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(8.3)Acid-Base Properties of Salt Solutions
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pH Review
RecallAcidic [H3O+] > [OH-]
Basic [H3O+] < [OH-]
Neutral [H3O+] = [OH-]
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Demo - pH Review
a) NaCl (s) Na+ (aq) + Cl-(aq) Neutral
a) NaOH (s) Na+ (aq) + OH-
(aq) Basic
c) NH4Cl (s) NH4+
(aq) + Cl-(aq) Acidic
Why? This is because of salt hydrolysis
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Salt Hydrolysis When added to water, salts dissociate
into ionsIf these ions react with H2O by
taking a proton or by giving a proton then the pH of the solution will be affected.
These ions are said to hydrolyze
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Salt Hydrolysis
There are 7 different ways to classify/predict the affect this dissociation of ions will have on the pH of the solution.
We must consider both ions from the salt in order to determine the effect on pH of an aqueous solution.
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1. Neutral Salt SolutionEx. NaCl (s) Na+
(aq) + Cl- (aq)
Na+ - cannot accept H+ from H2O
- no H+ to donate to form H3O+
- therefore won’t hydrolyze H2O
Cl- - no H+ to donate- possibly could accept H+
Cl-(aq) + H20 (l) HCl (aq) + OH-(aq)
BUT lets consider the conjugate pairs
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Neutral Salt Solution
Cl-(aq) + H20 (l) HCl (aq) + OH-(aq)
Recall: The stronger an acid, the weaker its conjugate base
Cl- could accept a proton, but HCl is a strong acid, therefore the H+ and Cl- ions will not come together.
Therefore, the ions do not upset balance of [H3O+] or [OH-] in H2O so solution is neutral, pH=7
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2. Basic Salt Solutions
Ex. Na2CO3(s) 2 Na+(aq) + CO3
2-(aq)
Na+ - no H+ to donate to H2O to form H3O+
- cannot accept H+
- therefore will not hydrolyze H2O
CO32-
- no H+ to donate to H2O to form H3O+
- possibly could accept H+
CO32-
(aq) + 2 H2O (l) H2CO3(aq) + 2 OH-(aq)
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Basic Salt Solutions
CO32-
(aq) +2H2O(l) H2CO3(aq)+ 2OH-(aq)
If we consider the conjugate pairs, this is a weak base.
Anions whose conjugate acids are weak will accept H+ from H2O creating OH-
Therefore it’s a BASIC solution.
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3. Acidic Salt Solutions
Ex. NH4Cl(s) NH4+
(aq) + Cl- (aq)
Cl- - no H+ to donate- possibly could accept H+ (forming HCl),
but HCl is a SA (dissociates completely) so won’t occur.
NH4+
- won’t accept H+ (positives repel) - possibly could donate H+ to H2O
NH4+
(aq) + H2O (l) NH3 (aq) + H3O+ (aq)
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Acidic Salt Solutions
NH4+
(aq) + H2O (l) NH3 (aq) + H3O+ (aq)
NH3 is the conjugate base and is weak. Therefore it won’t readily accept H+
Cations whose conjugate bases are weak will donate H+ to H2O creating H3O+
Therefore it’s an ACIDIC solution.
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4. If Both Salt Ions Hydrolyze
Ex. NH4CH3COO (s) NH4+
(aq) + CH3COO-(aq)
Acid NH4
+ (aq) + H2O (l) NH3 (aq) + H3O+
(aq) (Ka)
Basic CH3COO-
(aq) + H2O(l) CH3COOH (aq) + OH-(aq) (Kb)
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If Both Salt Ions Hydrolyze pH of the solution will depend on the
balance between the H3O+ and OH- created
Therefore, we use our Ka and Kb appendix.
Ka>Kb = acidicKa<Kb = basic
The higher value determines if the solution is acidic or basic
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5. Metal Cations
Ex. Al3+(aq) (Pg. 582, table 3)
- has a high charge density (large charge in small volume)- produce H+ ions indirectly- is hydrated in water
Al 3+ (aq) + 6H20 (l) Al(H2O)6 3+
(aq)
- high charge of Al 3+ increases polarity in –OH bond in the H2O molecules hydrating it
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Metal Cations
Al 3+ (aq) + 6H20 (l) Al(H2O)6 3+
(aq)
- therefore H in H2O hydrates becomes even more positive- so is more easily pulled off by solvent H2O molecules creating H3O+
Al(H2O)6 3+
(aq)+ H2O(l) H3O+(aq)+Al(H2O)5OH 2+
(aq)
- experiments show only one of the 6 waters will donate H+
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Homework
Read pg 581-594 Do pg 588 #1-5
Test Chapter 8 in 5 days (see review)
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(8.3)Acid-Base Properties of Salt Solutions
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6. Hydrolysis of Amphoteric Ions
Ex. NaHCO3 (s) Na+ (aq) + HCO3
-(aq)
Na+ - cannot donate or accept H+
HCO3- - possibly could donate or accept H+
HCO3-(aq) + H2O (l) CO3
2-(aq) + H3O+
(aq)
HCO3-(aq) + H2O (l) H2CO3 (aq) + OH-
(aq)
Again, we refer to the Ka and Kb values!
Kb>Ka therefore solution will be basic
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7. Metal and Nonmetal Oxides
Metal oxides react with water to produce basic solutions
Ex. CaO(s) + H2O(l) Ca2+ (aq) + 2OH- (aq)
Nonmetal oxides react with water to produce acidic solutions
Ex.CO2(g) +2H2O (l) HCO3-
(aq) + H3O+ (aq)
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Calculating pH of Salt Solutions
Ex 1. Calculate the pH of a 0.20mol/L NH4Cl solution.
Ans. 4.97
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Calculating pH of Salt Solutions
Ex 2. A 0.100mol/L NaCN solution has a pH of 11.15.
a) Calculate the Kb of CN-
Ans: 2.0x10-5
b) What is the Ka of its conjugate acid?Ans: 5.0x10 -10