CHEM&163 (S2016)
1.
Chapter 15 – Acids and Bases
Lecture Outline (M. Dunn)
Acid Base theory by Three Definitions
Lecture Slide
2. Acid/Base Theory:
•
Arrhenius Acids and Bases
•
Acid: Released H+ in water (H3O+)
•
Base: Releases OH- in water
•
Bronsted-Lowry
•
Acid: Proton (H+) Donor
•
Base: Proton (H+) Acceptor
•
Lewis
•
•
My notes/reminders etc.
Acid: Electron pair Acceptor
Base: Electron pair Acceptor
Which definition is the most narrow? Which is the most broad?
3. Write a reaction for HNO3(aq) showing that it is an Arrhenius acid:
What is the relationship between H+(aq) and H3O+(aq) ?
Why is H+ often referred to as a proton?
4. Rank the following solutions from most acidic to most basic (water molecules have
been omitted for
clarity).
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Explain why:
CHEM&163 (S2016)
Chapter 15 – Acids and Bases
5. Show the proton (H+) transfer in this Bronsted-Lowry Acid/Base reaction
H3O+ + NH3 ⇌ H2O + NH4+
Which compound is the acid?
Why?
Which compound is the base?
Why?
Could this example be used for an Arrhenius acid/base?
Conjugate Acid/Base Pairs:
Consider the backwards reaction
Which compound is the acid?
Why?
Which compound is the base?
Why?
Label the CONJUGATE Acid and CONJUGATE Base
Draw a line connecting the Acid to the Conjugate Base. What is the relationship
between these two species?
Draw a line connecting the Base to the Conjugate Acid. What is the relationship
between these two species?
6. Show the proton (H+) transfer in this Bronsted-Lowry Acid/Base reaction
HNO3
+
H2O
 H3O+ + NO3-
Which compound is the acid?
Why?
Which compound is the base?
Why?
Label the CONJUGATE Acid and CONJUGATE Base
How did you figure out the conjugates?
Could this example be used to illustrate an Arrhenius acid/base reaction? Explain
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Lecture Outline (M. Dunn)
CHEM&163 (S2016)
Chapter 15 – Acids and Bases
7. Codeine, C18H21O3N is a Bronsted-Lowry (BL) base…
What is the conjugate acid of codeine?
Write a net ionic equation to show codeine behaving as a BL base in water.
Label the acid/base pairs
Note Nitrogen containing compounds (neutral charge) often behave a BL bases.
8. Water is amphoteric. Define amphoteric:
Note: For our purposes, amphoteric is the same thing as amphiprotic
Referring to the reactions we’ve already examined,
•
Give an example of water acting as an ACID (BL)
•
Give an example of water acting as a BASE (BL)
9. Lewis Acid Base theory:
Reminder, in Lewis acid/base theory, the model is about accepting or donating
____________. The acid is the ____________________________.
The base is the _____________________________.
In the following reaction, label the Lewis acid and Lewis base:
The product contains a coordinate covalent bond. Define covalent bond?
Label the coordinate covalent bond in the reaction.
Could this reaction be used to illustrate Bronsted-Lowry Acid/Base theory? Why or
Why not?
Could this reaction be used to illustrate Arrhenius Acid/Base theory? Why or Why
not?
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Lecture Outline (M. Dunn)
CHEM&163 (S2016)
Chapter 15 – Acids and Bases
Lecture Outline (M. Dunn)
10. Ammonia is a base by ALL three definitions. Write a reaction equation
illustrating this according to each of the three acid/base theories.
Arrhenius:
Bronsted/Lowry:
Lewis:
2. Strong Vs Weak Acids and Bases
11.
1.
2.
12. Write the generic BL reaction equation for a strong
acid in water:
13. Write the generic BL reaction equation for a weak
acid in water.
14. Strength of Acids and Bases
• Strong Acid – Ionizes completely in aqueous
solution
• Strong Base – Ionizes completely in aqueous
solution
Solution composition:
•
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Weak Bases- Partially ionizes in aqueous
solution
NH3 + H2O ⇌ NH4+ + H3O+
Solution composition:
What do the symbols mean?
What are the differences between a strong acid and weak
acid:
a. Degree of ionization
b.
Composition of aqueous solution
c.
Conductor of electricity
d.
Anything else?
CHEM&163 (S2016)
Chapter 15 – Acids and Bases
15. The Strong Acids/Bases (These should be
memorized!)
If it doesn’t appear on this list it is WEAK!
Strong acids
Strong Bases
• HClO4
• LiOH
• H2SO4
• NaOH
• HNO3
• KOH
• HCl
• Ca(OH)2
• HBr
• Sr(OH)2
• HI
• Ba(OH)2
16 . Relative Strengths of Acids and Bases
The stronger an acid, the weaker its conjugate base.
The weaker an acid, the stronger its conjugate base.
18. Generic form of BL reaction equation for acid in
water.
Generic form for BL reaction equation for anion (base)
in water.
