Post on 04-May-2022
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Karen C. Timberlake
Lecture Presentation
Chapter 11
Acids and Bases
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Clinical laboratory technicians prepare specimens for the detection of cancerous tumors and type blood samples for transfusions. They must also interpret and analyze the test results, which are then passed on to the physician.
Chapter 11 Acids and Bases
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chapter 11 Readiness
Key Math Skills
• Solving Equations (1.4D)
• Writing Numbers in Scientific Notation (1.4F)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chapter 11 Readiness
Core Chemistry Skills
• Writing Ionic Formulas (6.2)
• Balancing a Chemical Equation (7.1)
• Using Concentrations as a Conversion Factor (9.4)
• Writing the Equilibrium Constant Expression (10.3)
• Calculating Equilibrium Concentrations (10.4)
• Using Le Châtelier’s Principle (10.5)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chapter 11 Acids and Bases
A soft drink contains phosphoric acid (H3PO4) and carbonic acid (H2CO3).
Learning Goal Describe and name acids and bases.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Arrhenius Acids
Arrhenius acids
• produce hydrogen ions (H+) when they dissolve in water.
HCl(g) H+(aq) + Cl−(aq)
• are also electrolytes, because they produce H+ in water.
• have a sour taste.
• turn blue litmus red.
• corrode some metals.
H2O(l)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Naming Acids
• Acids with a hydrogen ion (H+) and a nonmetal (or CN−) ion are named with the prefix hydro and end with ic acid.
HCl(aq) hydrochloric acid
• Acids with a hydrogen ion (H+) and a polyatomic ion are named by changing the end of the name of the polyatomic ion from
ate to ic acid or ite to ous acid
ClO3− chlorate ClO2
− chlorite
HClO3 chloric acid HClO2 chlorous acid
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Select the correct name for each of the following acids:1. HBr A. bromic acid
B. bromous acid C. hydrobromic acid
2. H2CO3 A. carbonic acidB. hydrocarbonic acidC. carbonous acid
3. HBrO2 A. bromic acidB. hydrobromous acidC. bromous acid
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Arrhenius Bases
Arrhenius bases
• produce hydroxide ions (OH−)in water.
• taste bitter or chalky.• are also electrolytes, because
they produce hydroxide ions (OH−) in water.
• feel soapy and slippery.• turn litmus indicator paper • blue and phenolphthalein
indicator pink.
An Arrhenius base produces
cations and OH− anions in an aqueous solution.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Naming Bases
Typical Arrhenius bases are named as hydroxides.
NaOH sodium hydroxide
KOH potassium hydroxide
Ba(OH)2 barium hydroxide
Al(OH)3 aluminum hydroxide
Calcium hydroxide, Ca(OH)2,
is used in the food industryto produce beverages, and in
dentistry as a filler for root canals.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Characteristics of Acids and Bases
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Match the formulas of acids and bases with their names.
1. ___ HNO2 A. iodic acid
2. ___ Ca(OH)2 B. sulfuric acid
3. ___ H2SO4 C. sodium hydroxide
4. ___ HIO3 D. nitrous acid
5. ___ NaOH E. calcium hydroxide
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
11.2 Brønsted–Lowry Acids and Bases
According to the Brønsted–Lowry theory,
• an acid is a substance that donates H+.
• a base is a substance that accepts H+.
Learning Goal Identify conjugate acid–base pairs for Brønsted–Lowry acids and bases.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
NH3, a Brønsted–Lowry Base
In the reaction of ammonia and water,
• NH3 acts as the base that accepts H+.
• H2O acts as the acid that donates H+.
Because the nitrogen atom of NH3 has a stronger attraction for H+ than oxygen, water acts as an acid by donating H+.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
In each of the following equations, identify the Brønsted–
Lowry acid and base in the reactants:
A. HNO3(aq) + H2O(l) H3O+(aq) + NO3
−(aq)
B. HF(aq) + H2O(l) H3O+ (aq) + F−(aq)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Identify each as a characteristic of
A. an acid or B. a base.
____ 1. has a sour taste
____ 2. produces OH− in aqueous solutions
____ 3. has a chalky taste
____ 4. is an electrolyte
____ 5. produces H+ in aqueous solutions
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Conjugate Acid–Base Pairs
In any acid–base reaction, there are two conjugate acid–
base pairs.
• Each pair is related by the loss and gain of H+.
• One pair occurs in the forward direction.
