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Acids and Bases
Section 16.1
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Models of Acids and Bases
• Arrhenius: Acids produce H+ ions in solution, bases produce OH– ions.
• Brønsted–Lowry: Acids are proton (H+) donors, bases are proton acceptors.
HCl + H2O → Cl– + H3O+
acid base
Acids and Bases
Section 16.1
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Acid in Water
HA(aq) + H2O(l) H3O+(aq) + A–(aq)
acid base conjugate conjugateacid base
• Conjugate base is everything that remains of the acid molecule after a proton is lost.
• Conjugate acid is formed when the proton is transferred to the base.
Acids and Bases
Section 16.1
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• Water acts as a base accepting a proton from the acid.
• Forms hydronium ion (H3O+).
Acids and Bases
Section 16.1
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Concept Check
Which of the following represent conjugate acid–base pairs?
a) HCl, HNO3
b) H3O+, OH–
c) H2SO4, SO42–
d) HCN, CN–
Acid Strength
Section 16.2
Strong Acid
• Completely ionized or completely dissociated
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Acid Strength
Section 16.2
Weak Acid
• Most of the acid molecules remain intact.
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Acid Strength
Section 16.2
Behavior of Acids of Different Strengths in Aqueous Solution
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Acid Strength
Section 16.2
• A strong acid contains a relatively weak conjugate base.
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Acid Strength
Section 16.2
Ways to Describe Acid Strength
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Acid Strength
Section 16.2
Common Strong Acids
• Sulfuric acid, H2SO4
• Hydrochloric acid, HCl
• Nitric acid, HNO3
• Perchloric acid, HClO4
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And HI and HBr
Acid Strength
Section 16.2
• Oxyacid – acidic proton is attached to an oxygen atom
• Organic acids – have a carbon atom backbone and commonly contain the carboxyl group:
Typically a weak acid
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Acid Strength
Section 16.2
Concept Check
Consider a 1.0 M solution of HCl.
Order the following from strongest to weakestbase and explain:
H2O(l)
A–(aq) (from weak acid HA)
Cl–(aq)
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Acid Strength
Section 16.2
Let’s Think About It…
• How good is Cl–(aq) as a base?
• Is A–(aq) a good base?
The bases from strongest to weakest are:
A–, H2O, Cl–
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Acid Strength
Section 16.2
Concept Check
Acetic acid (HC2H3O2) and HCN are both weak acids. Acetic acid is a stronger acid than HCN.
Arrange these bases from weakest to strongestand explain your answer:
H2O Cl– CN– C2H3O2–
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Acid Strength
Section 16.2
Let’s Think About It…
• H2O(l) + H2O(l) H3O+(aq) + OH–(aq)
acid base conjugate conjugateacid base
• At 25C, Kw = 1.0 x 10–14
The bases from weakest to strongest are:
Cl–, H2O, C2H3O2–, CN–
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Water as an Acid and a Base
Section 16.3
Water as an Acid and a Base
• Water is amphoteric:
Behaves either as an acid or as a base.
• At 25°C:
Kw = [H+][OH–] = 1.0 × 10–14
• No matter what the solution contains, the product of [H+] and [OH–] must always equal 1.0 × 10–14 at 25°C.
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Water as an Acid and a Base
Section 16.3
Three Possible Situations
• [H+] = [OH–]; neutral solution
• [H+] > [OH–]; acidic solution
• [H+] < [OH–]; basic solution
In each case, however,
Kw = [H+][OH–] = 1.0 × 10–14.
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Water as an Acid and a Base
Section 16.3
Concept Check
In an acidic aqueous solution, which statement below is correct?
a) [H+] < 1.0 × 10–7 Mb) [H+] > 1.0 × 10–7 Mc) [OH–] > 1.0 × 10–7 Md) [H+] < [OH–]
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Water as an Acid and a Base
Section 16.3
Exercise
In an aqueous solution in which [OH–] = 2.0 x 10–10 M, the [H+] = ________ M, and the solution is ________.
a) 2.0 × 10–10 M ; basicb) 1.0 × 10–14 M ; acidicc) 5.0 × 10–5 M ; acidicd) 5.0 × 10–5 M ; basic
[H+] = Kw/[OH–] = 1.0 × 10–14/2.0 × 10–10 = 5.0 × 10–5 M.Since [H+] is greater than [OH–], the solution is acidic.
