chapter 14 acids and bases ap*. section 14.8 acid-base properties of salts salts ionic compounds. ...
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Section 14.8Acid-Base Properties of Salts
Salts Ionic compounds. When dissolved in water, break up into its ions (which
can behave as acids or bases). Review the solubility rules (p. 156, table 4.1) Salts containing NO3
-, NH4+ and group 1A cations are
highly soluble. Example:
KCl (s) + H2O (l) K+ + Cl- + H2O (l)
Net equation: KCl (s) K+ + Cl-
Note, this reaction goes to completion, due to the high solubility of KCl.Copyright © Cengage Learning. All rights reserved 2
Section 14.8Acid-Base Properties of Salts
Salts The dissolved ions of salts can affect pH. The salt of a strong acid and a strong base gives a neutral
solution. For example, KCl & NaNO3 will NOT change the pH. Why is this true?KCl (s) K+ + Cl-
K+ + H2O KOH + H+
Cl- + H2O HCl + OH-
Since K+, Cl- , Na+ & NO3- are all conjugates of strong
acids/bases, these ions will not change the pH.Copyright © Cengage Learning. All rights reserved 3
1st identify the ions produced in solution.
2nd predict how the ions will
interact with water.
3rd any strong acid/base will drive the
reaction due to the large Ka or Kb
Only starting ions will be present in significant quantities, so pH will be neutral (due to H2O only)
Section 14.8Acid-Base Properties of Salts
Salts A basic solution is formed if the anion of the salt is the
conjugate base of a weak acid. For example, NaF & KC2H3O2 WILL change the pH. Why is this true?NaF (s) Na+ + F-
Na+ + H2O NaOH + H+
F- + H2O HF + OH-
Since Na+ is a conjugate of strong base = neutral pHSince F- is a conjugate of weak acid (HF), pH will be >7, because
Na+, F-, H2O, HF & OH- will all be present in solution.Copyright © Cengage Learning. All rights reserved 4
Section 14.8Acid-Base Properties of Salts
Salts An acidic solution is formed if the cation of the salt is the conjugate acid of a weak base. NH4Cl will produce an acidic solution. Why is this true?NH4Cl (s) NH4
+ + Cl-
NH4+ + H2O NH3
+ H3O+
Cl- + H2O HCl + OH-
Since NH4+ is a conjugate of weak base, pH will be <7, because
NH4+, H2O, NH4Cl & H3O+ will all be present in solution.
Since Cl- is a conjugate of strong acid = neutral pH
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Section 14.8Acid-Base Properties of Salts
Qualitative Prediction of pH of Salt Solutions (from Weak Parents)
Copyright © Cengage Learning. All rights reserved 7
If a salt contains BOTH the conjugate of a weak acid & the conjugate of a weak base, then the Ka & Kb values must be compared to determine the approximate pH.
*
Section 14.8Acid-Base Properties of Salts
HC2H3O2 Ka = 1.8 × 10-5
HCN Ka = 6.2 × 10-10
Calculate the Kb values for: C2H3O2− and CN−
Don’t forget: Ka × Kb = Kw
Kb (C2H3O2-) = 5.6 × 10-10
Kb (CN-) = 1.6 × 10-5
Note that as Ka increases, a conjugate Kb decreases!!Copyright © Cengage Learning. All rights reserved
EXERCISE!EXERCISE!
Section 14.8Acid-Base Properties of Salts
Arrange the following 1.0 M solutions from lowest to highest pH.
HBr NaOH NH4Cl
NaCN NH3 HCN
NaCl HF
Justify your answer.HBr, HF, HCN, NH4Cl, NaCl, NaCN, NH3, NaOH
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CONCEPT CHECK!CONCEPT CHECK!
Section 14.8Acid-Base Properties of Salts
Consider a 0.30 M solution of NaF. The Ka for HF is 7.2 × 10-4.
What are the major species?
Na+, F-, H2O
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CONCEPT CHECK!CONCEPT CHECK!
Section 14.8Acid-Base Properties of Salts
Let’s Think About It…
Q: Why isn’t NaF considered a major species? Answer: Because NaF is highly soluble. It will dissolve
completely into its ions. What are the possibilities for the dominant reactions? Answer: examine the next slide...
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Section 14.8Acid-Base Properties of Salts
Let’s Think About It…
The possibilities for the dominant reactions are:
1. F–(aq) + H2O(l) HF(aq) + OH–(aq)
2. H2O(l) + H2O(l) H3O+(aq) + OH–(aq)
3. Na+(aq) + H2O(l) NaOH + H+(aq)
4. Na+(aq) + F–(aq) NaF
Copyright © Cengage Learning. All rights reserved 12
Section 14.8Acid-Base Properties of Salts
Let’s Think About It…
How do we decide which reaction controls the pH? Answer: only reactions 1 & 2 can control pH. Reaction 3
produces a srong base (unreasonable) & reaction 4 will not be in equilibrium, since all the NaF will dissolve.
This leaves: F–(aq) + H2O(l) HF(aq) + OH–(aq)
H2O(l) + H2O(l) H3O+(aq) + OH–(aq) Determine the equilibrium constant for each reaction. The first reaction will dominate, due to the large Ka.
Copyright © Cengage Learning. All rights reserved 13
Section 14.8Acid-Base Properties of Salts
Calculate the pH of a 0.75 M aqueous solution of NaCN. Ka for HCN is 6.2 × 10–10.
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EXERCISE!EXERCISE!
Section 14.8Acid-Base Properties of Salts
Let’s Think About It…
What are the major species in solution?
Na+, CN–, H2O
Why isn’t NaCN considered a major species? Because it is highly soluble & will dissolve completely.
Copyright © Cengage Learning. All rights reserved 15
Section 14.8Acid-Base Properties of Salts
Let’s Think About It…
What are all possibilities for the dominant reaction? The possibilities for the dominant reaction are:
1. CN–(aq) + H2O(l) HCN(aq) + OH–(aq)
2. H2O(l) + H2O(l) H3O+(aq) + OH–(aq)
3. Na+(aq) + H2O(l) NaOH + H+(aq)
4. Na+(aq) + CN–(aq) NaCN
Which of these reactions really occur?Copyright © Cengage Learning. All rights reserved 16
Section 14.8Acid-Base Properties of Salts
Let’s Think About It…
How do we decide which reaction controls the pH?
CN–(aq) + H2O(l) HCN(aq) + OH–(aq)
H2O(l) + H2O(l) H3O+(aq) + OH–(aq)
Copyright © Cengage Learning. All rights reserved 17
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