Acids can be strong or weak acids. The strong acids are:

All other acids are considered weak acids. If you know the seven strong acids, you then will know all of the weak acids. A strong acid essentially dissociates completely where a weak acid only partially dissociates. Strong acids are strong electrolytes and weak acids are weak electrolytes.

In the table below, some acids and their conjugate bases are shown. The strong acids are at the top of the column. The strong acids in water, all have the same strength. The strongest acid in water is H₃O⁺ ion. The strongest base in water is OH^– ion. Notice, a stronger acid has a weak conjugate base. A weaker acid has a stronger conjugate base.

In the table above, the acid strength decreases as you go down the column, and the base strength increases as you go down a column. For example, HCl has a weaker conjugate base than does nitrous acid, HNO₂.

For a proton transfer reaction that involves a weak acid or weak base, the side of the equation with the weaker acid and base is favored. For example,

Looking at the equation, we see the H₃O⁺ is a stronger acid than HF. The equilibrium would favor the reactants — the side with the weaker acid and base. Water is a weaker base than F^–. In these reactions, you only need to determine the weaker acid or the weaker base as both will be on the same side of the equation. Again, the weaker acid and weaker base will be favored in a reaction.

Factors that Affect Acid Strength

There are many factors that affect acid strength. We will look at bond dissociation energy and electronegativity as applied to an H–X bond. In general, the weaker and more polar the bond, the stronger the acid. If we look at binary acids in a group of the periodic table, we see the acid strength increases as we go down a group. For example, the hydrohalic acids from Group 7A start with HF which is a weak acid down, HCl, HBr, and HI which is the strongest acid in the group. Electronegativity is not as important as it decreases when going down a group of the periodic table. Although all of the hydrohalic acids are polar, the bond strength is most important. As we go down the group, the bond strength decreases making the HI the strongest acid. The bond strength for HF is 570 kJ/mol where for HI it is 298 kJ/mol. In the figure below, we see the acid strength increases down the group because the bond strengths decrease going down the group.

As for binary acids in the same period, the changes in bond strength are not as significant as electronegativity. The bonds become more polar going from left to right across a period due to the higher electronegativity of the atom bonded to hydrogen. In the figure below, we see that acid strength increases going across a period of the periodic table. The atom, Y, bonded to hydrogen, H–Y, increases in electronegativity going across a period. Electronegativity is most important in the determination of acid strength for binary acids in a period.

As we move across a period, the H–A bond becomes more polar which makes it easier for a hydrogen ion to be transferred to a solvent water molecule or a base. The more easily the proton can be transferred, the stronger the acid.

Oxoacids

An oxoacid has the structure H–O–Y–. The acidic hydrogen atom is always bonded to an oxygen atom. The oxygen atom is also bonded to Y which can be a nonmetallic atom like C, N, Cl, Br, I, or S. There can be more than one oxygen atom in the formula. Some examples of oxoacids are HClO, HBrO₂, H₂SO₄ and so on.

For oxoacids with the same number of oxygen atoms, the acid strength depends on the electronegativity of Y. Take a look at the three acids below.

Recall, the electronegativity of the elements in Group 7A decreases going down the group on the periodic table. The HÇlO is the strongest acid.

In the case of oxoacids with the same atom, Y, but a different number of oxygen atoms, the acid strength increases with increasing number of oxygen atoms. The series of oxoacids that contain iodine is shown below.

The iodine becomes more electronegative with each added oxygen, therefore, the H atom becomes more acidic. The HIO₄ is the strongest acid. Below is the order from weakest to strongest acid.

If we look at H₂SO₃ and H₂SO₄, we see the H₂SO₄ is the stronger acid because it has one more oxygen than H₂SO₃.

Exercises

Exercise 1. Which acid in each pair has the strongest conjugate base?

Check Solution To Exercise 1

Exercise 2. Which base in each pair has the strongest conjugate acid?

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Exercise 3. Are products or reactants favored in the following reaction?

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Exercise 4. Are products or reactants favored in the following reaction?

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Exercise 5. Are products or reactants favored in the following reaction?

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Exercise 6. Order the following acids in each set from weakest to strongest acid strength. Explain.

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Exercise 7. Order the following acids in each set from weakest to strongest acid strength. Explain.

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Exercise 8. Order the following organic acids from weakest to strongest acid.

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