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