1. Acids, Bases, and Salts 19.1 Acid-Base Theories Chapter 19
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. Arrhenius Acids and Bases
How did Arrhenius define an acid and a base?
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3. Acids and bases have distinct properties.
Arrhenius Acids and Bases
Acids and bases have distinct properties.
Acids give foods a tart or sour taste.
Aqueous solutions of acids are strong or weak electrolytes.
Acids cause certain dyes, called indicators, to change color.
Many metals, such as zinc and magnesium, react with aqueous solutions of acids to produce hydrogen gas.
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4. Acids and bases have distinct properties.
Arrhenius Acids and Bases
Acids and bases have distinct properties.
Soap is a familiar material that has the properties of a base.
The bitter taste is a general property of bases.
The slippery feel of soap is another property of bases.
Bases will cause an indicator to change color.
Bases also form aqueous solutions that are strong or weak electrolytes.
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5. Arrhenius Acids and Bases
In 1887, the Swedish chemist Svante Arrhenius proposed a new way of defining and thinking about acids and bases.
According to Arrhenius, acids are hydrogen-containing compounds that ionize to yield hydrogen ions (H+) in solution. Bases are compounds that ionize to yield hydroxide ions (OH–) in aqueous solution.
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6. Arrhenius Acids Arrhenius Acids and Bases Some Common Acids Name
Acids vary in the number of hydrogens they contain that can form hydrogen ions.
Some Common Acids
Name
Formula
Hydrochloric acid
HCl
Nitric acid
HNO3
Sulfuric acid
H2SO4
Phosphoric acid
H3PO4
Ethanoic acid
CH3COOH
Carbonic acid
H2CO3
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7. Arrhenius Acids and Bases
A hydrogen atom that can form a hydrogen ion is described as ionizable.
Nitric acid (HNO3) has one ionizable hydrogen. Nitric acid is classified as a monoprotic acid.
The prefix mono- means “one,” and the stem protic reflects the fact that a hydrogen ion is a proton.
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8. Arrhenius Acids and Bases
A hydrogen atom that can form a hydrogen ion is described as ionizable.
Nitric acid (HNO3) has one ionizable hydrogen. Nitric acid is classified as a monoprotic acid.
Acids that contain two ionizable hydrogens, such as sulfuric acid (H2SO4), are called diprotic acids.
Acids that contain three ionizable hydrogens, such as phosphoric acid (H3PO4), are called triprotic acids.
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9. Not all compounds that contain hydrogen are acids.
Arrhenius Acids and Bases
Arrhenius Acids
Not all compounds that contain hydrogen are acids.
Only a hydrogen that is bonded to a very electronegative element can be released as an ion. Such bonds are highly polar.
When a compound that contains such bonds dissolves in water, it releases hydrogen ions.
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10. Arrhenius Acids and Bases
In an aqueous solution, hydrogen ions are not present. Instead, the hydrogen ions are joined to water molecules as hydronium ions.
A hydronium ion (H3O+) is the ion that forms when a water molecule gains a hydrogen ion.
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11. Arrhenius Acids and Bases
Methane (CH4) is an example of a hydrogen-containing compound that is not an acid.
The four hydrogen atoms in methane are attached to the central carbon atom by weakly polar C—H bonds.
Methane has no ionizable hydrogens and is not an acid.
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12. Arrhenius Acids and Bases
Ethanoic acid (CH3COOH), which is commonly called acetic acid, is an example of a molecule that contains both hydrogens that do not ionize and a hydrogen that does ionize.
Although its molecules contain four hydrogens, ethanoic acid is a monoprotic acid.
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13. Arrhenius Acids and Bases
The three hydrogens attached to a carbon atom are in weakly polar bonds.
They do not ionize.
Only the hydrogen bonded to the highly electronegative oxygen can be ionized.
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14. The table below lists four common bases.
Arrhenius Acids and Bases
Arrhenius Bases
The table below lists four common bases.
Some Common Bases
Name
Formula
Solubility in Water
Sodium hydroxide
NaOH
High
Potassium hydroxide
KOH
Calcium hydroxide
Ca(OH)2
Very low
Magnesium hydroxide
Mg(OH)2
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15. The base sodium hydroxide (NaOH) is known as lye.