• One pair occurs in the reverse direction.
Acid and conjugate base pair 1
HA + B A− + BH+
Base and conjugate acid pair 2
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Conjugate Acid–Base Pairs
In this acid–base reaction,
• the first conjugate acid–base pair is HF, which donates H+ to form its conjugate base, F−.
• the other conjugate acid–base pair is H2O, which accepts H+ to form its conjugate acid, H3O
+.
• each pair is related by a loss and gain of H+.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Conjugate Acid–Base Pairs
In the reaction of NH3 and H2O,
• one conjugate acid–base pair is NH3/NH4+.
• the other conjugate acid–base pair is H2O/H3O+.
Core Chemistry Skill Identifying Conjugate Acid–Base Pairs
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
1. Write the conjugate base for each of the following acids:
A. HBr
B. H2S
C. H2CO3
2. Write the conjugate acid of each of the following bases:
A. NO2−
B. NH3
C. OH−
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Identify the sets that contain acid–base conjugate pairs.
1. HNO2, NO2−
2. H2CO3, CO32−
3. HCl, ClO4−
4. HS−, H2S
5. NH3, NH4+
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Amphoteric Substances
Substances that can act as both acids and bases are
amphoteric or amphiprotic.
For water, the most common amphoteric substance, the acidic or basic behavior depends on the other reactant.
• Water donates H+ when it reacts with a stronger base.
• Water accepts H+ when it reacts with a stronger acid.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Guide to Writing Conjugate Acid–Base Pairs
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Identify the conjugate acid–base pairs in the following
reaction:
HNO3(aq) + NH3(aq) NO3−(aq) + NH4
+(aq)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Identify the conjugate acid–base pairs in the following reaction:
HNO3(aq) + NH3(aq) NO3−(aq) + NH4
+(aq)
STEP 1 Identify the reactant that loses H+ as the acid.
In the reaction, HNO3 donates H+ to NH3.
STEP 2 Identify the reactant that gains H+ as the base.
In the reaction, NH3 gains H+ to form NH4+. Thus,
• NH3 is the base and NH4+ is its conjugate acid.
• HBr is the acid and Br− is its conjugate base.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Identify the conjugate acid–base pairs in the following
reaction:
HNO3(aq) + NH3(aq) NO3−(aq) + NH4
+(aq)
STEP 3 Write the conjugate acid–base pairs.
HBr/Br− is the acid and conjugate base pair.
NH3/NH4+ is the base and conjugate acid pair.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
11.3 Strengths of Acids and Bases
Weak acids only partially
dissociate in water.
Hydrofluoric acid, HF, is
the only halogen that
forms a weak acid.
Learning Goal Write equations for the dissociation of strong and weak acids; identify the direction of reaction.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
11.3 Strengths of Acids and Bases
Weak acids only partially
dissociate in water.
Hydrofluoric acid, HF, is
the only halogen that
forms a weak acid.
Learning Goal Write equations for the dissociation of strong and weak acids; identify the direction of reaction.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Strong and Weak Acids
A strong acid completely ionizes (100%) in aqueous solutions.
HCl(g) + H2O(l) H3O+(aq) + Cl−(aq)
A weak acid dissociates only slightly in water to form a few ions in aqueous solutions.
H2CO3(aq) + H2O(l) H3O+(aq) + HCO3
− (aq)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Strong Acids
In water, the dissolved molecules
of HA, a strong acid,
• dissociate into ions 100%.
• produce large concentrations of H3O
+ and the anion (A−).
The strong acid HCl dissociates completely into ions:
HCl(g) + H2O(l)
H3O+(aq) + Cl−(aq)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Weak Acids
In weak acids, only a few
molecules dissociate.
• Most of the weak acid remains as the undissociated(molecular) form of the acid.
• The concentrations of H3O+
and the anion (A−) are small.
H2CO3 is a weak acid:
H2CO3(aq) + H2O(l)
H3O+(aq) + HCO3
−(aq)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Strong and Weak Acid Dissociation
• In an HCl solution,
the strong acid HCldissociates 100% to form H+ and Cl−.
• A solution of the weak acid HC2H3O2
contains mostly molecules of HC2H3O2 and a few ions of H+ and C2H3O2
−.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Strong and Weak Acid Dissociation
Figure 11.2 ▶ After dissociation in water, (a) the strong acid HI has highconcentrations of H3O
+ and I–, and (b) the weak acid HF has a high
concentration of HF and low concentrations of H3O+ and F
–
.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Diprotic Acids: Carbonic Acid
• Some weak acids, such as carbonic acid, are diprotic
acids that have two H+, which dissociate one at a time.
