Strong and Weak Acids and Bases Lesson 7 8.2

STRENGTH OF ACIDS AND BASES A strong acid or base completely dissociates in water, therefore [H3O+(aq)] or [OH-(aq)] is equal to the [strong acid] or [strong base], respectively Therefore the concentrations are the same A weak acid or base only slightly dissociates in water.

Various Ways to Describe Acid Strength Property Strong Acid Weak Acid Value of acid ionization constant Ka Ka is large Ka is small Position of the ionization equilibrium Far to the right Far to the left Equilibrium concentration of H+(aq) compared with original concentration of HA [H+(aq)]equilibrium = [HA(aq)]initial [H+(aq)]equilibrium<<[HA(aq)]initial

STRENGTH OF ACIDS AND THEIR CONJ. BASE A strong acid or base has a very weak conjugate. A weak acid or base has a weak conjugate A very weak acid or base has a strong conjugate.

WEAK BASES Many compounds are bases even though they do not contain the hydroxide ion. These compounds increase the concentration of hydroxide ions in aqueous solution because of their reaction with water. Example:

BASE IONIZATION CONSTANT Recall the general equation for the reaction of a base B with water: Base Ionization Constant Kb : Equilibrium constant for the ionization of a base; also called the base dissociation constant The Equilibrium Law Expression can be written as follows:

EQUILIBRIUM OF WEAK ACIDS AND BASES

EQUATIONS! *you will be given these on a test* Kw = [H3O+][OH-] Kw = KaKb pH = -log[H3O+] pOH = -log[OH-] pH + pOH = 14 [H3O+] = 10-pH [OH-] = 10-pOH

Determining Kb Example Problem #1 The hydrogen phosphate ion, HPO4-2(aq) has a Ka of 1.3 x 10-13 at SATP. What is the base ionization constant, Kb, for the phosphate ion, PO4-3(aq)?

Determining [OH-] Example Problem #2 [H3O+] is 1   Determining [OH-] Example Problem #2 [H3O+] is 1.7 x 10-14 mol/L in a solution of calcium hydroxide. What is the concentration of the hydroxide ion OH-(aq)?

  Determining pH Example Problem #3 A solution of ammonia has an [OH-] of 8.2 x 10-2mol/L. What is the pH of the solution?

Finding pH, pOH, [H3O+] and [OH-] Example Problem #4 A chemist dissolved some Aspirin in water. The chemist then measured the pH of the solution and found it to be 2.73 at 25oC. What are [H3O+] and [OH] of the solution?

Finding pH, pOH, [H3O+] and [OH-] Example Problem #4 A chemist dissolved some Aspirin in water. The chemist then measured the pH of the solution and found it to be 2.73 at 25oC. What are [H3O+] and [OH] of the solution?

Putting bases in order of strength Example Problem #5 Arrange the following entities according to increasing strength as bases F-, Cl- , NO2-, CN-. Use the table to help.

Homework Answer Practice Problems Pg 502 #1-2, Pg. 505 #1-3,