1. Define the terms strong and weak as they are applied to acids and bases.
2. Explain the relationship between the relative strength of an acid and the strength of its conjugate base.
3. Predict the direction of a given acid-base reaction.
4. Calculate the pH of given solutions of strong acids and strong bases.

Strong Acid and Bases. The stronger the acid, the more completely it ionizes in water, producing H₃O⁺(aq) and an anion. Strong acids are strong electrolytes and ionize completely in water. HBr, for example, is a strong acid.

HBr(aq) + H₂O(l) H₃O⁺(aq) + Br^–(aq)

A solution of hydrobromic acid consists of H₃O⁺(aq) and Br^–(aq) ions. The concentration of HBr molecules is zero because all have ionized. The concentration of H₃O⁺ is equal to the initial concentration of the acid.

Like a strong acid, a strong base is one that ionizes completely in aqueous solution. KOH, a commonly used base, is an example. It is purchased as a white solid. It dissolves readily in water to give a solution of K⁺ and OH^– ions:

KOH(aq) K⁺(aq) + OH^–(aq)

A solution of 0.10 M KOH is made by dissolving 0.10 mol of KOH in 1 L of solution. In this solution, [K⁺] is 0.10 M, [OH^–] is 0.10 M, and [KOH] is essentially zero. Of the strong bases, only NaOH and KOH are commonly used in the laboratory.

Weak Acids and Bases. Not all acids are strong proton donors in aqueous solutions. A weak acid is one that is only partially ionized in water. Hydrofluoric acid is a typical weak acid in water:

HF(aq) + H₂O(l) H₃O⁺(aq) + F^–(aq)

Since HF is only partially ionized the concentration of HF molecules is much greater than the concentrations of H⁺(aq) and F^–(aq) ions. For example, a 1.0 M HF solution has the following concentrations: [H⁺] = [F^–] = 0.026 M, and [HF] = 0.974 M. About 97% of the initial HF molecules are not ionized. For weak acids, the ionization equilibrium lies to the left. The extent of reaction is small. A number of weak acids are listed in Table 15.2 of the text. Figure 15.1 compares the relative concentrations of HA, H⁺, and A^– in solutions of strong and weak acids.

Figure 15.1 Illustration of the extent of ionization of (a) a strong acid, HA, that ionizes 100%, and (b) a weak acid, HA, that ionizes a few percent. A^– represents the anion. The height of the bar represents the concentration of the species present.

Some bases are not ionized completely in aqueous solution. A weak base ionizes to a limited extent in water. Ammonia is an example of a weak base. In a 0.10 M solution of ammonia, only about 1% of the ammonia molecules are ionized. An important reverse reaction means that the equilibrium lies to the left.

NH3(aq)	+	H2O(l)	NH4+(aq) + OH–(aq)
base		acid

Strength of Conjugate Bases. The strength of an acid is related to the strength of its conjugate base. Let's compare the ionization of HCl, a strong acid, and HF, a weak acid.

HCl(aq) + H₂O(l) H₃O⁺(aq) + Cl^–(aq)

HF(aq) + H₂O H₃O⁺(aq) + F^–(aq)

Chloride ion is the conjugate base of HCl. Here we see that it must be a very weak base because it has no tendency to accept a proton from H₃O⁺ in the reverse reaction. In the ionization of the weak acid HF, the reverse reaction occurs to a much greater extent than for HCl. Fluoride ion accepts a proton from H₃O⁺. The F^– ion is a much stronger base than Cl^–. The conjugate bases of strong acids have no measurable base strength.

When comparing the strength of different acids we see that: as acid strength increases, the strength of its conjugate base decreases. Table 15.2 in the text lists several important conjugate acid-base pairs, according to their relative strengths. A short version is reproduced here in Table 15.2.

Table 15.2 Relative Strengths of Conjugate Acid-Base Pairs

The strengths of the following acids increase in the order HCN < HF < HNO₃. Arrange the conjugate bases of these acids in order of increasing base strength.