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The pH Scale
Section 16.4
• pH = –log[H+]
• A compact way to represent solution acidity.
• pH decreases as [H+] increases.
• Significant figures:
The number of decimal places in the log is equal to the number of significant figures in the original number.
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pOH = -log[OH-]
The pH Scale
Section 16.4
pH Range
• pH = 7; neutral
• pH > 7; basic
Higher the pH, more basic.
• pH < 7; acidic
Lower the pH, more acidic.
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The pH Scale
Section 16.4
The pH Scale and pH Values of Some Common Substances
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The pH Scale
Section 16.4
Exercise
Calculate the pH for each of the following solutions.
a) 1.0 × 10–4 M H+
pH = –log[H+] = –log(1.0 × 10–4 M) = 4.00
b) 0.040 M OH–
Kw = [H+][OH–] = 1.00 × 10–14 = [H+](0.040 M) = 2.5 × 10–13 M H+
pH = –log[H+] = –log(2.5 × 10–13 M) = 12.60
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The pH Scale
Section 16.4
Exercise
The pH of a solution is 5.85. What is the [H+] for this solution?
[H+] = 1.4 × 10–6 M
[H+] = 10^–5.85 = 1.4 × 10–6 M
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The pH Scale
Section 16.4
pH and pOH
• Recall:
Kw = [H+][OH–]
–log Kw = –log[H+] – log[OH–]
pKw = pH + pOH
14.00 = pH + pOH
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The pH Scale
Section 16.4
Exercise
Calculate the pOH for each of the following solutions.
a) 1.0 × 10–4 M H+
pH = –log[H+] = –log(1.0 × 10–4 M) = 4.00;
So, 14.00 = 4.00 + pOH; pOH = 10.00
b) 0.040 M OH–
pOH = –log[OH–] = –log(0.040 M) = 1.40
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The pH Scale
Section 16.4
Exercise
The pH of a solution is 5.85. What is the [OH–] for this solution?
[OH–] = 7.1 × 10–9 M
14.00 = 5.85 + pOH; pOH = 8.15
[OH–] = 10^–8.15 = 7.1× 10–9 M
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Calculating the pH of Strong Acid Solutions
Section 16.5
• Determine the [H+].
2.0 M HCl → 2.0 M H+ and 2.0 M Cl–
• pH = log[H+]
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Calculating the pH of Strong Acid Solutions
Section 16.5
Concept Check
Consider an aqueous solution of 2.0 × 10–3
M HCl. What is the pH?
pH = 2.70
2.0 × 10–3 M HCl → 2.0 × 10–3 M H+ and 2.0 × 10–3 M Cl–
pH = –log[H+] pH = –log(2.0 × 10–3 M)pH = 2.70
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Calculating the pH of Strong Acid Solutions
Section 16.5
Concept Check
Calculate the pH of a 1.5 × 10–11 M solution of HCl.
pH = 7.00
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Calculating the pH of Strong Acid Solutions
Section 16.5
Concept Check
Calculate the pH of a 1.5 × 10–2 M solution of HNO3.
pH = 1.82
pH = –log[H+] pH = –log(1.5 × 10–2 M)pH = 1.82
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Buffered Solutions
Section 16.6
• Buffered solution – resists a change in its pH when either an acid or a base has been added.
Presence of a weak acid and its conjugate base buffers the solution.
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You are NOT responsible for Buffers…
Buffered Solutions
Section 16.6
The Characteristics of a Buffer
1. The solution contains a weak acid HA and its conjugate base A–.
2. The buffer resists changes in pH by reacting with any added H+ or OH– so that these ions do not accumulate.
3. Any added H+ reacts with the base A–.
H+(aq) + A–(aq) → HA(aq)
4. Any added OH– reacts with the weak acid HA.
OH–(aq) + HA(aq) → H2O(l) + A–(aq)
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Buffered Solutions
Section 16.6
Concept Check
If a solution is buffered with NH3 to which has been added NH4Cl, what reaction will occur if a strong base such as NaOH is added?
a) NaOH + NH3 → NaNH4Ob) NaOH + NH4
+ → Na+ + NH3 + H2Oc) NaOH + Cl– → NaCl + OH–
d) NaOH + H2O → NaH3O2
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