applications of aqueous equilibrium chapter 15 -na+ + f hf -h+ + f what effect does the naf have on...
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NaF Na+ + F-
HF H+ + F-
What effect does the NaF have on the dissociation of HF?
HF H+ + F-
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
Solutions of Acids or Bases Containing a Common Ion
add F-
from NaF
fewer H+
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.
Common Ion Sample Problem 15.1 p. 682
Compare the percent dissociation of 0.100 M propanoic acid (HC3H5O2, Ka = 1.3 x 10-5 with the percent dissociation of a mixture containing 0.100M HC3H5O2 and 0.100M NaC3H5O2. Explain the large difference in precent dissociation of the acid.
Common Ion Practice Exercise #25 page 740
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
A buffered solution contains 0.50 M acetic acid (HC2H3O2, Ka = 1.8 x 10-5) and 0.50 M sodium acetate (NaC2H3O2). Calculate the pH of this buffer.
Buffer Calculations Sample Problem 15.2 p. 684
Calculate the change in pH that occurs when 0.010 mol solid NaOH is added to 1.0L of the buffered solution described in Sample Exercise 15.2. Compare this pH change with that which occurs when 0.010 mol solid NaOH is added to 1.0 L of water.
Buffer Calculations Sample Problem 15.3 p. 685
HA H+ + A-
When OH- is added to the system it reacts completely with the HA to produce water and A-.
OH- + HA H2O + 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
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
How does buffering work when base is added?
Try
[HA]
[A]
[HA]
[A]
[HA]
[A]
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
How does buffering work when acid is added?
Try
Useful for calculating the pH of systems where [HA]/[A-] is known.
Henderson Hasselbalch Equation
Try
[H+] = Ka [HA]
[A-]
-log [H+] = -log Ka [HA]
[A-] - log
pH = pKa [HA]
[A-] - log
pH = pKa [HA]
[A-] + log
pH = pKa [acid]
[base] + log Henderson Hasselbalch
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, HC3H5O3, is a common constituent of biological systems. For example, it is found in milk and is present in human muscle tissue during exertion.
Buffer Calculations Sample Problem 15.4 p. 689
A buffered solution contains 0.25 M NH3 (Kb = 1.8 x 10-5) and 0.40 M NH4Cl. Calculate the pH of this solution.
Buffer Calculations Sample Problem 15.5 p. 690
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 15.5. (A buffered solution contains 0.25 M NH3 (Kb = 1.8 x 10-5) and 0.40 M NH4Cl.)
Buffer Calculations Sample Problem 15.6 p. 691
• 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- + H2O or OH- + BH+ B + H2O
• 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.
Summary of Most Important Characteristics of Buffered Solutions
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-].
Buffering Capacity
Calculate the change in pH that occurs when 0.010 mole gaseous HCl is added to 1.0L of
Solution A: 5.00 M HC2H3O2 and 5.00 M NaC2H3O2
Solution B: 0.050 M HC2H3O2 and 0.050 M NaC2H3O2
Ka for HC2H3O2 = 1.8 x 10-5
Buffer Calculations Sample Problem 15.7 p. 693
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 4.30. Which system will work best?
Buffer Calculations Sample Problem 15.8 p. 695
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