Bases

1. Bases • According to the Arrhenius concept, a base is a substance that produce OH- ions in aqueous solution. • According to the Brønsted-Lowry model, a base is a proton acceptor. • A strong base dissociates completely when dissolved in aqueous solution: • NaOH(s) → Na+(aq) + OH-(aq) • All the hydroxides of the Group I elements (LiOH, NaOH, KOH, RbOH, and CsOH) are strong bases. • The alkaline earth (Group 2) hydroxides – Ca(OH)2, Ba(OH)2, and Sr(OH)2 – are strong bases.

2. Weak Bases • Many types of proton acceptors (bases) do not contain the hydroxide ion. • However, when dissolved in water, these substances increase the concentration of hydroxide ions because of their reaction with water; thus, they yield a basic solution. • Note below the ammonia molecule accepts a proton and thus functions as a base.

3. The general reaction between a base B and water is given by • B(aq) + H2O(l) ⇌ BH+(aq) + OH-(aq) Conjugate acid Conjugate base Base Acid • The equilibrium constant for this general reaction is • where Kb always refers to the reaction of a base with water to form the conjugate acid and the hydroxide ion. • Bases of the type represented by B are called weak bases.

4. pH of Weak Bases • pH calculations for solutions of weak bases are very similar to those for weak acids.

5. Polyprotic Acids • Some important acids, such as sulfuric acid (H2SO4) and phosphoric acid (H3PO4), can furnish more than one proton and are called polyprotic acids. • A polyprotic acid always dissociates in a stepwisemanner, one proton at a time. • For example, the diprotic acid carbonic acid (H2CO3), dissociates in the following steps: • H2CO3(aq) ⇌ H+(aq) + HCO3-(aq) Ka1 = 4.3 x 10-7 • HCO3(aq) ⇌ H+(aq) + CO32-(aq) Ka2 = 5.6 x 10-11

6. Characteristics of Weak Polyprotic Acids • Typically, successive Ka values are so much smaller than the first value that only the first dissociation step makes a significant contribution to the equilibrium concentration and the calculation of pH for a solution of a typical weak polyprotic acid is identical to that for a solution of a weak monoprotic acid. • Sulfuric acid is unique in being a strong acid in its first dissociation step and a weak acid in its second step. For relatively concentrated solutions of sulfuric acid (1.0 M or higher), the large concentration of H+ from the first dissociation step represses the second step, which can be neglected as a contributor of H+ ions. For dilute solutions of sulfuric acid, the second step does make a significant contribution, and the quadratic equation must be used.

7. Acid-Base Properties of Salts • Salt is simply another name for ionic compound. • When a salt dissolves in water, we assume it breaks up into ions, which are independent units. • Under certain conditions, these ions can behave as acids or bases.

8. Salts that Produce Neutral Solutions • Salts that consist of the cations of strong bases and the anions of strong acids have no effect on [H+] when dissolved in water. • Aqueous solutions of salts such as KCl, NaCl, NaNO3, and KNO3 are neutral (pH = 7). • For example, • KCl(s) → K+(aq) + Cl-(aq) cation of strong base, KOH anion of strong acid, HCl

9. Salts that Produce Basic Solutions • In an aqueous solution of sodium acetate (NaC2H3O2), the major species are • Na+,C2H3O2-, and H2O • What are the acid-base properties of each component? • The Na+ ion has neither acid nor base properties. • The C2H3O2- ion is the conjugate base of acetic acid, a weak acid. This means that C2H3O2- has a significant affinity for a proton and is a base. • Water is a weakly amphoteric substance. • The pH of this solution will be determined by the C2H3O2- ion, since it is a base.

10. In this solution, water is the only source of protons to react with the base, C2H3O2-, and the reaction is: • C2H3O2-(aq) + H2O(l) ⇌ HC2H3O2(aq) + OH-(aq) • Since OH- is produced, Kbis defined as the equilibrium constant; therefore, • How can we obtain the Kb value for the acetate ion? • The value of Ka for acetic acid is known (1.8 x 10-5) and can be used along with Kw to obtain Kb for the acetate ion.

11. For any weakacid and its conjugate base, • Ka x Kb = Kw • Thus, when either Ka or Kb is known, the other can be calculated. • For the acetate ion,

12. Salts that Produce Acidic Solutions • Some salts produce acidic solutions when dissolved in water. • For example, when solid NH4Cl is dissolved in water, NH4+ and Cl- ions are present, with NH4+ behaving as a weak acid: • NH4+(aq) ⇌ NH3(aq) + H+(aq) • conjugate weak • acid base • The Cl- ion does not affect the pH of the solution. • In general, salts in which the anion is not a base and the cation is the conjugate acid of a weak base produce acidic solutions.

13. A second type of salt that produces an acidic solution is one that contains a highly charged metal ion. • For example, when solid aluminum chloride (AlCl3) is dissolved in water, the resulting solution is highly acidic. • Although the Al3+ ion is not itself a Brønsted-Lowry acid, the hydrated ion Al(H2O)63+ formed in water is a weak acid: • Al(H2O)63+(aq) ⇌ Al(OH)(H2O)52+(aq) + H+(aq) • Typically, the higher the charge on the metal ion, the stronger the acidity of the hydrated ion.

14. Solutions in Which both Ions can Affect pH • For many salts, such as ammonium acetate (NH4C2H3O2), both ions can affect the pH of the aqueous solution. • Compare Ka and Kb values for the acidic and basic ions and predict based on table below.