H2CO3(aq) + H2O(l) H3O+(aq) + HCO3
−(aq)
• Because HCO3− is also a weak acid, a second dissociation
can take place to produce another hydronium ion and the carbonate ion, CO3
2−.
HCO3−(aq) + H2O(l) H3O
+(aq) + CO32−(aq)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Diprotic Acids: Sulfuric Acid
• Some strong acids, such as sulfuric acid, are
diprotic acids that have two H+, which dissociate
one at a time.
H2SO4(aq) + H2O(l) H3O+(aq) + HSO4
−(aq)
• Because HSO4− is a weak acid, a second
dissociation can take place to produce another H+
and the sulfate ion, SO42− .
HSO4−(aq) + H2O(l) H3O
+(aq) + SO42−(aq)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Strong Bases
Strong bases as strong electrolytes
• are formed from metals of Groups 1A (1) and 2A (2).
• include LiOH, NaOH, KOH, Ba(OH)2, Sr(OH)2, and Ca(OH)2.
• dissociate completely in water.
KOH(s) K+(aq) + OH−(aq)
• are found in household products used to remove grease and unclog drains.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Weak Bases
Weak bases are weak electrolytes
• that are poor acceptors of H+ ions.
• produce very few ions in solution.
• include ammonia.
NH3(g) + H2O(l) NH4+(aq) + OH−(aq)
Ammonia Ammonium hydroxide
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Strong and Weak Bases
Strong Bases
Lithium hydroxide LiOHSodium hydroxide NaOHPotassium hydroxide KOHRubidium hydroxide RbOHCesium hydroxide CsOHCalcium hydroxide Ca(OH) 2*Strontium hydroxide Sr(OH) 2*Barium hydroxide Ba(OH)2*
*Low solubility, but they dissociate completely
Bases in Household Products
Weak BasesWindow cleaner, ammonia, NH3
Bleach, NaOClLaundry detergent, Na2CO3, Na3PO4
Toothpaste and baking soda, NaHC3
Baking powder, scouring powder, Na2CO3
Lime for lawns and agriculture, CaCO3
Laxatives, antacids, Mg(OH) 2, Al(OH)3
Strong BasesDrain cleaner, oven cleaner, NaOH
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Direction of Reaction
Strong acids have weak conjugate bases that do not readily
accept H+. • As the strength of the acid decreases, the strength of its
conjugate base increases.
In any acid–base reaction, there are two acids and two bases.• However, one acid is stronger than the other acid, and one
base is stronger than the other base.• By comparing their relative strengths, we can determine
the direction of the reaction.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Direction of Reaction: H2SO4
Sulfuric acid, H2SO4, is a strong acid that readily gives up H+
to water.
H2SO4(aq) + H2O(l) H3O+(aq) + HSO4
−(aq)
Stronger Stronger Weaker Weaker
acid base acid base
• The hydronium ion H3O+ produced is a weaker acid than
H2SO4.• The conjugate base HSO4
− is a weaker base than water.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Direction of Reaction: CO32−
The carbonate ion from carbonic acid, H2CO3, reacts with water.
• Water donates one H+ to carbonate, CO32− to form HCO3
−
and OH−.
• From Table 11.3, we see that HCO3− is a stronger acid than H2O.
• We also see that OH− is a stronger base than CO32−.
To reach equilibrium, the strong acid and strong base react in the
direction of the weaker acid and weaker base.
CO32− (aq) + H2O(l) OH−(aq) + HCO3
−(aq)
Weaker Weaker Stronger Stronger
acid base base acid
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Identify each of the following as a strong or weak acid
or base:
A. HBr
B. HNO2
C. NaOH
D. H2SO4
E. Cu(OH)2
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Using Table 11.3, identify the stronger acid in each pair.
A. HNO2 or H2S
B. HCO3− or HBr
C. H3PO4 or H3O+
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
11.4 Dissociation Constants for Acids and Bases
HCHO2(aq) + H2O(l) H3O+(aq) + CHO2
−(aq)
Learning Goal Write the expression for the dissociation constant of a weak acid or weak base.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Dissociation of a Weak Acid
Because the dissociation of strong acids in water is essentially complete, the reaction is not considered to be an equilibrium process.
