BASE IONIZATION Section 8.3

Base-Ionization Constant - K b ■The following equation represents a weak base, B, dissolving in water: ■B (aq) + H 2 O (l)  BH + (aq) + OH - (aq) ■The base accepts the donated proton from water. Since ammonia and amines have a nitrogen atom that can also accept a proton, they are considered bases. ■NH 3 (aq) + H 2 O (l)  NH 4 + (aq) + OH - (aq) ■Notice that the base does not dissociate but rather, it ionizes. ■Therefore, the equilibrium constant for a base can be referred to as either the base- ionization constant or the base-dissociation constant, K b.

See Table 8.4 and Appendix B for Base- Ionization Constants ■To solve problems, follow the same procedure as you do with acids, except you calculate the concentration of the hydroxide ion rather than the hydronium ion.

Example – When a solution containing 5.0 g/L of a base, C 6 H 5 NH 2 was prepared, the pH was determined to be Calculate the base-ionization constant. ■Step 1 – Calculate the molar mass of the base. ■93.14 g/mol ■Step 2 - Calculate the molar concentration of the solution ■ mol/L ■Step 3 – Set up an ICE Table. ■Use the pH to determine the pOH and the pOH to determine the [OH - ]. ■4.79 x ■Write the expression and calculate K b. ■4.3 x

Practice problems – page 523- even #s

Ka, Kb and Kw ■There is an important relationship between the Ka of an acid, the Kb of its conjugate base and the ion-product constant of water. ■At a constant temperature, the overall equilibrium constant is the product of the individual equilibrium constants. ■KaKb=Kw ■Remember, the ion product constant of water (Kw) is 1.0 x

Calculate Kb for the conjugate base of benzoic acid, C 6 H 5 COOH(aq) Calculate Ka for the conjugate acid of ethylamine, C 2 H 5 NH 2 (aq) ■Step 1 – Identify the conjugate base of benzoic acid. ■It is the benzoate ion (C 6 H 5 COO - ) ■Step 2 – Use Appendix B to find the value of Ka for benzoic acid and use the equation to find Kb. ■Kb = 1.6 x ■Step 3 – Identify the conjugate acid of ethylamine. ■It is C 2 H 5 NH 3 + ■Step 4 – Find the value of Kb in Appendix B and use it to find Ka. ■It is 2.2 x

Practice Problems – p. 526 – even #s