1. Chapter 19 Acids, Bases, and Salts 19.3 Strengths of Acids and Bases
19.1 Acid-Base Theories
19.2 Hydrogen Ions and Acidity
19.3 Strengths of Acids and Bases
19.4 Neutralization Reactions
19.5 Salts in Solution
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2. What makes one acid safer than another?
CHEMISTRY & YOU
What makes one acid safer than another?
Lemon juice, which contains citric acid, has a pH of about 2.3. Yet, you consume lemon juice.
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3. Strong and Weak Acids and Bases
How are acids and bases classified as either strong or weak?
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4. Strong and Weak Acids and Bases
Acids and bases are classified as strong or weak based on the degree to which they ionize in water.
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5. In general, a strong acid is completely ionized in aqueous solution.
Strong and Weak Acids and Bases
In general, a strong acid is completely ionized in aqueous solution.
Hydrochloric and sulfuric acid are examples of strong acids.
HCl(g) + H2O(l) → H3O+(aq) + Cl–(aq) %
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6. A weak acid ionizes only slightly in aqueous solution.
Strong and Weak Acids and Bases
A weak acid ionizes only slightly in aqueous solution.
The ionization of ethanoic acid (CH3COOH), a typical weak acid, is not complete.
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7. Interpret Data
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8. Interpret Graphs
Dissociation of an acid (HA) in water yields H3O+ and an anion, A–. The bar graphs compare the extent of the dissociation of a strong acid and a weak acid.
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9. Acid Dissociation Constant
Strong and Weak Acids and Bases
Acid Dissociation Constant
A strong acid, such as hydrochloric acid, completely dissociates in water.
As a result, [H3O+] is high in an aqueous solution of strong acid.
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10. Acid Dissociation Constant
Strong and Weak Acids and Bases
Acid Dissociation Constant
By contrast, weak acids remain largely undissociated.
In an aqueous solution of ethanoic acid, less than 1 percent of the molecules are ionized.
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11. Acid Dissociation Constant
Strong and Weak Acids and Bases
Acid Dissociation Constant
You can use a balanced equation to write the equilibrium-constant expression for a reaction.
The equilibrium-constant expression shown below is for ethanoic acid.
Keq=
[H3O+] × [CH3COO–]
[CH3COOH] × [H2O]
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12. Acid Dissociation Constant
Strong and Weak Acids and Bases
Acid Dissociation Constant
The acid dissociation constant (Ka) is the ratio of the concentration of the dissociated form of an acid to the concentration of the undissociated form.
The dissociated form includes both the H3O+ and the anion.
Keq × [H2O] = Ka =
[H3O+] × [CH3COO–]
[CH3COOH] × [H2O]
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13. Acid Dissociation Constant
Strong and Weak Acids and Bases
Acid Dissociation Constant
The acid dissociation constant (Ka) reflects the fraction of an acid that is ionized.
For this reason, dissociation constants are sometimes called ionization constants.
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14. Acid Dissociation Constant
Strong and Weak Acids and Bases
Acid Dissociation Constant
If the degree of dissociation or ionization of the acid is small, the value of the dissociation constant will be small.
Weak acids have small Ka values.
If the degree of ionization of an acid is more complete, the value of Ka will be larger.
The stronger an acid is, the larger its Ka value will be.
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15. Acid Dissociation Constant
Strong and Weak Acids and Bases
Acid Dissociation Constant
Nitrous acid (HNO2) has a Ka of 4.4 × 10−4, but ethanoic acid (CH3COOH) has a Ka of 1.8 × 10−5.
This means that nitrous acid is more ionized in solution than ethanoic acid.
Nitrous acid is a stronger acid than ethanoic acid.
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16. Interpret Data
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17. Acid Dissociation Constant
Strong and Weak Acids and Bases
Acid Dissociation Constant
Some acids have more than one dissociation constant because they have more than one ionizable hydrogen.
Oxalic acid is a diprotic acid.
It loses two hydrogens, one at a time.
Therefore, it has two dissociation constants.
Oxalic acid is found naturally in certain herbs and vegetables.
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18. Acid Dissociation Constant
Strong and Weak Acids and Bases
Acid Dissociation Constant
Observe what happens to the Ka with each ionization.
The Ka decreases from first ionization to second.
It decreases again from second ionization to third.
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19. Calculating Dissociation Constants
Strong and Weak Acids and Bases
Calculating Dissociation Constants
To calculate the acid dissociation constant (Ka) of a weak acid, you need to know the initial molar concentration of the acid and the [H+] (or alternatively, the pH) of the solution at equilibrium.
You can use these data to find the equilibrium concentrations of the acid and the ions.
These values are then substituted into the expression for Ka.