• Weak acids partially dissociate in water as the ion products reach equilibrium with the undissociated weak acid molecules.
• Formic acid is a weak acid that dissociates in water to form hydronium ion, H3O
+, and formate ion, CHO2−.
HCHO2 (aq) + H2O(l) H3O+(aq) + CHO2
−(aq)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Writing Dissociation Constants
As with other dissociation expressions,
• the molar concentration of the products is divided by the molar concentration of the reactants.
• water is a pure liquid with a constant concentration and is omitted.
• the expression is called acid dissociation constant, Ka.
HCHO2 (aq) + H2O(l) H3O+(aq) + CHO2
−(aq)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Acid Dissociation Constant, Ka
When the value of the Ka
• is small, the equilibrium lies to the left, favoring the reactants.
• is large, the equilibrium lies to the right, favoring the products.
Weak acids have small Ka values, while strong acids have very large Ka values.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Base Dissociation Constant, Kb
When the value of the Kb,
• is small, the equilibrium lies to the left, favoring the reactants.
• is large, the equilibrium lies to the right, favoring the products.
The stronger the base, the larger the Kb value.
CH3—NH2(aq) + H2O(l) CH3—NH3+(aq) + OH−(aq)
The concentration of water is omitted from the base dissociation constant expression.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Write the acid dissociation constant expression for nitrous acid, HNO2.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
11.5 Dissociation of Water
The equilibrium reached between the conjugate acid–base
pairs of water produces both H3O+ and OH−.
H2O(l) + H2O(l) H3O+(aq) +
OH−(aq)
Learning Goal Use the water dissociation constant to calculate the [H3O
+] and [OH−] in an aqueous solution.
+ ++ -
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Dissociation Constant of Water, Kw
Water is amphoteric—it can act as an acid or a base. In water,
• H+ is transferred from one H2O molecule to another.
• one water molecule acts as an acid, while another acts as a base.
• equilibrium is reached between the conjugate acid–base pairs.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Writing the Dissociation Constant, Kw
In the equation for the dissociation of water, there is both a
forward and a reverse reaction.
H2O(l) + H2O(l) H3O+(aq) + OH−(aq)
• In pure water, the concentrations of H3O+ and OH− at 25
°C are each 1.0 × 10−7 M.
[H3O+] = [OH−] = 1.0 × 10–7 M
Kw = [H3O+] [OH−]
Kw = (1.0 × 10−7 M) (1.0 × 10−7 M) = 1.0 × 10–14 at 25 °C
Base Acid Conjugate Conjugate
acid base
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Dissociation Constant, Kw
The ion product constant for water, Kw, is defined as
• the product of the concentrations of H3O+ and OH−.
• equal to 1.0 × 10−14 at 25 °C (the concentration units
are omitted).
When
• [H3O+] and [OH−] are equal, the solution is neutral.
• [H3O+] is greater than the [OH−], the solution is acidic.
• [OH−] is greater than the [H3O+], the solution is basic.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Using Kw to Calculate [H3O+] and [OH−]
• If we know the [H3O+] of a solution, we can use the
Kw to calculate the [OH−].
• If we know the [OH−] of a solution, we can use the Kw to calculate the [H3O
+].
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Pure Water Is Neutral
[H3O+] = 1.0 × 10−7 M
[OH−] = 1.0 × 10−7 M
[H3O+] = [OH−]
Pure water is neutral.
In pure water, the ionization of water molecules produces small but equal quantities of H3O
+ and OH− ions.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Acidic Solutions
Adding an acid to pure water
• increases the [H3O+].
• causes the [H3O+] to exceed
1.0 × 10−7 M.
• decreases the [OH−].
[H3O+] > [OH−]
The solution is acidic.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Basic Solutions
Adding a base to pure water
• increases the [OH−]
• causes the [OH−] to exceed 1.0 × 10−7 M
• decreases the [H3O+]
[H3O+] < [OH−]
The solution is basic.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Comparison of [H3O+] and [OH−]
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Guide to Calculating [H3O+] and [O–] in
Aqueous Solutions
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Calculating [H3O+]
What is the [H3O+] of a solution if [OH−] is 5.0 × 10−8 M?
STEP 1 State the given and needed quantities.
STEP 2 Write the Kw for water and solve for the unknown [H3O
+].
