Acids & Bases Lesson 2 Strong and Weak (Bases). Review of Bronsted- Lowry Acids.
Chapter 13: Acids and Bases€¦ · 13-3 Chapter 13 Topics 1. What Are Acids and Bases? 2. Strong...
Transcript of Chapter 13: Acids and Bases€¦ · 13-3 Chapter 13 Topics 1. What Are Acids and Bases? 2. Strong...
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Chapter 13:
Acids and
Bases
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Questions for Consideration
1. How do acids and bases differ from other
substances?
2. How do strong and weak acids differ?
3. How can we compare the different strengths of
weak acids?
4. What causes aqueous solutions to be acidic or
basic?
5. How is pH related to acidity and basicity?
6. What is a buffered solution?
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Chapter 13 Topics
1. What Are Acids and Bases?
2. Strong and Weak Acids and Bases
3. Relative Strengths of Weak Acids
4. Acidic, Basic, and Neutral Solutions
5. The pH Scale
6. Buffered Solutions
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Acids and Bases Affect Our Lives
Figure 13.1
Figure 13.2
Figure 13.3
Figure from p. 536
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13.1 What are Acids and Bases?
◼ Acids and bases have properties that differ
from other substances:
◼ Acids taste sour.
◼ Bases taste bitter and feel slippery.
◼ Both change the color of some dyes.
◼ Acids cause many metals to corrode.
◼ Acids and bases combine to neutralize each
other.
CAUTION: Do not taste laboratory chemicals.
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Acid and Base Definitions
◼ 1800’s Arrhenius Model
◼ An acid in water produces hydrogen (H+) ions.
◼ A base in water produces hydroxide (OH−) ions.
◼ HCl(g) H+(aq) + Cl−(aq)
◼ NaOH(s) Na+(aq) + OH−(aq)
◼ Arrhenius earned the Nobel prize for his work that showed that H+(aq) and OH−(aq) ions are important in acid-base chemistry.
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The Hydronium Ion
◼ Problem with Arrhenius
Model:
◼ H+ does not exist
completely free in
aqueous solution. It
associates strongly with
other water molecules.
◼ Chemists recognize this
by representing an
aqueous H+ ion as
H3O+(aq), the hydronium
ion.
Figure 13.4
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Acid and Base Definitions
◼ 1923 Brønsted-Lowry definitions:
◼ An acid donates an H+ ion to another substance.
◼ A base accepts an H+ from another substance.
HCl(g) + H2O(l) → H3O+(aq) + Cl−(aq)
NH3(aq) + H2O(l) NH4+(aq) + OH−(aq)
◼ In an acid-base reaction, the acid donates (transfers) an H+ to a base.
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Activity: Brønsted-Lowry Acids and
Bases
◼ For each reaction, identify the Brønsted-Lowry acid
and base reactants.
1. OCl-(aq) + H2O(l) HOCl(aq) + OH-(aq)
2. H2SO4(aq) + F-(aq) → HSO4-(aq) + HF(aq)
3. NH4+(aq) + H2O(l) NH3(aq) + H3O
+(aq)
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Activity Solutions: Brønsted-Lowry
Acids and Bases◼ For each reaction, identify the Brønsted-Lowry acid
and base reactants.
1. OCl-(aq) + H2O(l) HOCl(aq) + OH-(aq)base acid
The water (H2O) donates an H+ to the OCl- to form
HOCl. The water is an acid. The OCl- accepts an H+
from the water (H2O), therefore it is the base.
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Activity Solutions: Brønsted-Lowry
Acids and Bases2. H2SO4(aq) + F-(aq) → HSO4
-(aq) + HF(aq)acid base
H2SO4, sulfuric acid, is the acid and it donates an H+
to the F-. F- accepts H+ from the sulfuric acid and is therefore the base.
3. NH4+(aq) + H2O(l) NH3(aq) + H3O
+(aq)acid base
NH4+ donates an H+ to the water and thus is the
acid. H2O accepts the H+ and is the base.
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Acid-base reactions can take place
without water:
◼ NH3(g) + HCl(g) NH4+(g) + Cl−(g)
◼ NH4+(g) + Cl−(g) NH4Cl(s)
Figure from p. 532
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Brønsted-Lowry Acids and Bases
◼ Many anions from dissolved ionic compounds act as bases:
Na2CO3(s) 2Na+(aq) + CO32−(aq)
◼ The carbonate ion acts as a base in water:
CO32−(aq) + H2O(l) HCO3
−(aq) + OH−(aq)base acid
◼ and with other acids:
HF(aq) + CO32−(aq) F−(aq) + HCO3
−(aq)acid base
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Conjugate Acid-Base Pairs
◼ Imagine the following reaction going in the reverse
direction. What would be the acid and what
would be the base?