Arrhenius Acids and Bases
Arrhenius Bases
The base sodium hydroxide (NaOH) is known as lye.
Sodium hydroxide is an ionic solid.
It dissociates into sodium ions in aqueous solution.
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16. The base sodium hydroxide (NaOH) is known as lye.
Arrhenius Acids and Bases
Arrhenius Bases
The base sodium hydroxide (NaOH) is known as lye.
Sodium hydroxide is extremely caustic.
A caustic substance can burn or eat away materials with which it comes in contact.
This property is the reason that sodium hydroxide is a major component of products that are used to clean clogged drains.
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17. Potassium hydroxide (KOH) is another ionic solid.
Arrhenius Acids and Bases
Arrhenius Bases
Potassium hydroxide (KOH) is another ionic solid.
It dissociates to produce potassium ions and hydroxide ions in aqueous solution.
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18. Arrhenius Acids and Bases
Arrhenius Bases
Sodium and potassium are Group 1A elements. Elements in Group 1A, the alkali metals, react violently with water.
The products of these reactions are aqueous solutions of a hydroxide and a hydrogen gas.
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19. Sodium hydroxide and potassium hydroxide are very soluble in water.
Arrhenius Acids and Bases
Arrhenius Bases
Sodium hydroxide and potassium hydroxide are very soluble in water.
The solutions would typically have the bitter taste and slippery feel of a base, but you would not want to test these properties.
The solutions are extremely caustic to the skin. They can cause deep, painful, slow-healing wounds if not immediately washed off.
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20. Ammonia is a base, and bases are caustic in high concentrations.
CHEMISTRY & YOU
Visitors to Bracken Cave wear protective gear to keep ammonia gas out of their eyes and respiratory tracts. Think about the properties of bases. Why are high levels of ammonia harmful?
Ammonia is a base, and bases are caustic in high concentrations.
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21. Arrhenius Acids and Bases
Arrhenius Bases
Calcium hydroxide, Ca(OH)2, and magnesium hydroxide, Mg(OH)2, are compounds of Group 2A metals.
These compounds are not very soluble in water. Their solutions are always very dilute, even when saturated.
The low solubility of magnesium hydroxide makes the suspension safe to consume.
Some people use this suspension as an antacid.
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22. Can every hydrogen from every molecule form hydrogen ions, therefore acting as an Arrhenius acid?
No. Only hydrogens that are bonded to a very electronegative element can be released as ions. That means that only molecules containing hydrogens bonded to very electronegative elements are Arrhenius acids.
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23. Brønsted-Lowry Acids and Bases
What distinguishes an acid from a base in the Brønsted-Lowry theory?
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24. Brønsted-Lowry Acids and Bases
Sodium carbonate (Na2CO3) and ammonia (NH3) act as bases when they form aqueous solutions.
Neither of these compounds is a hydroxide-containing compound, so neither would be classified as a base by the Arrhenius definition.
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25. Brønsted-Lowry Acids and Bases
In 1923, the Danish chemist Johannes Brønsted and the English chemist Thomas Lowry were working independently.
Each chemist proposed the same definition of acids and bases.
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26. Brønsted-Lowry Acids and Bases
According to the Brønsted-Lowry theory, an acid is a hydrogen-ion donor and a base is a hydrogen-ion acceptor.
This theory includes all the acids and bases that Arrhenius defined.
It also includes some compounds that Arrhenius did not classify as bases.
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27. Brønsted-Lowry Acids and Bases
You can use the Brønsted-Lowry theory to understand why ammonia is a base.
When ammonia dissolves in water, hydrogen ions are transferred from water to ammonia to form ammonium ions and hydroxide ions.
Ammonia is a Brønsted-Lowry base because it accepts hydrogen ions.
Water is a Brønsted-Lowry acid because it donates hydrogen ions.
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28. Conjugate Acids and Bases
Brønsted-Lowry Acids and Bases
Conjugate Acids and Bases
When the temperature of an aqueous solution of ammonia is increased, ammonia gas is released.