ANALYZE Given Need Know
THE [OH−] = 5.0 × 10−8 M [H3O+] Kw = [H3O
+][OH−]
PROBLEM = 1.0 × 10−14
ANALYZE Given Need Know
THE [OH−] = 5.0 × 10−8 M [H3O+] Kw = [H3O
+][OH−]
PROBLEM = 1.0 × 10−14
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Calculating [H3O+]
What is the [H3O+] of a solution if [OH−] is 5.0 × 10−8 M?
STEP 3 Substitute in the known [H3O+] or [OH−] and
calculate.
Because the [H3O+] of 2.0 × 10–7 M is larger than the
[OH−] of 5.0 × 10–8 M, the solution is acidic.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
If lemon juice has [H3O+] of 2.0 × 10–3 M, what is the [OH−] of
the solution?
A. 2.0 × 10−11 M
B. 5.0 × 10−11 M
C. 5.0 × 10−12 M
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
11.6 The pH Scale
Learning Goal Calculate the pH from [H3O+]; given the
pH, calculate [H3O+] and [OH−] of a solution.
The pH scale is used to describe the acidity of solutions.
A dipstick is used to measure the pH of urine.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
The pH Scale
The pH of a solution
• is used to indicate the acidity of a solution.
• has values that usually range from 0 to 14.
• is acidic when the values are less than 7.
• is neutral at a pH of 7.
• is basic when the values are greater than 7.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
The pH Scale
The pH of a solution is commonly measured using
• a pH meter in the laboratory.
• pH paper, an indicator that turns specific colors at a specific pH value.
The pH of a solution is found by comparing the colors of indicator paper to a chart.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
pH Measurement
The pH of a solution can be determined using (a) a pH meter,
(b) pH paper, and (c) indicators that turn different colors corresponding to different pH values.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
pH of Common Substances
On the pH scale,
values below 7.0 are acidic, a value of 7.0 is neutral, and values above 7.0 are basic.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Identify each solution as acidic, basic, or neutral.
A. ___ HCl with a pH = 1.5
B. ___ pancreatic fluid, [H3O+] = 1 × 10−8 M
C. ___ Sprite soft drink, pH = 3.0
D. ___ pH = 7.0
E. ___ [OH−] = 3 × 10−10 M
F. ___ [H3O+ ] = 5 × 10−12
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Calculating the pH of Solutions
The pH scale
• is a logarithmic scale that corresponds to the [H3O+] of
aqueous solutions.
• is the negative logarithm (base 10) of the [H3O+].
pH = −log[H3O+]
To calculate the pH, the negative powers of 10 in the molar concentrations are converted to positive numbers. If [H3O
+] is 1.0 × 10−2 M,
pH = −log[1.0 × 10−2 ] = −(−2.00) = 2.00
Key Math Skill Calculating pH from [H3O+]
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
pH: Significant Figures
To determine the number of significant figures in the pH value,
• the number of decimal places in the pH value is the same as the number of significant figures in the coefficient of [H3O
+].
• the number to the left of the decimal point in the pH value is the power of 10.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
pH Scale and [H3O+]
Because pH is a log scale,
• a change of one pH unit corresponds to a tenfold change in [H3O
+].
• pH decreases as the [H3O+] increases.
pH 2.00 is [H3O+] = 1.0 × 10−2 M
pH 3.00 is [H3O+] = 1.0 × 10−3 M
pH 4.00 is [H3O+] = 1.0 × 10−4 M
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Guide to Calculating pH of Solutions
The pH of a solution is calculated from the [H3O+] by using the
log key in your calculator and changing the sign.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
pH Calculation
Aspirin, which is acetylsalicylic acid, was the first nonsteroidal
anti-inflammatory drug used to alleviate pain and fever. If a solution of aspirin has a [H3O
+] = 1.7 × 10−3 M, what is the pH of the solution?
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
pH Calculation
If a solution of aspirin has a [H3O+] = 1.7 × 10−3 M, what is
the pH of the solution?
STEP 1 State the given and needed quantities.
ANALYZE Given Need Know
THE [H3O+] = 1.7 × 10−3 M pH of solution pH = −log[H3O
+]
PROBLEM
ANALYZE Given Need Know
THE [H3O+] = 1.7 × 10−3 M pH of solution pH = −log[H3O
+]
PROBLEM
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
pH Calculation
If a solution of aspirin has a [H3O+] = 1.7 × 10−3 M, what is the
pH of the solution?
