chapter 14 acids and bases ap*. section 14.8 acid-base properties of salts salts ionic compounds. ...

Chapter 14

Acids and Bases

AP*

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


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)


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


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

Copyright © Cengage Learning. All rights reserved 5



*


Qualitative Prediction of pH of Salt Solutions (from Weak Parents)


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.

*


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!


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


CONCEPT CHECK!CONCEPT CHECK!


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


CONCEPT CHECK!CONCEPT CHECK!


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...




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




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.



Calculate the pH of a 0.75 M aqueous solution of NaCN. Ka for HCN is 6.2 × 10–10.


EXERCISE!EXERCISE!



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.




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



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)



Steps Toward Solving for pH

Kb = 1.6 × 10–5

pH = 11.54

CN–(aq) + H2O HCN(aq) + OH–(aq)

Initial 0.75 M 0 ~ 0

Change –x +x +x

Equilibrium 0.75–x x x

chapter 14 acids and bases ap*. section 14.8 acid-base properties of salts salts ionic compounds. ...

Documents