Note: A number in [brackets] indicates a subscript, a number or sign in {braces} indicates a superscript.

CN{-} F{-} NO[3]{-} < CN{-} F{-} NO[3]{-} < CN{-} F{-} NO[3]{-}

          Correct!

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Predicting the Direction of Acid-Base Reactions. The information in Table 15.2 can be used to predict the direction of an acid-base reaction. When an acid reacts with a base will the reaction tend to go from left to right and favor the products, or will it tend to go from right to left and favor the reactants. For instance, when HF reacts with water, on which side will the equilibrium lie?

HF(aq) + H₂O H₃O⁺(aq) + F^–(aq)

To decide compare the strength of HF to that of the acid in the reverse reaction, H₃O⁺. According to Table 15.2, H₃O⁺ is the stronger acid. Compare the base strength of H₂O to that of the base in the reverse reaction, F^– ion. F^– ion is the stronger base. The reaction can be summarized as:

HF(aq)	+	H2O		H3O+(aq)	+	F–(aq)
weaker acid		weaker base		stronger acid		stronger base

All proton transfers proceed from the stronger acid and stronger base toward the weaker acid and weaker base. Therefore in this case the reaction proceeds from right to left and equilibrium lies to the left. There will be more HF at equilibrium than H₃O⁺ and F^– ion. A simplification is to just compare the strengths of the two bases. The reaction can be viewed as a competition of two bases for the proton. Who wins? The stronger base, of course. The stronger F^– ions accept protons more readily than do H₂O molecules, and so HF predominates over H₃O⁺ in solution. The weaker acid and weaker base predominate at equilibrium. Example 15.8 applies this principle to another reaction.

To which side does the equilibrium lie in the following acid–base reaction?

HF(aq) + (aq) HNO₂(aq) + F^–(aq)

reactants
products

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HF(aq)	+	NO2-(aq)		HNO2(aq)	+	F–(aq)
stronger acid		stronger base		weaker acid		weaker base

Since proton transfer reactions proceed from the stronger acid and stronger base toward the weaker acid and weaker base, the reaction favors HNO₂ and F^– ion. The equilibrium lies to the right.

The pH of Strong Acid and Strong Base Solutions. The pH of a strong acid solution depends on the hydrogen ion concentration. What is the pH of a 0.0052 M HBr solution? When hydrobromic acid is added to water, it ionizes completely.

HBr(aq) + H₂O(l) H₃O⁺(aq) + Br^–(aq)

For simplicity we can write the reaction without the H₂O molecules.

HBr(aq) H⁺(aq) + Br^–(aq)

Before any ionization occurs the HBr concentration is 0.0052 M, but after ionization its concentration is zero. Since each mole of HBr that reacts yields 1 mol of H⁺ and 1 mol of Br^–, the concentrations of H⁺ and Br^– ions are both 0.0052 M. 0.0052 mol HBr reacts to give 0.0052 mol H⁺ ion and 0.0052 mol Br^– ion per liter of solution.

The pH is given by:

The calculation of the pH of a strong base is given in Example 15.9.

What is the H⁺ ion concentration in 0.0025 M NaOH?

[H⁺] = x 10^ M

          Correct!

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Complete the following questions to check your understanding of the material. Select the check button to see if you answered correctly.

9. What are the H⁺ ion and OH^– ion concentrations in a 0.0033 M NaOH solution?

10. Calculate the pH of a 0.00051 M HCl solution.
11. Calculate the pH of a 0.125 M NaOH solution.
12. What is the pH of a 0.0025 M solution of Ca(OH)₂?
13. What is the pH of a solution made from 3.0 g HCl dissolved in 3.0 L of solution?
14. Which of the following acids has the strongest conjugate base?
    

    HNO₂
    HCN
    H₃O⁺

15. Predict the direction in which the equilibrium will lie for the following reaction.
    HNO₂(aq) + (aq) (aq) + HClO₄(aq)