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20. Calculating Dissociation Constants
Strong and Weak Acids and Bases
Calculating Dissociation Constants
You can find the Ka of an acid in water by substituting the equilibrium concentrations of the acid, [HA], the anion from the dissociation of the acid, [A−], and the hydrogen ion, [H+], into the equation below.
Fix equation so bracket is above the fraction-bar line
Ka =
[H+][A−]
[HA]
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21. Calculating a Dissociation Constant
Sample Problem 19.6
Calculating a Dissociation Constant
In a M solution of ethanoic acid, [H+] = 1.34 × 10−3M. Calculate the Ka of this acid. Refer to the table for the ionization equation for ethanoic acid.
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22. Analyze List the knowns and the unknown.
Sample Problem 19.6
Analyze List the knowns and the unknown. 1
KNOWNS
[ethanoic acid] = M
[H+] = 1.34 × 10−3M
UNKNOWN
Ka = ?
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23. Calculate Solve for the unknown.
Sample Problem 19.6
Calculate Solve for the unknown. 2
Start by determining the equilibrium concentration of the ions.
[H+] = [CH3COO−] = 1.34 × 10−3M
Each molecule of CH3COOH that ionizes gives an H+ ion and a CH3COO– ion.
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24. Calculate Solve for the unknown.
Sample Problem 19.6
Calculate Solve for the unknown. 2
Determine the equilibrium concentrations of each component.
( – )M = M
Concentration
[CH3COOH]
[H+]
[CH3COO−]
Initial
0.1000
Change
−1.34 × 10−3
1.34 × 10−3
Equilibrium
0.0987
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25. Calculate Solve for the unknown.
Sample Problem 19.6
Calculate Solve for the unknown. 2
Substitute the equilibrium values into the expression for Ka.
Ka =
[H+] × [CH3COO–]
[CH3COOH] =
(1.34 × 10−3M) × (1.34 × 10−3M)
0.0987
= 1.82 × 10−5
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26. Evaluate Does the result make sense?
Sample Problem 19.6
Evaluate Does the result make sense? 3
The calculated value of Ka is consistent with that of a weak acid.
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27. Base Dissociation Constant
Strong and Weak Acids and Bases
Base Dissociation Constant
Just as there are strong acids and weak acids, there are strong bases and weak bases.
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28. Base Dissociation Constant
Strong and Weak Acids and Bases
Base Dissociation Constant
Just as there are strong acids and weak acids, there are strong bases and weak bases.
A strong base dissociates completely into metal ions and hydroxide ions in aqueous solution.
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29. Base Dissociation Constant
Strong and Weak Acids and Bases
Base Dissociation Constant
Just as there are strong acids and weak acids, there are strong bases and weak bases.
A strong base dissociates completely into metal ions and hydroxide ions in aqueous solution.
A weak base reacts with water to form the conjugate acid of the base and hydroxide ions.
For a weak base, the amount of dissociation is relatively small.
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30. Base Dissociation Constant
Strong and Weak Acids and Bases
Base Dissociation Constant
Ammonia is an example of a weak base.
Window cleaners often use a solution of ammonia in water to clean glass.
NH3(aq) + H2O(l) NH4+(aq) + OH–(aq)
Ammonia
Water
Ammonium ion
Hydroxide ion
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31. Base Dissociation Constant
Strong and Weak Acids and Bases
Base Dissociation Constant
When equilibrium is established, only about 1 percent of the ammonia is present as NH4+.
This ion is the conjugate acid of NH3.
The concentrations of NH4+ and OH− are low and equal.
NH3(aq) + H2O(l) NH4+(aq) + OH–(aq)
Ammonia
Water
Ammonium ion
Hydroxide ion
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32. Base Dissociation Constant
Strong and Weak Acids and Bases
Base Dissociation Constant
The equilibrium-constant expression for the dissociation of ammonia in water is as follows:
Keq =
[NH4+] × [OH−]
[NH3] × [H2O]
NH3(aq) + H2O(l) NH4+(aq) + OH–(aq)
Ammonia
Water
Ammonium ion
Hydroxide ion
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33. Base Dissociation Constant
Strong and Weak Acids and Bases
Base Dissociation Constant
Recall that the concentration of water is constant in dilute solutions.
This constant can be combined with the Keq for ammonia to give a base dissociation constant (Kb) for ammonia.
Keq × [H2O] = Kb =
[NH4+] × [OH−]
[NH3]
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34. [conjugate acid] × [OH−]
Strong and Weak Acids and Bases
Base Dissociation Constant
The base dissociation constant (Kb) is the ratio of the concentration of the conjugate acid times the concentration of the hydroxide ion to the concentration of the base.