Write the BL reaction of formic acid with water.
Write the BL reaction of Acetic acid in water.
Write the BL reaction equation of formate ion in water.
Write the reaction of acetate ion in water.
Formic acid, HCHO2, is a stronger acid than acetic acid,
HC2H3O2. Which is the stronger base: formate ion,
CHO2−, or acetate ion, C2H3O2−?
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17.
Lecture Outline (M. Dunn)
CHEM&163 (S2016)
POGIL: Relative Strength of Acids
Chapter 15 – Acids and Bases
Lecture Outline (M. Dunn)
Why? Many molecules of interest behave as acids or bases. However, not all acids and bases have the same
strength. Since the chemical behavior of acids and bases depends upon their strength, knowledge of relative
acidity and basicity can be the key to predicting chemical reactivity.
Learning Objective:
Relate previous learned concepts to the relative strength of acids and bases, including electronegativity,
resonance and bond energy, and energy profile diagrams.
Information:
The diagram in Model 1 shows the free energy change for the ionization of an acid. The energy difference
between the products and reactants determines strength of an acid. The underlying principle is that the
systems seek the lowest energy possible. If the products are lower in energy, the reaction will favor the
products (equilibrium lies to the right). If the reactants are lower in energy, the reaction will favor the
reactants (equilibrium lies to the left). All weak acids will have a positive value for ΔErxn.
MODEL 1: Reaction Profile showing the Energy Change for the ionization of a weak
Acid
EA
Energy
ΔErxn
Reaction progress
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CHEM&163 (S2016)
Questions:
Chapter 15 – Acids and Bases
Lecture Outline (M. Dunn)
1. What is more stable, the acid HA or the ionized products (H+ + A-)? ? How do you know this?
2. The equilibrium constant for the dissociation of weak acids is known as Ka. Write the equilibrium
expression (aka law of mass action) for the disassociation of the weak acid HA.
3. Is the Ka value larger than, smaller than, or equal to 1?
4. Draw a sketch similar to the energy profile show in in Model 1 for the weak acid, H-B, which is a
stronger acid than H-A. Pay particular attention to the difference in energy between the reactants and
products of the two reactions.
5. Which is stronger base, B- or A-?
6. Sketch a reaction profile similar to that in the model but for a strong acid that completely ionizes in
water: H-X. Clearly show the energy difference between the reactant and products.
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CHEM&163 (S2016)
Chapter 15 – Acids and Bases
Structure Affects the Strength of an Acid
19. In determining the strength of the acid, the main issue is how
easily the proton is removed
Easily removed = _______________________
Tightly held = __________________________
There are THREE guidelines
20. Guideline #1
Strength of bond depends on size of atom
How does bond strength correspond to acid strength?
•
•
H-X; large X = strong acid
Within a column (group), size is the main issue
Compare acid strength
HF
HCl
HBr
HI
21. Guideline #2
The first one is easy…… (polyprotic acids)
H2SO4 vs HSO4-
H3PO4 vs H2PO4 vs HPO42-
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Lecture Outline (M. Dunn)
CHEM&163 (S2016)
Chapter 15 – Acids and Bases
22. Guideline #3
The more polarized the bond is, the greater the acid strength
•
Polarized bond
•
Electron with drawing groups weaken the bond to H
•
Think electronegativity
•
When comparing two anions in the same row,
electronegativity differences are more important than size
differences. When comparing within a column size is more
important.
23. Electron withdrawing groups weaken the bond (see guideline #3)
COMPARE: H3N, H2O, HF
24. Sometimes you need to look down chain to find a difference in
electron withdrawing power (See guideline #3)
Which acid is strongest/weakest? Why?
25. Which acid is strongest/weakest? Why?
26. Not all H’s within a structure are equally acidic.
Draw a Lewis structure for sulfuric acid and acetic acid. Which H’s
are acidic? Why?
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Lecture Outline (M. Dunn)
CHEM&163 (S2016)
Chapter 15 – Acids and Bases
Lecture Outline (M. Dunn)
27. The issues summarized.
1. In a column/group SIZE matters (as X gets bigger, acid gets
stronger)
Only the size of the atom directly attached to the H
matters. (Down chain doesn’t matter)
2.
In a row/period ELECTRON withdrawing matters (more
electronegativity = stronger acid)
If there is no difference in atoms directly attached to the H,
look down chain
3.
In a polyprotic acid, the first one is easy (more H’s=
stronger acid)
28. Concept Check: In each pair, consider the structures then predict which acid is the stronger one.
Acids
1. NH3 or PH3
2.
HI or H2Te
3.
H2SO3 or HSO3-
4.
H3AsO4 or H3PO4
5.
HSO4- or HSeO4-
Key Issue
Conclusion
Justification
INFORMATION: Resonance structures that increase stability are those that delocalize electrons over the
molecule and place negative charge on electronegative atoms. For example, HNO3 is a strong acid and HNO2
is a weak acid because HNO3 has more resonance structures that place the negative charge on difference
oxygen atoms.