STEP 2 Enter the [H3O+] into the pH equation and
calculate.
pH = −log[H3O+] = −log[1.7 × 10−3]
Calculator Procedure:
1.7 3
Calculator Display:
EE or Exp +/−log =
2.769551079
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
pH Calculation
If a solution of aspirin has a [H3O+] = 1.7 × 10−3 M, what is the
pH of the solution?
STEP 3 Adjust the number of SFs on the right of the decimal point.
Coefficient Power of ten
1.7 × 10–3
Two SFs Exact
pH = −log[1.7 × 10−3] = 2.77
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Find the pH of a solution with a [H3O+] of 4.0 × 10−5.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Calculating [H3O+] from pH
Given the pH of a solution, we can reverse the calculation to
obtain the [H3O+].
• For whole number pH values, the negative pH value is the power of 10 in the [H3O
+] concentration.
[H3O+] = 10−pH
• For pH values that are not whole numbers, the calculation requires the use of the 10x key, which is usually a 2nd function key.
Key Math Skill Calculating [H3O+] from pH.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Guide to Calculating [H3O+] from pH
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Determine the [H3O+] for a solution that has a pH
of 3.42.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
11.7 Reactions of Acids and Bases
Gastric acid contains
HCl and is produced by parietal cells that line the stomach. When protein enters the stomach, HClis secreted until the pH reaches 2, the optimum pH for digestion.
Learning Goal Write balanced equations for reactions of acids with metals, carbonates, and bases.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Reactions of Acids
Acids react with
• metals to produce salt and hydrogen gas.
• bases to produce a salt and water.
• bicarbonate and carbonate ions to produce carbon dioxide gas.
A salt is an ionic compound that does not have H+ as the cation or OH− as the anion.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Acids and Metals
Acids react with metals to produce hydrogen gas and the salt
of the metal.
2K(s) + 2HCl(aq) 2KCl(aq) + H2(g)
Metal Acid Salt Hydrogen
Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)
Metal Acid Salt Hydrogen
Magnesium reacts
rapidly with acid and
forms H2 gas and a
salt of magnesium.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Acids, Carbonates, and Bicarbonates
Acids react with carbonates and hydrogen carbonates to produce
carbon dioxide gas, a salt, and water:
2HCl(aq) + CaCO3(s) CO2(g) + CaCl2(aq) + H2O(l)
Acid Carbonate Carbon Salt Water
dioxide
HCl(aq) + NaHCO3(s) CO2(g) + NaCl(aq) + H2O(l)
Acid Bicarbonate Carbon Salt Water
dioxide
Core Chemistry Skill Writing Equations for Reactions of Acids
and Bases
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Acids and Hydroxides: Neutralization
In a neutralization reaction,
• an acid reacts with a base to produce salt and water.
• the salt formed is the anion from the acid and the cationfrom the base.
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
Acid Base Salt Water
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Acids and Hydroxides: Neutralization
In neutralization reactions, one H+ always reacts with
one OH−.
• If we write the strong acid and strong base as ions,
HCl(aq) + NaCl(aq) NaOH(aq) + H2O(l)
we see that H+ reacts with OH− to form water, leaving the ions Na+ and Cl− in solution:
H+(aq) + Cl−(aq) + Na+(aq) + OH−(aq)
Na+(aq) + Cl−(aq) + H2O(l)
• The overall reaction occurs as the H+ from the acid and OH−
from the base form water:
H+(aq) + OH−(aq) H2O(l) Net ionic equation
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Guide to Balancing an Equation for Neutralization
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Balancing Neutralization Reactions
Write the balanced equation for the neutralization of solid magnesium hydroxide and nitric acid.
STEP 1 Write the reactants and products.
Mg(OH)2(s) + HNO3(aq) salt + H2O(l)
STEP 2 Balance the H+ in the acid with the OH− inthe base.
Mg(OH)2(s) + 2HNO3(aq) salt + H2O(l)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Balancing Neutralization Reactions
Write the balanced equation for the neutralization of solid magnesium hydroxide and nitric acid.
STEP 3 Balance the H2O with H+ and the OH−.
Mg(OH)2(s) + 2HNO3(aq) salt + 2H2O(l)
STEP 4 Write the salt from the remaining ions.
