chapter 14: acids & bases dr. aimée tomlinson chem 1212
TRANSCRIPT
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CHAPTER 14: ACIDS & BASES
Dr. Aimée TomlinsonChem1212
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Acid-Base Concepts:The Brønsted-Lowry
Theory
Section 14.1
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Three Theories for Acids & Bases
Arrhenius acids & bases
Brønsted-Lowry acids & bases
Lewis acids & bases
(aq) (aq) (aq)acid: an H+ donor HA H A
(aq) (aq) (aq)base: an OH- donor MOH M OH
We will see this type at the end of the chapter
(aq) (aq) (aq)acid: an H+ donor HA H A
(aq) (aq) (aq)base: H+ acceptor H B BH
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Conjugate Acid-Base Pairs
conjugate acid: the acid that is created after the Brønsted-Lowry base has accepted the proton, BH+
conjugate base: the base that is created after the Brønsted-Lowry acid has donated the proton, A-
Examples
53( ) 2 ( ) 4( ) ( ): 1.76 10g l aq aq b
base acid conj acid conj base
basic NH H O NH OH K
2 3: 1aacid conj basebase conj acid
acidic HCl H O Cl H O K
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Acid & Base Strength
Section 14.2
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Strong Acids
A strong acid will completely dissociate/ionize: All the reactant goes to product/single-headed arrow The product is a very weak conjugate acid/base pair
List of Strong acids: HCl, HBr, H2SO4, HI, HClO4, HClO3, HNO3
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Strong Bases
A strong base will completely dissociate/ionize:
List of Strong bases: MOH (M=alkali metal), NH2-, H-
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Weak Acids
Only partially dissociate
The eq constant is called Ka where “a” for acid There is always some reactant still present at
eq unlike the strong acid case The larger the Ka the stronger the acid
E.g. Ka >> 1 for HNO3
We will come back to this in a little bit
42( ) ( ) 2( ) 4.0 10aq aq aq aHNO H NO K
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Hydrated Protons & Hydronium Ions
Section 14.3
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Meet Hydronium
H3O+ is acidified water or what truly happens when H+ is in H2O
We call this ion hydronium
We use H+ and H3O+ interchangeably
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Amphoterism
Defn: A species that may act as both an acid and a base
Water as a base:
Water as an acid:
(aq) 2 (l) 3 (aq) (aq)HA H O H O A
(aq) 2 (l) (aq) (aq)B H O BH OH
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Dissociation of Water
Section 14.4
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What’s in Water & What it Means
=
2𝐻2𝑂 (𝑙 )⇌𝑂𝐻 (𝑎𝑞)− +𝐻 3𝑂 (𝑎𝑞 )
+¿ ¿
NOTE H2O(l) as always is not in the equilibrium expression
Relationship between [OH-(aq)] and [H3O+
(aq)]:
For both ions their concentrations at 298 K is 1.0 x 10-
7M making the Kw = 1.0 x 10-14
3 ( ) ( )
3 ( ) ( )
3 ( ) ( )
[ ] [ ]
[ ] [ ]
[ ] [ ]
aq aq
aq aq
aq aq
H O OH acidic
H O OH neutral
H O OH basic
7 7 14(1.0 10 )(1.0 10 ) 1.0 10wK
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Example
Determine the hydroxide concentration in a solution with =1.89 x 10-4 M.
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The pH Scale
Section 14.5
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Power of Hydrogen aka pH
pH < 7.0 acidic
3 aqpH log H O
pH = 7.0 neutral
pH > 7.0 basic
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Power of Hydroxide aka pOH
pH > 7.0 acidic
aqpH log OH
pH = 7.0 neutral
pH < 7.0 basic
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Relationship for pH, pOH & Kw
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Measuring pH
Section 14.6
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pH Indicators
More relevant in Chapter 15 so we will address it more fully there
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pH in Solutions of Strong Acids & Strong
Bases
Section 14.7
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The Strong Completely Dissociate
H3O+/OH- concentrations will become whatever those of the strong acids or bases were
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Example I
EXAMPLE: Write the balanced equation for each of the following and determine the pH.1.) 0.5000 M HClO4(aq)
2.) 0.0256 M LiOH(aq)
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Example II
Determine the hydronium ion concentration for a 0.01500 M Ca(OH)2 assuming complete dissociation.
