8.3 Bases

• Similar to weak acids, weak bases react with water to a solution of ions at equilibrium.

• The general equation is:B(aq) + H2O(l) HB+

(aq) + OH-(aq)

where, B represents any baseThe equilibrium expression for this reaction is:

Kc = [HB+][OH-] [B][H2O]

Base Dissociation Constant, Kb

• Just as with weak acids, the concentration of water is almost constant in a solution of weak base.

• Therefore, we can group the constant terms in the equation.

[H2O] Kc = [HB+][OH-] = Kb

[B]• Table 8.3 (Pg. 404 lists some common base

dissociation constants, Kb)

Calculating pH and pOH

• An aqueous solution of ammonia has a concentration of 0.105 mol/L. Calculate the pH of the solution.

1. Write the balanced chemical equation.NH3(aq) + H2O(l) NH4

+(aq) + OH-

(aq)

Given: [NH3] = 0.105 mol/L

2. Set up an ICE table

P.P. #29, Continued

• Write the equation for the base dissociation constant, Kb

Kb =[NH4+][OH-]

[NH3]From the table, Kb(NH3) = 1.8 x 10-5

Concentration (mol/L)

NH3(aq) H2O(l) NH4+

(aq) OH- (aq)

Initial 0.105 0 ~0

Change -x +x +x

Equilibrium 0.105 - x +x +x

P.P. #29, Continued

3. Substitute Kb and the equilibrium values from the ICE table into the equation. Solve for x.

1.8 x 10-5 = (x) (x)

(0.105 – x)

4. Look at [NH3]/Kb = 0.105/1.8x10-5 = 5833 > 100, so the amount of NH3 that

dissociates is negligible compared to the initial [NH3]

Therefore, (0.105 – x), becomes (0.105) solve for x

P.P. #29, Continued

5. Solve for x1.8 x 10-5 = (x) (x)

(0.105)

(0.105)1.8 x 10-5 = x2

x = √1.89 x 10-6 = 1.374 x 10-3

6. Solve for pOHpOH = -log[OH-] = -log[1.374 x 10-3] = 2.832

P.P. #29, Continued

7. Solve for pHpH = 14 – pOH = 14 – 2.832 = 11.168

Therefore, the pH of the ammonia solution is 11.14

The relationship b/n Ka and Kb

• For any acid and its conjugate baseKa(Kb) = [H3O+][OH-] = Kw

• What does this mean?– The stronger an acid is, the weaker its conjugate

base is.The conjugate of a strong acid is always a weak base.The conjugate of a strong base is always a weak acid.

Buffer Solutions• Contain a mixture of:– A weak acid and its conjugate base

OR– A weak base and its conjugate acid

• Buffer solutions resist a change in pH when a moderate amount of acid or base is added.

• Buffer solutions are made 2 ways:– Use a weak acid and one of its salts• Eg. Mix acetic acid and sodium acetate

– Use a weak base and one of its salts• Eg. Mix ammonia and ammonium chloride

Making a Buffer

• http://www.chembio.uoguelph.ca/educmat/chm19104/chemtoons/chemtoons5.htm

How do buffers work?

• Consider a buffer made with acetic acid and sodium acetate.– Acetic acid is weak, so [CH3COOH] is high

– Sodium acetate is very soluble, so [CH3COO-] is high– Adding and acid or base has only a slight effect on pH

because the H3O+ or OH- ions are removed by one of the components of the buffer solution.

– http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/buffer12.swf

Importance of Buffers

• Very important in biological systems• For example:• pH of arterial blood is ~7.4• It must stay b/n 7.0 and 7.5 or the organism

may die.• Blood is buffered by the equilibrium between

CO3 ions and HCO3 ions. (formed by the reaction of dissolved CO2 and H2O)

8.4 Acid-Base Titration Curves

• Acid – Base Titration Curve = Graph of the pH of an acid (or base) against

the volume of and added base (or acid)• Titration reactions are used to find the

equivalence point• Equivalence point – point where the acid and

base completely react with one another.• If,– V1, V2 and C1 are known– C2 can be found (C1V1 = C2V2)

How pH indicators show equivalence points

• pH changes rapidly near the equivalence point

– A single drop of titrant can change the pH by 2 pH units.

– The colour change indicates equivalence even if the colour change happens at pH of 8

– As long as it is in the steep section of the curve

• Strong acid titrated with strong base

Selecting a pH IndicatorWeak acid titrated with strong base.

Section Review

• Which indicator should be used for each curve?

• Is this the titration curve for a weak acid titrated with a strong base? Explain.