Download 24 Common Ion Effect on Acid Ionization

Common Ion Effect on Acid Ionization
How is the ionization of a weak acid affected by other ion species in solution?
Why?
The ionization constant (Ka) for a weak acid allows chemists to predict the concentration of ions in solution
at equilibrium. At this point you have learned to solve these types of problems if the weak acid is ionized in
water. But what if the beaker contains a solution that already has some ions dissolved in it? Can the weak
acid still ionize to the same extent? Does the percent ionization change?
Model 1 – Weak Acid Equilibrium
Note: Proceed to Question 1 on the next page. Do not fill in these tables until instructed to do so.
HCH3COO (aq) + H2O → H3O1+ (aq) + CH3COO1− (aq) Ka = 1.76 × 10 −5
Beaker 1
Beaker 2
HCH3COO
Initial
Change
H3O1+ CH3COO1−
1.00 M
0M
0M
−x
+x
+x
Equilibrium
Initial
Change
HCH3COO
H3O1+
CH3COO1−
1.00 M
0M
0.200 M
−x
+x
+x
HCH3COO
H3O1+
CH3COO1−
1.00 M
0.200 M
0M
−x
+x
+x
Equilibrium
Percent Acid Ionization = Percent Acid Ionization = Beaker 3
Beaker 4
HCH3COO
Initial
Change
H3O1+ CH3COO1−
1.00 M
0M
0.500 M
−x
+x
+x
Equilibrium
Percent Acid Ionization = Common Ion Effect on Acid Ionization
Initial
Change
Equilibrium
Percent Acid Ionization = 1
1. Match each of the following descriptions with one of the beakers in Model 1. In each case assume
that the change in volume as the solid(s) are added is minimal. Hint: Calculate the molarity of
any soluble ions after the solids have dissolved or solutions have been mixed.
a.Beaker contains 500.0 mL of 1.00 M acetic acid.
b. Beaker contains 0.250 mole of solid sodium acetate, NaCH3COO, dissolved into
500.0 mL of 1.00 M acetic acid.
c.Beaker contains 0.0500 mole of solid zinc acetate, Zn(CH3COO)2, dissolved into
500.0 mL of 1.00 M acetic acid.
d.Beaker contains 250.0 mL of 0.40 M hydrochloric acid mixed with 250.0 mL of
2.00 M acetic acid.
2. Based on the acid ionization constant, Ka, for acetic acid provided in Model, 1 do you expect
most of the acid molecules to ionize in any of the four beakers?
3. Three of the beakers in Model 1 contain a common ion in solution.
a. Which beakers in Model 1 contain a common ion?
b. Consider the source of the common ion in each beaker. Does the common ion come from the
weak acid molecules? If no, explain the source of the common ion.
4. Predict what will be in Beakers 1 and 2 when the systems in those beakers reach equilibrium. Use
particulate drawings below to communicate your predictions. You may omit water molecules for
clarity.
2POGIL™ Activities for AP* Chemistry
5. Write the equation for the acid ionization constant, Ka, for this acid.
6. Divide the work among group members to fill in each of the tables in Model 1. Use the acid ionization constant to find the final concentrations of the molecular and ionic species in each beaker
(excluding water). Add this to Model 1. (Work space for these calculations has been provided in
Model 1.)
7. Calculate the percent ionization of the weak acid in each beaker in Model 1 using the information from the tables. Add this information to Model 1.
8. Does the presence of a common ion with a weak acid increase or decrease the percent ionization
of the weak acid? Support your answer with evidence from Model 1.
9. According to Model 1, as the concentration of the common ion is increased, how does the percent ionization of the weak acid change? Support your answer with evidence from Model 1.
10. Are the data in Model 1 consistent with LeChâtelier’s Principle? Support your answer with specific examples from Model 1.
11. Consider the four beakers below. Rank the beakers in terms of percent ionization of the weak
acid from lowest to highest percent. The weak acid ionization constant for hydrofluoric acid is
6.31 × 10−4.
Beaker 1
0.025 M HF
Beaker 2
0.025 M HF
0.010 M NaF
Common Ion Effect on Acid Ionization
Beaker 3
0.025 M HF
0.010 M BaF2
Beaker 4
0.025 M HF
0.010 M HCl
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12. The acid ionization constant for benzoic acid (C6H5COOH) is 6.46 × 10 −5. Compare the percent ionization of 1.00 M benzoic acid in water with its percent ionization in 0.500 M sodium
benzoate solution. Support your comparison with calculations similar to those in Model 1.
4POGIL™ Activities for AP* Chemistry
Extension Questions
13. The pH of the solutions in the four beakers in Model 1 was measured. Match each pH with the
correct beaker.
pH = 0.7 Beaker pH = 4.0 Beaker pH = 2.4 Beaker pH = 4.5 Beaker 14. You have seen that the presence of a common ion in solution will decrease the percent ionization
of a weak acid in solution. Provide two things that could be done to a solution to increase the
percent ionization of a weak acid.
Common Ion Effect on Acid Ionization
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