Mg(OH)2(s) + 2HNO3(aq) Mg(NO3)2(aq) + 2H2O(l)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Write a balanced equation for the following reaction:
Mg(OH)2(s) + HBr(aq)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Select the correct group of coefficients for each of the
following neutralization equations:
1. HCl(aq) + Al(OH)3(aq) AlCl3(aq) + H2O(l)
A. 1, 3, 3, 1 B. 3, 1, 1, 1 C. 3, 1, 1, 3
2. Ba(OH)2(aq) + H3PO4(aq) Ba3(PO4)2(s) + H2O(l)
A. 3, 2, 2, 2 B. 3, 2, 1, 6 C. 2, 3, 1, 6
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Select the correct group of coefficients for each of the
following neutralization equations:
STEP 4 Write the salt from the remaining ions.
1. 3HCl(aq) + Al(OH)3(aq) AlCl3(aq) + 3H2O(l)
The answer is C, 3, 1, 1, 3.
2. 3Ba(OH)2(aq) + 2H3PO4(aq)
Ba3(PO4)2(s) + 6H2O(l)
The answer is B, 3, 2, 1, 6.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Select the correct group of coefficients for each of the
following neutralization equations:
STEP 4 Write the salt from the remaining ions.
1. 3HCl(aq) + Al(OH)3(aq) AlCl3(aq) + 3H2O(l)
The answer is C, 3, 1, 1, 3.
2. 3Ba(OH)2(aq) + 2H3PO4(aq)
Ba3(PO4)2(s) + 6H2O(l)
The answer is B, 3, 2, 1, 6.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health: Antacids
Antacids are substances that
• are used to neutralize excess stomach acid.• are made of aluminum hydroxide and magnesium
hydroxide mixtures.
These hydroxides are not very soluble in water, so the levels of available OH−
are not damaging to theintestinal tract.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
11.8 Acid–Base Titration
The titration of an acid. A known volume of an acid is placed in
a flask with an indicator and titrated with a measured volume of a base solution, such as NaOH, to the neutralization endpoint.
Learning Goal Calculate the molarity or volume of an acid or base
from titration information.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Acid–Base Titration
Titration
• is a laboratory procedure used to determine the molarity of an acid.
• uses a base such as NaOH to neutralize a measured volume of an acid.
• requires a few drops of an indicator such as phenolphthalein to identify the endpoint.
Core Chemistry Skill Calculating Molarity or Volume of an Acid or
Base in a Titration
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Acid–Base Titration
In the following titration, a specific volume of acidic solution is titrated to the endpoint with a known concentration of NaOH.
Base �
NaOH
Acid �
Solution
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Indicator
The indicator phenolphthalein
• is added to identify the
endpoint.
• turns pink when the solution is neutralized.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Endpoint of Titration
At the endpoint of the titration,
• the moles of base are equal to the moles of acid in the solution.
• the concentration of the base is known.
• the volume of the base used to reach the endpoint is measured.
• the molarity of the acid is calculated using the neutralization equation for the reaction.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Guide to Calculating Boiling Point Elevation, Freezing Point Lowering
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Acid–Base Titration Calculations
What is the molarity of an HCl solution if 18.5 mL of
0.225 M NaOH is required to neutralize 0.0100 L of HCl?
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
STEP 1 State given and needed quantities and concentrations.
ANALYZE Given Need Equation
THE 18.5 mL of molarity of HCl(aq) + NaOH(aq) �
PROBLEM 0.225 M NaOH HCl solution NaCl(aq) + H2O(l)
0.0100 L HCl
ANALYZE Given Need Equation
THE 18.5 mL of molarity of HCl(aq) + NaOH(aq) �
PROBLEM 0.225 M NaOH HCl solution NaCl(aq) + H2O(l)
0.0100 L HCl
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Acid–Base Titration Calculations
What is the molarity of an HCl solution if 18.5 mL of
0.225 M NaOH is required to neutralize 0.0100 L of HCl?
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
STEP 2 Write a plan to calculate molarity or volume.
mL NaOH
solution
Metric
factorL NaOH
solutionMolarity
moles of
NaOH
Mole–Mole
factor
moles of
NaOH
moles of
HClDivide by
liters
molarity of
HCl solution
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Acid–Base Titration Calculations
What is the molarity of an HCl solution if 18.5 mL of 0.225 M
NaOH is required to neutralize 0.0100 L of HCl?
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
STEP 3 State equalities and conversion factors including concentration.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Acid–Base Titration Calculations
What is the molarity of an HCl solution if 18.5 mL of 0.225 M
NaOH is required to neutralize 0.0100 L of HCl?
