Applications of Aqueous Equilibrium Chapter 15. Common Ion Effect & Buffers Sections 1-3

Transcription

1 Applications of Aqueous Equilibrium Chapter 15 Common Ion Effect & Buffers Sections 1-3

2 Solutions of Acids or Bases Containing a Common Ion NaF Na + + F - HF H + + F - What effect does the NaF have on the dissociation of HF? add F - from NaF HF H + + F - fewer H + Common Ion Effect a shift in equilibrium that occurs because of the addition of an ion already in the equilibrium Important in polyprotic acids The production of H + in the first step inhibits the production of H + in subsequent steps

3 Common Ion Sample Problem 15.1 p. 682 In Section 14.5 we found the equilibrium concentration of H + in a 1.0 M HF solution is 2.7 x 10-2 M, and the percent dissociation of HF is 2.7%. Calculate the [H + ] and the percent dissociation of HF in a solution containing 1.0 M HF (Ka = 7.2 x 10-4 ) and the 1.0 M NaF.

4 Common Ion Practice Exercise #25 page 740 Compare the percent dissociation of M propanoic acid (HC 3 H 5 O 2, Ka = 1.3 x 10-5 with the percent dissociation of a mixture containing 0.100M HC 3 H 5 O 2 and 0.100M NaC 3 H 5 O 2. Explain the large difference in precent dissociation of the acid.

5 Resists changes in its ph when H + or OH - are added A weak acid and its salt Or A weak base and its salt Buffers

6 Buffer Calculations Sample Problem 15.2 p. 684 A buffered solution contains 0.50 M acetic acid (HC 2 H 3 O 2, K a = 1.8 x 10-5 ) and 0.50 M sodium acetate (NaC 2 H 3 O 2 ). Calculate the ph of this buffer.

7 Buffer Calculations

8 Buffer Calculations Sample Problem 15.3 p. 685 Calculate the change in ph that occurs when mol solid NaOH is added to 1.0L of the buffered solution described in Sample Exercise Compare this ph change with that which occurs when mol solid NaOH is added to 1.0 L of water.

9 How does buffering work when base is added? HA H + + A - When OH - is added to the system it reacts completely with the HA to produce water and A -. OH - + HA H 2 O + A - For the original buffering system Ka = [H + ][A - ]/[HA] Rearrange [H + ] = Ka x [HA]/[A - ] Ka will not change so.. [H + ] (and ph) is governed by [HA]/[A - ] When OH - is added the HA is converted to A - [HA]/[A - ] decreases so [H + ] decreases [HA] [A] [HA] [A] [HA] [A] The change in [HA]/[A - ] is small so the change in [H + ] and ph is small The essence of buffering is that [HA] & [A - ] are large compared to [OH - ] added Try

10 How does a buffer work when OH - is added?

11 How does a buffer work when OH - is added?

12 How does buffering work when acid is added? HA H + + A - When H + is added to the system it reacts completely with the A - to produce HA. H + + A - HA For the original buffering system Ka = [H + ][A - ]/[HA] Rearrange [H + ] = Ka x [HA]/[A - ] Ka will not change so.. [H + ] (and ph) is governed by [HA]/[A - ] When OH - is added the A - is converted to HA [HA]/[A - ] increases so [H + ] decreases The change in [HA]/[A - ] is small so the change in [H + ] and ph is small The essence of buffering is that [HA] & [A - ] are large compared to [H + ] added Try

13 Henderson Hasselbalch Equation Useful for calculating the ph of systems where [HA]/[A - ] is known. [H + ] = Ka [HA] [A - ] -log [H + ] = -log Ka - log [HA] [A - ] ph = pka - log [HA] [A - ] ph = pka + log [A - ] [HA] Henderson Hasselbalch ph = pka + log [base] [acid] Try

14 Buffer Calculations Sample Problem 15.4 p. 689 Calculate the ph of a solution containing 0.75 M lactic acid (Ka = 1.4 x 10-4 ) and 0.25 M sodium lactate. Lactic acid, HC 3 H 5 O 3, is a common constituent of biological systems. For example, it is found in milk and is present in human muscle tissue during exertion.

15 Buffer Calculations Sample Problem 15.5 p. 690 A buffered solution contains 0.25 M NH 3 (Kb = 1.8 x 10-5 ) and 0.40 M NH 4 Cl. Calculate the ph of this solution.

16 Buffer Calculations Sample Problem 15.6 p. 691 Calculate the ph of the solution that results when 0.10 mole gaseous HCl is added to 1.0 L of the buffered solution from exercise (A buffered solution contains 0.25 M NH 3 (Kb = 1.8 x 10-5 ) and 0.40 M NH 4 Cl.)

17 Summary of Most Important Characteristics of Buffered Solutions Contains large concentrations of weak acid and weak base weak acid, HA & conjugate base, A Weak base B & conjugate acid, BH + When H + is added, it reacts to completion with the weak base. H + + A - HA or H + + B BH + When OH - is added, it reacts to completion with the weak acid. OH - + HA A - + H 2 O or OH - + BH + B + H 2 O The ph of the buffered solution is determined by the ratio of [HA] to [A - ] or [B] to [BH + ] these concentrations are large compared to the amount of H + or OH - added.

18 Buffering Capacity The amount of H + that can be absorbed with out significant change in ph The ph of a buffer is determined by the ratio [A - ]/[HA]. The capacity of the buffer is determined by the magnitudes of [HA] & [A - ].

19 Buffer Calculations Sample Problem 15.7 p. 693 Calculate the change in ph that occurs when mole gaseous HCl is added to 1.0L of Solution A: 5.00 M HC 2 H 3 O 2 and 5.00 M NaC 2 H 3 O 2 Solution B: M HC 2 H 3 O 2 and M NaC 2 H 3 O 2 Ka for HC 2 H 3 O 2 = 1.8 x 10-5

20 Comparing Solution A and Solution B

21 Comparing Solution A and Solution B

22 Buffer Calculations Sample Problem 15.8 p. 695 A chemist needs a solution buffered at a ph 4.30 and can choose from the following acids and their sodium salts. a. chloroacetic acid, Ka = 1.35 x 10-3 b. propanoic acid, Ka = 1.3 x 10-5 c. benzoic acid, Ka = 6.4 x 10-5 d. hypochlorous acid, Ka = 1.35 x 10-8 Calculate the ratio [HA]/[A - ] required for each system to yield a ph of Which system will work best?