Kaplan MCAT General Chemistry Review

Once again, we are dealing with an equilibrium process here and can apply those principles to the problems dealing with weak acids and bases on the exam.

Acetic acid is a weak acid, so the concentration of CH₃COOH at equilibrium is equal to its initial concentration, 2.0 M, less the amount dissociated, x. Likewise [H⁺] = [CH₃COO^-] = x, because each molecule of CH₃COOH dissociates into one H⁺ ion and one CH₃COO^- ion. Thus, the equation can be rewritten as follows:

MCAT Expertise

This estimation makes life easy and will get you the correct answer on Test Day.

We can approximate that 2.0 - x 2.0 because acetic acid is a weak acid and only slightly dissociates in water. This simplifies the calculation of x:

The fact that [x] is so much less than the initial concentration of acetic acid (2.0 M) validates the approximation; otherwise, it would have been necessary to solve for x using the quadratic formula. That sounds rather unpleasant, doesn’t it? Fortunately for you, the MCAT test writers select examples of weak acids and bases that allow you to make this approximation. (A rule of thumb is that the approximation is valid as long as x is less than 5 percent of the initial concentration.)

Salt Formation

Acids and bases may react with each other, forming a salt (and often, but not always, water), in what is termed a neutralization reaction (see Chapter 4, Compounds and Stoichiometry). For example,

HA + BOH BA + H₂O

Bridge

Remember the reaction types discussed in Chapter 4? Well, here is our neutralization reaction.

The salt may precipitate out or remain ionized in solution, depending on its solubility and the amount produced. Neutralization reactions generally go to completion. The reverse reaction, in which the salt ions react with water to give back the acid or base, is known as hydrolysis.

Four combinations of strong and weak acids and bases are possible:

1. Strong acid + strong base: e.g., HCl + NaOH NaCl + H₂O

2. Strong acid + weak base: e.g., HCl + NH₃ NH₄Cl

3. Weak acid + strong base: e.g., HClO + NaOH NaClO + H₂O

4. Weak acid + weak base: e.g., HClO + NH₃ NH₄ClO

The products of a reaction between equal concentrations of a strong acid and a strong base are a salt and water. The acid and base neutralize each other, so the resulting solution is neutral (pH = 7), and the ions formed in the reaction do not react with water.

The product of a reaction between a strong acid and a weak base is also a salt, but often no water is formed because weak bases are usually not hydroxides. However, in this case, the cation of the salt will react with the water solvent, re-forming the weak base in hydrolysis. For example,

HCl (aq) + NH₃ (aq) NH₄⁺ (aq) + Cl^- (aq) Reaction I

NH₄⁺ (aq) + H₂O (l) NH₃ (aq) + H₃O⁺ (aq) Reaction II

NH₄⁺ is the conjugate acid of a weak base (NH₃), which is stronger than the conjugate base (Cl^-) of the strong acid HCl. NH₄⁺ will then transfer a proton to H₂O to form the hydronium ion. The increase in the concentration of the hydronium ion will cause the water system to shift away from auto-ionization, thereby reducing the concentration of hydroxide ion. Consequently, the concentration of the hydrogen ion will be greater than that of the hydroxide ion at equilibrium, and as a result, the pH of the solution will fall below 7.

On the other hand, when a weak acid reacts with a strong base, the pH of the solution at equilibrium is in the basic range because the salt hydrolyzes to re-form the acid, with the concurrent formation of hydroxide ion from the hydrolyzed water molecules. The increase in the concentration of the hydroxide ion will cause the water system to shift away from auto-ionization, thereby reducing the concentration of the hydrogen ion. Consequently, the concentration of the hydroxide ion will be greater than that of the hydrogen ion at equilibrium, and as a result, the pH of the solution will rise above 7. Consider the reaction of acetic acid CH₃COOH (weak acid) with sodium hydroxide NaOH (strong base):

CH₃COOH (aq) + NaOH (aq) Na⁺ (aq) + CH₃COO^- (aq) + H₂O (l) Reaction I

CH ₃COO^- (aq) + H₂O (l) CH₃COOH (aq) + OH^- (aq) Reaction II

The pH of a solution containing a weak acid and a weak base depends on the relative strengths of the reactants. For example, the acid HClO has a K_a = 3.2 × 10^-8, and the base NH₃ has a K_b = 1.8 × 10^-5. Thus, an aqueous solution of HClO and NH₃ is basic because the K_a for HClO is less than the K_b for NH₃ (that is to say, HClO is weaker as an acid than NH₃ is as a base, and consequently, at equilibrium, the concentration of hydroxide ions will be greater than the concentration of hydrogen ions in the aqueous solution).

Polyvalence and Normality