Kb =
[conjugate acid] × [OH−]
[base]
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35. Base Dissociation Constant
Strong and Weak Acids and Bases
Base Dissociation Constant
The magnitude of Kb indicates the ability of a weak base to compete with the very strong base OH– for hydrogen ions.
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36. Base Dissociation Constant
Strong and Weak Acids and Bases
Base Dissociation Constant
The magnitude of Kb indicates the ability of a weak base to compete with the very strong base OH– for hydrogen ions.
Because bases such as ammonia are weak relative to the hydroxide ion, the Kb for such a base is usually small.
The Kb for ammonia is 1.8 × 10−5.
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37. Base Dissociation Constant
Strong and Weak Acids and Bases
Base Dissociation Constant
The magnitude of Kb indicates the ability of a weak base to compete with the very strong base OH– for hydrogen ions.
Because bases such as ammonia are weak relative to the hydroxide ion, the Kb for such a base is usually small.
The Kb for ammonia is 1.8 × 10−5.
The smaller the value of Kb, the weaker the base.
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38. Concentration Versus Strength
Strong and Weak Acids and Bases
Concentration Versus Strength
Sometimes people confuse the concepts of concentration and strength.
The words concentrated and dilute indicate how much of an acid or base is dissolved in solution.
These terms refer to the number of moles of the acid or base in a given volume.
The words strong and weak refer to the extent of ionization or dissociation of an acid or base.
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39. Comparing Concentration and Strength of Acids Quantitative (or Molar)
Interpret Data
Concentration Versus Strength
The table below shows four possible combinations of concentration and strength for acids.
Comparing Concentration and Strength of Acids
Acidic solution
Concentration
Strength
Quantitative (or Molar)
Relative
Hydrochloric acid
12M HCl
Concentrated
Strong
Gastric juice
0.8M HCl
Dilute
Ethanoic acid
17M CH3COOH
Weak
Vinegar
0.2M CH3COOH
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40. Concentration Versus Strength
Strong and Weak Acids and Bases
Concentration Versus Strength
The gastric juice in your stomach is a dilute solution of HCl.
The relatively small number of HCl molecules in a given volume of gastric juice are all dissociated into ions.
Even when concentrated hydrochloric acid is diluted with water, it is still a strong acid.
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41. Concentration Versus Strength
Strong and Weak Acids and Bases
Concentration Versus Strength
Conversely, ethanoic acid (acetic acid) is a weak acid because it ionizes only slightly in solution.
Vinegar is a dilute solution of ethanoic acid.
Even at a high concentration, ethanoic acid is still a weak acid.
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42. CHEMISTRY & YOU
Despite its relatively low pH, lemon juice is safe to consume because citric acid is a weak acid. Citric acid has three Ka values. What does this information tell you about citric acid?
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43. CHEMISTRY & YOU
Despite its relatively low pH, lemon juice is safe to consume because citric acid is a weak acid. Citric acid has three Ka values. What does this information tell you about citric acid?
This information tells you that citric acid has three ionizable hydrogen atoms. It is a triprotic acid.
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44. Strong and Weak Acids and Bases
Concentration Versus Strength
The same concepts apply to bases.
A solution of ammonia can be either dilute or concentrated.
However, in any solution of ammonia, the relative amount of ionization will be small.
Thus, ammonia is a weak base at any concentration.
Likewise, sodium hydroxide is a strong base at any concentration.
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45. In strong acids, are all ionizable hydrogens completely ionized
In strong acids, are all ionizable hydrogens completely ionized? In weak acids?
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46. In strong acids, are all ionizable hydrogens completely ionized
In strong acids, are all ionizable hydrogens completely ionized? In weak acids?
In strong acids, all ionizable hydrogens are completely ionized. In weak acids, all ionizable hydrogens are partially ionized.
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47. Key Concept & Key Equation
Acids and bases are classified as strong or weak based on the degree to which they ionize in water.
[H+][A−]
Ka =
[HA]
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48. weak acid: an acid that is only slightly ionized in aqueous solution
Glossary Terms
strong acid: an acid that is completely (or almost completely) ionized in aqueous solution
weak acid: an acid that is only slightly ionized in aqueous solution
acid dissociation constant (Ka): the ratio of the concentration of the dissociated form of an acid to the undissociated form; stronger acids have larger Ka values than weaker acids
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49. Glossary Terms
strong base: a base that completely dissociates into metal ions and hydroxide ions in aqueous solution
weak base: a base that reacts with water to form the hydroxide ion and the conjugate acid of the base
base dissociation constant (Kb): the ratio of the concentration of the conjugate acid times the concentration of the hydroxide ion to the concentration of the base
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50. END OF 19.3
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