HNH4+ reacts with OH– to form more NH3 and H2O.
In the reverse reaction, ammonium ions donate hydrogen ions to hydroxide ions.
NH4+ (the donor) acts as a Brønsted-Lowry acid, and OH− (the acceptor) acts as a Brønsted-Lowry base.
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29. Conjugate Acids and Bases
Brønsted-Lowry Acids and Bases
Conjugate Acids and Bases
In essence, the reversible reaction of ammonia and water has two acids and two bases.
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30. Conjugate Acids and Bases
Brønsted-Lowry Acids and Bases
Conjugate Acids and Bases
A conjugate acid is the ion or molecule formed when a base gains a hydrogen ion.
NH4+ is the conjugate acid of the base NH3.
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31. Conjugate Acids and Bases
Brønsted-Lowry Acids and Bases
Conjugate Acids and Bases
A conjugate base is the ion or molecule that remains after an acid loses a hydrogen ion.
OH– is the conjugate base of the acid H2O.
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32. Conjugate Acids and Bases
Brønsted-Lowry Acids and Bases
Conjugate Acids and Bases
Conjugate acids are always paired with a base, and conjugate bases are always paired with an acid.
A conjugate acid-base pair consists of two ions or molecules related by the loss or gain of one hydrogen ion.
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33. Conjugate Acids and Bases
Brønsted-Lowry Acids and Bases
Conjugate Acids and Bases
The ammonia molecule and the ammonium ion are a conjugate acid-base pair.
The water molecule and the hydroxide ion are also a conjugate acid-base pair.
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34. Conjugate Acids and Bases
Brønsted-Lowry Acids and Bases
Conjugate Acids and Bases
In this reaction, hydrogen chloride is the hydrogen-ion donor and is by definition a Brønsted-Lowry acid. Water is the hydrogen-ion acceptor and a Brønsted-Lowry base.
The chloride ion is the conjugate base of the acid HCl.
The hydronium ion is the conjugate acid of the water base.
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35. Conjugate Acids and Bases
Brønsted-Lowry Acids and Bases
Conjugate Acids and Bases
The figure below shows the reaction that takes place when sulfuric acid dissolves in water.
The products are hydronium ions and hydrogen sulfate ions.
Use the figure to identify the two conjugate acid-base pairs.
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36. Some Conjugate Acid-Base Pairs
Brønsted-Lowry Acids and Bases
Some Conjugate Acid-Base Pairs
Acid
Base
HCl
Cl–
H2SO4
HSO4–
H3O+
H2O
SO42–
CH3COOH
CH3COO–
H2CO3
HCO3−
HCO3–
CO32–
NH4+
NH3
OH–
Amphoteric Substances
Note that water appears in both the list of acids and the list of bases.
Sometimes water accepts a hydrogen ion.
At other times, it donates a hydrogen ion.
How water behaves depends on the other reactant.
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37. Amphoteric Substances
Brønsted-Lowry Acids and Bases
Amphoteric Substances
A substance that can act as either an acid or a base is said to be amphoteric.
Water is amphoteric.
In the reaction with hydrochloric acid, water accepts a proton and is therefore a base.
In the reaction with ammonia, water donates a proton and is therefore an acid.
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38. How can one substance, such as water, be both an acid and a base, according to the Brønsted-Lowry definition?
Because water can act as both a hydrogen-ion donator and a hydrogen-ion acceptor, it can act as both an acid and a base according to the Brønsted-Lowry definition.
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39. Lewis Acids and Bases Lewis Acids and Bases
How did Lewis define an acid and a base?
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40. Lewis Acids and Bases
According to Gilbert Lewis, an acid accepts a pair of electrons and a base donates a pair of electrons during a reaction.
This definition is more general than those offered by Arrhenius or by Brønsted and Lowry.
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41. Lewis Acids and Bases
A Lewis acid is a substance that can accept a pair of electrons to form a covalent bond.
A Lewis base is a substance that can donate a pair of electrons to form a covalent bond.
The Lewis definitions include all the Brønsted-Lowry acids and bases.
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42. Lewis Acids and Bases Consider the reaction of H+ and OH–.
