· AP Chemistry Name ______
Period ___ Date ___/___/___
15 · Reactions between Acids and Bases
ALKA SELTZER™ DEMO-LABETTE
Equipment:
4 plastic cups / 1 Alka Seltzer tablet / glass stir rodboiled distilled water / 1 M HCl
universal indicator / 1 M NaOH
I. pH Change of Distilled Water:
1. Pour distilled water into the four cups until they are about half-filled.
To each cup add 30 drops of universal indicator. Stir. Set two cups aside for Part II.
Add a drop of 1 M HCl to one cup. Stir. Can you observe any color (pH) change? Repeat.
Add a drop of 1 M NaOH to the other cup. Stir. Do you observe any color (pH) change? Repeat.
2. Write your observations about how easily the color changes with added acid or base:
II. pH Change of Alka Seltzer Solution:
3. Break the Alka Seltzer tablet into two halves.
Add one half to each of the two cups with distilled water and universal indicator.
Add a drop of 1 M HCl to one cup. Stir. Can you observe any color (pH) change? Repeat.
Add a drop of 1 M NaOH to the other cup. Stir. Do you observe any color (pH) change? Repeat.
4. Write your observations about how different the Alka Seltzer solution acts compared to water:
III. Why Does This Happen?
Alka Seltzer contains the weak acid, citric acid, which we will write as HA.
The dissociation equilibrium for HA is: HA H+ + A-
5. HA is a proton ______(donor / acceptor) A- is a proton ______(donor / acceptor)
6. Using Le Châtelier's Principle, indicate the effect of the following changes:
HA H+ + A-
add H+ ______
add OH- ______
7. In Alka Seltzer, the lack of color change is due to the fact that the added H+ ions are almost
all used up and the removed H+ ions are almost all replaced.
Added H+ ions are almost all absorbed by ______.
H+ ions (removed by OH-) are almost all replaced by ______.
The solution contains large numbers of donors (weak acid) and acceptors (conjugate base).
HA H+ + A-
8. Solutions with large concentrations of acids (proton donors) and bases (proton acceptors)
that do not neutralize each other are called ______.
VI. Buffer Calculations
9. The equilibrium of a buffer is the same as the equilibrium of the dissociation of an acid.
Write the dissociation equation and the Ka expression for acetic acid, HAc.
(HAc is an abbreviation for HC2H3O2.) Ka = 1.8 x 10-5.
10. Now, re-write your expression, solving for [H+] (or [H3O+]).
11. The most effective buffer you can make has large, equal amounts of the acid and its conjugate base.
If [HAc] = [Ac-], the above expression changes to:
12. If we take the –log of each side, [H+] becomes ______. The –log of Ka is called the pKa.
What is the significance of the pKa?
13. The most effective buffer is made from large equal concentrations of the acid and its conjugate base.
In this case, the pH = pKa. What is the pH of the best buffer made from the following acids?
a) HCN Ka = 4.0 x 10-10 The pH of the best buffer is _____.
The buffer would contain equal concentrations of HCN and _____.
b) HF Ka = 7.2 x 10-4 The pH of the best buffer is _____.
The buffer would contain equal concentrations of HF and _____.
c) HClO Ka = 3.5 x 10-8 The pH of the best buffer is _____.
The buffer would contain equal concentrations of HClO and _____.
14. For HAc, the pH of the best buffer is: ______If you wanted a buffer with a pH of 5.00, however,
would you need to add more acid, HAc, or more base, Ac-? ______
15. Determine the ratio of [Ac-] to [HAc] that would form a buffer with a pH of 5.00? ______
Use the expression in Question 10 above. What is the [H+] when the pH is 5.00?
16. Of course, a buffer can be made from a weak base and its conjugate acid.
NH3 should be mixed with _____ to make a buffer. Kb = 1.8 x 10-5.
What is the pKb of this base? ______
What is the pOH of the best buffer made from these chemicals? ______
What is the pH of this buffer? ______
17. What is the pH of the best buffer made from the following bases? pOH = pKb. pH = 14 - pOH.
a) NH3 Kb = 1.8 x 10-5 The pH of the best buffer is _____.
The buffer would contain equal concentrations of NH3 and ______.
b) CH3NH2 Kb = 4.4 x 10-4 The pH of the best buffer is _____.
The buffer would contain equal concentrations of CH3NH2 and ______.
c) C6H5NH2 Kb = 3.8 x 10-10 The pH of the best buffer is _____.
The buffer would contain equal concentrations of C6H5NH2 and ______.
HF Ka = 6.6 x 10-4 ammonia, NH3 Kb = 1.8 x 10-5HC2H3O2 Ka = 1.8 x 10-5 methylamine, CH3NH2 Kb = 4.4 x 10-4
HCN Ka = 6.2 x 10-10 aniline, C6H5NH2 Kb = 3.8 x 10-10
18. Choose an appropriate acid for the preparation of a buffer with a pH equal to 3.33.
Calculate the relative quantities of the acid and its conjugate base required for the buffer solution.
19. Repeat question 18 for a buffer requiring a pH of 9.25.