HF(aq) + CO32−(aq) F−(aq) + HCO3
−(aq)
We call the acid and base products the conjugates
of the base and acid that formed them.
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Conjugate Acid-Base Pairs
HF(aq) + CO32−(aq) F−(aq) + HCO3
−(aq)
acid base conjugate conjugatebase acid
◼ A conjugate acid-base pair differs only by one
proton.
◼ The conjugate base of H3PO4 is H2PO4-, not PO4
3-.
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Activity: Conjugate Acid-Base Pairs
◼ Determine the formula of the conjugate base of each acid.
◼ Acid Conjugate Base
H2CO3
H3PO4
HPO42-
NH4+
H2O
HCO3−
H2PO4-
PO43−
NH3
OH−
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Activity: Conjugate Acid-Base Pairs
◼ Determine the formula of the conjugate acid of each base.
◼ Base Conjugate Acid
SO42−
HCO3−
NH2−
ClO2−
H2PO4 −
HSO4−
H2CO3
NH3
HClO2
H3PO4
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Activity: Conjugate Acid-Base Pairs
◼ Identify the acid and base reactants and their
conjugate acid and base:
◼ HCO3- + H3PO4 H2CO3 + H2PO4
-
base acid conjugate conjugateacid base
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Amphoteric Substances
◼ A substance that can act as either an acid or a base
◼ Water is the most common amphoteric substance.
Another common amphoteric substance is the
bicarbonate ion, HCO3-:
HCO3-(aq) + OH-(aq) → CO3
2-(aq) + H2O(l)
acid base conjugate conjugatebase acid
HCO3-(aq) + H3O
+(aq) → H2CO3(aq) + H2O(l)
base acid conjugate conjugateacid base
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Acidic Hydrogen Atoms
◼ If an acid has more
than one hydrogen
atom, we need to
determine which
hydrogen atoms are
acidic.
◼ In oxoacids, the acidic
hydrogen atoms are
bonded to hydrogen
atoms:.
Figure 13.5
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13.2 Strong and Weak Acids and
Bases
◼ Strong and weak acids and bases differ in
the extent of ionization.
◼ Strong acids ionize completely.
◼ Weak acids and bases ionize to only a small
extent – a small fraction of the molecules
ionize.
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Strong Acids
◼ Strong acids are strong electrolytes.
Figure 13.6
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Strong Acids
Table 13.1
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Strong Bases
◼ Strong bases are strong electrolytes.
Figure 13.7
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Strong Bases
*Although the group IIA (2) metal hydroxides are not completely water soluble, they
are strong bases because the amount that dissolves dissociates almost completely.
Table 13.2
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Weak Acids
◼ Weak acids are weak electrolytes
Figure 13.8
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Common Weak Acids
Table 13.3
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Weak Bases
◼ Weak bases are weak electrolytes
Figure 13.9
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Common Weak Bases
Table 13.4
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Strong and Weak Acids and Bases
HCl(aq) HF(aq)
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Identify each as a strong or weak
acid or base:
Figure from p. 563
A. weak base
B. weak acid
C. strong acid
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Activity: Acids and Bases on the
Molecular Level
◼ One of the diagrams below represents HClO4, and
the other represents an aqueous solution of HSO4-.
Which is which? Explain your reasoning.
Figure from p. 540
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Activity Solutions: Acids and Bases on
the Microscopic Level
◼ The picture on the left (A) shows no acid molecules and therefore shows an acid that has completely dissociated.
◼ Diagram A would then be a strong acid and of the two choices, perchloric acid (HClO4) is the strong one.
◼ Diagram B (the picture on the right) shows acid molecules that have not completely dissociated and are therefore the weak acid, HSO4
-.
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13.5 The pH Scale
◼ The pH of a solution is defined as the negative logarithm (base 10) of the H3O
+ concentration:
pH = −log[H3O+]
◼ What is the pH of solutions with the following hydronium ion concentrations?
1. [H3O+] = 10−1 M pH = 1
2. [H3O+] = 10−5 M pH = 5
3. [H3O+] = 10−7 M pH = 7
4. [H3O+] = 10−11 M pH = 11
Which are acidic
and
which are basic?
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The pH Scale
◼ Most aqueous solutions
have pH values that
range from 1-14.
◼ However, some
concentrated solutions
fall outside of this
range.
◼ What is the pH of a 1.0
M HCl solution?Figure 13.13
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The pH Scale
◼ By what factor does
[H3O+] change when
the pH changes
◼ from 8.0 to 9.0?
◼ from 4.0 to 2.0?
Figure 13.13
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Measuring pH
◼ pH meters and pH
indicators are often
used to determine the
pH of a solution.
Figure 13.17
Figure 13.19