The hydrogen ion donates itself to the hydroxide ion.
H+ is a Brønsted-Lowry acid, and OH− is a Brønsted-Lowry base.
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43. Lewis Acids and Bases Consider the reaction of H+ and OH–.
The hydroxide ion can bond to the hydrogen ion because it has an unshared pair of electrons.
OH− is also a Lewis base, and H+, which accepts the pair of electrons, is a Lewis acid.
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44. Lewis Acids and Bases
A second example of a reaction between a Lewis acid and a Lewis base is what happens when ammonia dissolves in water.
Hydrogen ions from the dissociation of water are the electron-pair acceptor and the Lewis acid.
Ammonia is the electron-pair donor and the Lewis base.
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45. Acid-Base Definitions
Interpret Data
This table compares the definitions of acids and bases.
Acid-Base Definitions
Type
Acid
Base
Arrhenius
H+ producer
OH– producer
Brønsted-Lowry
H+ donor
H+ acceptor
Lewis
electron-pair acceptor
electron-pair donor
The Lewis definition is the broadest.
It extends to compounds that the Brønsted-Lowry theory does not classify as acids and bases.
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46. Identifying Lewis Acids and Bases
Sample Problem 19.1
Identifying Lewis Acids and Bases
Identify the Lewis acid and the Lewis base in this reaction between ammonia and boron trifluoride.
NH3 + BF3 → NH3BF3
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47. Analyze Identify the relevant concepts.
Sample Problem 19.1
Analyze Identify the relevant concepts. 1
When a Lewis acid reacts with a Lewis base, the base donates a pair of electrons and the acid accepts the donated pair.
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48. Solve Apply concepts to this problem.
Sample Problem 19.1
Solve Apply concepts to this problem. 2
Identify the reactant with the unshared pair of electrons and the reactant that can accept the pair of electrons.
Draw electron dot structures to identify which reactant has an unshared pair of electrons.
Ammonia has an unshared pair of electrons to donate.
The boron atom can accept the donated electrons.
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49. Solve Apply concepts to this problem.
Sample Problem 19.1
Solve Apply concepts to this problem. 2
Classify the reactants based on their behavior.
Lewis bases donate a pair of electrons, so ammonia is the Lewis base.
Lewis acids accept a pair of electrons, so boron trifluoride is the Lewis acid.
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50. Are hydrogen-ion donors also electron-pair acceptors?
Yes. All substances defined as acids by the Brønsted-Lowry definition (an acid is a hydrogen-ion donor) are also defined as acids by the Lewis definition (an acid is an electron-pair acceptor). That means that these substances are both hydrogen-ion donors and electron-pair acceptors.
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51. Key Concepts
According to Arrhenius, acids are hydrogen-containing compounds that ionize to yield hydrogen ions in aqueous solution. Bases are compounds that ionize to yield hydroxide ions in aqueous solution.
According to Brønsted-Lowry theory, an acid is a hydrogen-ion donor and a base is a hydrogen-ion acceptor.
According to Lewis, an acid accepts a pair of electrons and a base donates a pair of electrons.
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52. Glossary Terms
hydronium ion (H3O+): the positive ion formed when a water molecule gains a hydrogen ion
conjugate acid: the particle formed when a base gains a hydrogen ion; NH4+ is the conjugate acid of the base NH3
conjugate base: the particle that remains when an acid has donated a hydrogen ion; OH– is the conjugate base of the acid water
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53. Glossary Terms
conjugate acid-base pair: two substances that are related by the loss or gain of a single hydrogen ion; ammonia (NH3) and the ammonium ion (NH4+) are a conjugate acid-base pair
amphoteric: a substance that can act as both an acid and a base
Lewis acid: any substance that can accept a pair of electrons to form a covalent bond
Lewis base: any substance that can donate a pair of electrons to form a covalent bond
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54. BIG IDEA
Reactions
Chemists define acids and bases according to the ions they yield in aqueous solution.
Chemists also define acids and bases based on whether they accept or donate hydrogen ions, and whether they are electron-pair donors or acceptors.
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55. END OF 19.1
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