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Equilibria of Weak Acids
Section 14.8
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Weak Acids & Equilibrium
Unlike the strong they only partially dissociate in water hence HA is still present at eq:
( ) 2 ( ) 3 ( ) ( ) ( ) ( ) ( )aq l aq aq aq aq aqHA H O H O A or HA H A
3 ( ) ( ) ( ) ( )aq aq aq aq
a
H O A H AK
HA HA
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Ka & Acid Strength
The larger the Ka:
More strongly the eq will lie toward product
More likely the acid is to dissociate
The larger the [H3O+]
The lower the pH
The stronger the acid
Ka is large for strong acid HCl but very small for weak acid CH3OH
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Section 14.9
Calculating the Equilibria of Weak Acids
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Weak Acids & Equilibrium
Calculate [H+] and the pOH of 0.050M of benzoic acid. Ka = 6.5 x 10-5
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Weak Acid Flowchart
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Section 14.10
Percent Dissociation of Weak Acids
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Percent Dissocation
Degree of ionization/dissociation: percentage that an acid ionizes
Example: Determine the percent dissociation of 0.050M of benzoic acid.
( ) ( ) ( )
[ ]100%
[ ]aq aq aq
HHA H A
HA
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Section 14.11
Polyprotic Acids
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Polyprotic Acids
Acids which possess more than one proton
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Polyprotic Acid Example
Calculate the [H+] of 0.050M of sulfuric acid.
2 4( ) ( ) 4( )
2 24( ) ( ) 4( )
1
1.2 10
aq aq aq a
aq aq aq a
H SO H HSO K
HSO H SO K
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Polyprotic Acid Flowchart
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Why Ka1 > Ka2
2 4( ) ( ) 4( )
2 24( ) ( ) 4( )
1
1.2 10
aq aq aq a
aq aq aq a
H SO H HSO K
HSO H SO K
Electrostatically it is more difficult to remove H+ from SO42- than
from HSO4-
Hence Ka2 is always smaller than Ka1 and so on
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Section 14.12
Equilibria of Weak Bases
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Weak Base Equilibria
Calculate pH of 0.050M of ammonia. Kb = 1.8 x 10-5
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Weak Base Flowchart
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Section 14.13
The Relationship Between Ka & Kb
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The Link Between Ka & Kb is Kw
2 3
2
2 3
2
a
b
w
HCN H O CN H O K
CN H O HCN OH K
H O H O OH K
a b wK K K
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Example
Determine the Kb of HCN if Ka = 4.9 x 10-10.
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Section 14.14
Acid/Base Properties of Salts
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Stronger Partner Dominates
Strong acid + weak base = acidic solution Weak acid + strong base = basic solution Strong acid + strong base = neutral
solution
Example: Classify each of the following as acidic, basic, or neutral.
1.) KBr 2.) NaNO2
3.) NH4Cl
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What if both are weak?
Example II: Classify NH4CN as acidic, basic, or neutral.
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Finding pH/pOH of a Salt Solution
Calculate the pH of a 0.25M NaC2H3O2, Ka = 1.76x10-5
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Salt Flowchart
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Section 14.15
Factors that Affect Acid Strength
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Recall Electronegativity Trend
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EN Trend I
As we go down a column we decrease EN We thereby weaken the H-X bond Allows H+ to more readily go into solution Acid strength: HF < HCl < HBr < HI
Increasing acid strength going down the table:
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EN Trend II
As we go across we increase EN We make the H-X bond polar This eventually gives an EN difference which
leads to H+ Acid strength: CH4 < NH3 < H2O < HF
Increasing acid strength from left to right in the table:
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Oxoacids Trend I – more EN
As increase the EN of the halogen X we weaken the O-H bond
This is done by pulling electron density from the O-atom
This will allow the H+ to break-away more eqsily and go into solution
Acid strength: HOI < HOBr < HOCl < HOF
An oxoacid is any acid with acidic proton connected to an O-atom – they have the form HnXOm
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Oxoacids Trend II
As increase the number of O-atoms weakens the O-H bond
Again this is done by pulling electron density from the O-atom
This will allow the H+ to break-away more easily and go into solution
Acid strength: HOCl < HO2Cl < HO3Cl < HO4Cl
Increasing the number of O-atoms increases acid strength
Acid Oxidiation State of Cl
Ka
HClO +1 2.9 x 10-8
HClO2 +3 1.1 x 10-2
HClO3 +5 1HClO4 +7 1 x 108
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Amine Base Trends
Increasing the number of electro-donating groups will increase base strength
Increasing the number of electron-withdrawing/EN groups will decrease base strength
3 2 3 3 32 3NH NH CH NH CH N CH
3 2 2 3NH NH Cl NHCl NCl
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Section 14.16
Lewis Acids and Bases
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Definitions
Lewis AcidElectron-pair acceptor
Lewis BaseElectron-pair donor
3 3 3 3NH BF H NBF