19.3 Strengths of Acids and Bases > 1 Copyright © Pearson Education, Inc., or its affiliates. All...

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

19.3 Strengths of Acids and Bases >19.3 Strengths of Acids and Bases >

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Chapter 19Acids, Bases, and Salts

19.1 Acid-Base Theories19.2 Hydrogen Ions and Acidity

19.3 Strengths of Acids and Bases

19.4 Neutralization Reactions19.5 Salts in Solution



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Lemon juice, which contains citric acid, has a pH of about 2.3. Yet, you consume lemon juice.

CHEMISTRY & YOUCHEMISTRY & YOU

What makes one acid safer than another?



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Strong and Weak Acids and Bases

How are acids and bases classified as either strong or weak?

Strong and Weak Strong and Weak Acids and BasesAcids and Bases



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Acids and bases are classified as strong or weak based on the degree to which

they ionize in water.




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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)100%




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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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Interpret DataInterpret Data



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Interpret GraphsInterpret 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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A strong acid, such as hydrochloric acid, completely dissociates in water.

• As a result, [H3O+] is high in an aqueous solution of strong acid.

Acid Dissociation Constant




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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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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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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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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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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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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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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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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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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.

Calculating Dissociation Constants




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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.

Calculating Dissociation Constants

Ka =[H+][A−]

[HA]




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Sample Problem 19.6Sample Problem 19.6

Calculating a Dissociation Constant

In a 0.1000M 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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KNOWNS

[ethanoic acid] = 0.1000M

[H+] = 1.34 × 10−3M

UNKNOWN

Ka = ?

Analyze List the knowns and the unknown.1




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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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Determine the equilibrium concentrations of each component.

(0.1000 – 0.00134)M = 0.0987M

Concentration [CH3COOH] [H+] [CH3COO−]

Initial 0.1000 0 0

Change −1.34 × 10−3 1.34 × 10−3 1.34 × 10−3

Equilibrium 0.0987 1.34 × 10−3 1.34 × 10−3




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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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Evaluate Does the result make sense?3

The calculated value of Ka is consistent with that of a weak acid.




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Just as there are strong acids and weak acids, there are strong bases and weak bases.

Base Dissociation Constant




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• A strong base dissociates completely into metal ions and hydroxide ions in aqueous solution.





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• 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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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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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.





Hydroxide ion



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The equilibrium-constant expression for the dissociation of ammonia in water is as follows:


Keq =[NH4

+] × [OH−]

[NH3] × [H2O]




Hydroxide ion



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Recall that the concentration of water is constant in dilute solutions.


Keq × [H2O] = Kb =[NH4

+] × [OH−]

[NH3]


• This constant can be combined with the Keq for ammonia to give a base dissociation constant (Kb) for ammonia.



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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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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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• 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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• 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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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.

Concentration Versus Strength




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The table below shows four possible combinations of concentration and strength for acids.


Comparing Concentration and Strength of Acids

Acidic solution

Concentration

StrengthQuantitative (or Molar) Relative

Hydrochloric acid 12M HCl Concentrated Strong

Gastric juice 0.8M HCl Dilute Strong

Ethanoic acid 17M CH3COOH Concentrated Weak

Vinegar 0.2M CH3COOH Dilute Weak




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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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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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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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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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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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In strong acids, are all ionizable hydrogens completely ionized? In weak acids?



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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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Key Concept & Key Concept & Key EquationKey Equation

Acids and bases are classified as strong or weak based on the degree to which they ionize in water.

Ka =[H+][A−]

[HA]



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• 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

Glossary TermsGlossary Terms



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• 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

Glossary TermsGlossary Terms



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19.3 Strengths of Acids and Bases > 1 Copyright © Pearson Education, Inc., or its affiliates. All...

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