond up to oxygen, and a single bond to the right to oxygen, which has a single bond to hydrogen. The single bond between E and O is labeled Bond a and the single bond between oxygen and hydrogen is labeled Bond b. The second structure to the right is labeled Sulfuric Acid. There is a central Sulfur atom with single bonds to oxygen to the left and right and a double bond to oxygen to the top and bottom. The oxygens to the left and right are also single bonded to one hydrogen each.

If the atom E has a relatively high electronegativity, it strongly attracts the electrons it shares with the oxygen atom, making bond a relatively strong. The oxygen-hydrogen bond, bond b, is thereby weakened because electrons are displaced toward E. Bond b is polar and readily releases hydrogen ions to the solution, so the material behaves as an acid. These compounds are commonly called oxyacids.

Key Concepts and Summary

In this section, we look at how to compare the strength of acids qualitatively for two classes of acids. The first are binary acids, where hydrogen is bonded to one other element (like HF, H2O, PH3). Within periods (or rows), the more electronegative the element is, the stronger the acid is due to the polarity of the bond created. The strongest acids are on the right of the periodic table. Within families (or columns), the larger the element is, the stronger the acid is due to a weaker bond created with larger atoms. The strongest acids are towards the bottom of the periodic table.

The second class of acids are oxyacids, which contain a hydrogen attached to an oxygen and other elements. Carboxylic acids are a common example of this type of acid. The strength of these acids are only determined by electronegativity. The more electronegative the elements are within the molecule, the stronger the acid. The increased polarity of the molecule weakens the O-H bond and allows the proton to dissociate more.

Even though we have focused primarily on the strength of acids in this section, the stronger the acid, the weaker the conjugate base after the proton dissociates. The weaker the acid, the stronger the conjugate base after the proton dissociates.

Glossary

binary compound
In this course, a compound with hydrogen and one other element

oxyacid
an inorganic compound that contains a O-H bond and other elements, such as a carboxylic acid

Chemistry End of Section Exercises

  1. Predict which acid in each of the following pairs is the stronger and explain your reasoning for each:
    1. H2O or HF
    2. HCl or HF
    3. HSO3 or HSO4
    4. NH3 or H2S
    5. CH3COOH or CCl3COOH

Answers to Chemistry End of Section Exercises

  1. (a) HF, Both are in the same row so electronegativity is the greatest determining factor. F is more electronegative than O;
    (b) HCl, Both F and Cl are in the same group so size is the greatest determining factor. Cl is larger than F;
    (c) HSO4, These are oxyacids so greater electronegativity of the central atom increases strength. A greater number of oxygens connected to the central atom increases electronegativity.;
    (d) H2S, Consider this in steps: Compare H2S to H2O, S is larger than O so H2S is a stronger acid. O is more electronegative than N so H2O is a stronger acid than NH3. Therefore, H2S is stronger than NH3.
    (e) CCl3COOH, The central atom is more electronegative with Cl atoms bounds to it rather than H atoms.
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