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Chapter 14: Acids & Bases. Dr. Aimée Tomlinson. Chem 1212. Section 14.1. Acid-Base Concepts: The Brønsted -Lowry Theory. Three Theories for Acids & Bases. Arrhenius acids & bases. Brønsted -Lowry acids & bases. Lewis acids & bases. Conjugate Acid-Base Pairs. - PowerPoint PPT Presentation
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CHAPTER 14: ACIDS & BASES
Dr. Aimée TomlinsonChem1212
Acid-Base Concepts:The Brønsted-Lowry
Theory
Section 14.1
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
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-
Examples5
3( ) 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
Acid & Base Strength
Section 14.2
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
Strong Bases
A strong base will completely dissociate/ionize:
List of Strong bases: MOH (M=alkali metal), NH2-, H-
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
Hydrated Protons & Hydronium Ions
Section 14.3
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
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
Dissociation of Water
Section 14.4
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
ExampleDetermine the hydroxide concentration in a solution with =1.89 x 10-4 M.
The pH Scale
Section 14.5
Power of Hydrogen aka pH
pH < 7.0 acidic
3 aqpH log H O
pH = 7.0 neutral
pH > 7.0 basic
Power of Hydroxide aka pOH
pH > 7.0 acidic
aqpH log OH
pH = 7.0 neutral
pH < 7.0 basic
Relationship for pH, pOH & Kw
Measuring pH
Section 14.6
pH IndicatorsMore relevant in Chapter 15 so we will address it more fully there
pH in Solutions of Strong Acids & Strong
Bases
Section 14.7
The Strong Completely DissociateH3O+/OH- concentrations will become whatever those of the strong acids or bases were
Example IEXAMPLE: 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)
Example IIDetermine the hydronium ion concentration for a 0.01500 M Ca(OH)2 assuming complete dissociation.
Equilibria of Weak Acids
Section 14.8
Weak Acids & EquilibriumUnlike 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
Ka & Acid StrengthThe 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
Section 14.9
Calculating the Equilibria of Weak Acids
Weak Acids & EquilibriumCalculate [H+] and the pOH of 0.050M of benzoic acid. Ka = 6.5 x 10-5
Weak Acid Flowchart
Section 14.10
Percent Dissociation of Weak Acids
Percent DissocationDegree of ionization/dissociation: percentage that an acid ionizes
Example: Determine the percent dissociation of 0.050M of benzoic acid.
( ) ( ) ( )[ ] 100%[ ]aq aq aqHHA H AHA
Section 14.11
Polyprotic Acids
Polyprotic AcidsAcids which possess more than one proton
Polyprotic Acid ExampleCalculate the [H+] of 0.050M of sulfuric acid.
2 4( ) ( ) 4( )
2 24( ) ( ) 4( )
1
1.2 10aq aq aq a
aq aq aq a
H SO H HSO K
HSO H SO K
Polyprotic Acid Flowchart
Why Ka1 > Ka2
2 4( ) ( ) 4( )
2 24( ) ( ) 4( )
1
1.2 10aq 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
Section 14.12
Equilibria of Weak Bases
Weak Base EquilibriaCalculate pH of 0.050M of ammonia. Kb = 1.8 x 10-5
Weak Base Flowchart
Section 14.13
The Relationship Between Ka & Kb
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
ExampleDetermine the Kb of HCN if Ka = 4.9 x 10-10.
Section 14.14
Acid/Base Properties of Salts
Stronger Partner Dominates Strong acid + weak base = acidic solution Weak acid + strong base = basic solution Strong acid + strong base = neutral
solutionExample: Classify each of the following as acidic, basic, or neutral.
1.) KBr 2.) NaNO23.) NH4Cl
What if both are weak?Example II: Classify NH4CN as acidic, basic, or neutral.
Finding pH/pOH of a Salt SolutionCalculate the pH of a 0.25M NaC2H3O2, Ka = 1.76x10-5
Salt Flowchart
Section 14.15
Factors that Affect Acid Strength
Recall Electronegativity Trend
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:
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:
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
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
Amine Base TrendsIncreasing 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
Section 14.16
Lewis Acids and Bases
Definitions
Lewis AcidElectron-pair acceptor
Lewis BaseElectron-pair donor
3 3 3 3NH BF H NBF