Questions:
7. Draw the Lewis resonance structures of NO3- and NO2- and use them to explain why nitric acid is a strong
acid and nitrous acid is a weak acid.
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CHEM&163 (S2016)
Application:
Chapter 15 – Acids and Bases
1. Explain why ethanol (
Lecture Outline (M. Dunn)
) is considerably more acidic than ethane (CH3CH3).
2. Give one reason why acetic acid (
) is a stronger acid than ethanol?
3. Predict which hydrogen in aspartic acid,
your reasoning.
, is the most acidic. Explain
Determining the position of equilibrium based on Acid Strength
31-34. The crabapple analogy
•
Think of the “crab apple” as the proton
•
Strength of acid refers to how quickly or aggressively those crab apples are thrown.
•
This is a “WAR” of the acids
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CHEM&163 (S2016)
Chapter 15 – Acids and Bases
Lecture Outline (M. Dunn)
35. What we learn from the crab apple wars: How
do we determine the equilibrium position?
Remember what “position” of equilibrium means?
Example:
CH3COOH
+
NH3
⇌
CH3COO- + NH4+
1.
Identify the acids (one on each side)
2.
Which acid is stronger: (See table)
3.
Identify the stronger base (the stronger acid
gives the weaker conjugate base)
4.
Strongest acid and base react to give weaker
acid and base
36. You try one: For the following reaction,
equilibrium lies to the _______ because _________
is the stronger acid.
C6H5OH + H2PO4-
⇌
H3PO4 + C6H5O-
Acidic and Basic Properties of Pure Water
37-38. Write the reaction for the autoionization of water.
This means there are small amounts of ions present.
So…. What is the Keq expression?
What does Kw mean?
Meaning of
 ‘pH’

SIG FIG RULE of Logs:
# of digits in the mantissa = # of sig figs in the original
number
mantissa
‘pOH’
log(3.000x104) should be rounded to 4.4771
log(3x104) = 4.477121, but this value should be rounded to 4.5
antilog(0.301) = 1.9998, which should be rounded to 2.00,
antilog(0.30) = 1.9998, which should be rounded to 2.0
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CHEM&163 (S2016)
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Chapter 15 – Acids and Bases
Lecture Outline (M. Dunn)
CHEM&163 (S2016)
Chapter 15 – Acids and Bases
Lecture Outline (M. Dunn)
Proof of the relationship between pH and pOH
Acidic/basic
pH
pOH
Example solution
0
1 M HCl
1
Gastric juices pH=1.0-3.0
2
Lemon Juice
3
Vinegar/Diet Coke
4
Tomato Juice
5
Coffee
6
Unpolluted rain
7
Pure water
8
Pancreatic fluid
9
Soap
10
Milk of Magnesia
11
12
Household Ammonia
13
14
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Lye (1.0 M NaOH)
[H+] (M)
[ OH-] (M)
CHEM&163 (S2016)
Chapter 15 – Acids and Bases
pH, pOH
[H+] (M)
Lecture Outline (M. Dunn)
[OH-] (M)
[H+][OH-]= 1X10-14
( this is sometimes given as
[H3O+])
pOH
pH+pOH=14
pH=7 neutral
pH>7 base
p<7 acid
[H+]
[OH-]
pH
pOH
1.0E-6 M
1.0E-8 M
9.00
7.00
2.5E-9M
3.3E-5 M
3.25
9.6
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Acid/Base/or
Neutral
pOH=-log[OH-]
[OH-]=10-pOH
pH=-log[H+]
[H+]=10-pH
pH
CHEM&163 (S2016)
[H+]
Chapter 15 – Acids and Bases
[OH-]
pH
Lecture Outline (M. Dunn)
pOH
3.8E-6 M
9.4E-8 M
9.55
6.53
6.8E-9M
1.6E-5 M
2.25
6.75
Answer key to be published on CANVAS.
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Acid/Base/or
Neutral
CHEM&163 (S2016)
Chapter 15 – Acids and Bases
Lecture Outline (M. Dunn)
Strategy Questions: Acid/Base pH calculations
1.
You want to make up 3.00 L of aqueous hydrochloric acid, HCl(aq), that has a pH of 2.00. How many grams of
concentrated hydrochloric acid will you need? Concentrated hydrochloric acid contains 37.2 % HCl (wt/wt).
This problem is an application of the following concepts:




pH scale
Degree of ionization of a strong acid
Concentration terms, definitions and conversion between different terms (strict unit analysis).
Dilution
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CHEM&163 (S2016)
Chapter 15 – Acids and Bases
Lecture Outline (M. Dunn)
What is the pH of a solution in which 15 mL of 0.1 M NaOH is added to 25 mL of 0.10 M HCl?



Limiting reactant problem:
RICE TABLE method
There is a dilution factor
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CHEM&163 (S2016)
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Chapter 15 – Acids and Bases
Lecture Outline (M. Dunn)