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
STEP 4 Set up the problem to calculate the needed quantity.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
What is the molarity of an HCl solution if 25.5 mL of 0.438 M
NaOH is required to neutralize 0.0250 L of HCl?
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
11.9 Buffers
A buffer solution maintains the pH by neutralizing small amounts
of added acid or base.
An acid must be present to react with any OH− added, and a
base must be present to react with any H3O+ added.
Learning Goal Describe the role of buffers in maintaining
the pH of a solution; calculate the pH of a buffer.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Buffers
When an acid or a base
is added to water, the pH changes drastically.
In a buffer solution, the pH is maintained; pH does not change when acids or bases are added.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
How Buffers Work
Buffers work because
• they resist changes in pH from the addition of an acid or a base.
• in the body, they absorb H3O+ or OH− from foods and
cellular processes to maintain pH.
• they are important in the proper functioning of cells and blood.
• they maintain a pH close to 7.4 in blood.
A change in the pH of the blood affects the uptake of oxygen and cellular processes.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Components of a Buffer
A buffer solution
• contains a combination of acid–base
conjugate pairs, a weak acid and a salt of its
conjugate base, such as
HC2H3O2(aq) and C2H3O2−(aq)
• has equal concentrations of a weak acid and
its salt.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
How Buffers Work
In the buffer with acetic acid (HC2H3O2) and sodium
acetate (NaC2H3O2),
• the salt produces acetate ions and sodium ions.
NaC2H3O2(aq) C2H3O2−(aq) + Na+(aq)
• the salt is added to provide a higher concentration of the conjugate base C2H3O2
− than from the weak acid alone.
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O
+(aq) Large amount Large amount
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Function of a Weak Acid in a Buffer
If a small amount of base is added to this same buffer
solution, it is neutralized by the acetic acid, HC2H3O2, which shifts the equilibrium in the direction of the products acetate ion and water.
HC2H3O2(aq) + OH−(aq) C2H3O2−(aq) + H2O(l)
Equilibrium shifts in the direction of the products.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Function of Conjugate Base in a Buffer
When a small amount of acid is added, the additional H3O+
combines with the acetate ion, C2H3O2−, causing the equilibrium to
shift in the direction of the reactants, acetic acid and water.
The acetic acid produced contributes to the available weak acid.
HC2H3O2(aq) + H2O(l) C2H3O2− (aq) + H3O
+(aq)
Equilibrium shifts in the direction of the reactants.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Working Buffers
The buffer described here consists of about equal concentrations of
acetic acid (HC2H3O2) and its conjugate base, acetate ion (C2H3O2−).
• Adding H3O+ to the buffer reacts with the salt, C2H3O2
−, whereas
adding OH− neutralizes the acid HC2H3O2.
• The pH of the solution is maintained as long as the added
amounts of acid or base are small compared to the
concentrations of the buffer components.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
By rearranging the Ka expression to give [H3O+], we can obtain
the ratio of the acetic acid/acetate buffer and calculate the pH.
Solving for H3O+ gives
Weak acid
Conjugate base
Core Chemistry Skill Calculating the pH of a Buffer
Calculating the pH of a Buffer
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
The Ka for acetic acid, HC2H3O2, is 1.8 × 10–5. What is the pH of a buffer prepared with 1.0 M HC2H3O2 and 1.0 M C2H3O2
−?
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O
+(aq)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
The Ka for acetic acid, HC2H3O2, is 1.8 × 10–5. What is the pH of a buffer prepared with 1.0 M HC2H3O2 and 1.0 M C2H3O2
−?
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O
+(aq)
STEP 1 State the given and needed quantities.
ANALYZE Given Need
THE [HC2H3O2] = 1.0 M pH of solution
PROBLEM [C2H3O2−] = 1.0 M
Equation
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O
+(aq)
ANALYZE Given Need
THE [HC2H3O2] = 1.0 M pH of solution
PROBLEM [C2H3O2−] = 1.0 M
Equation
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O
+(aq)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
The Ka for acetic acid, HC2H3O2, is 1.8 × 10–5. What is the pH of a buffer prepared with 1.0 M HC2H3O2 and 1.0 M C2H3O2
−?
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O
+(aq)
STEP 2 Write the Ka expression and rearrange for [H3O+].
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
The Ka for acetic acid, HC2H3O2, is 1.8 × 10–5. What is the pH
of a buffer prepared with 1.0 M HC2H3O2 and 1.0 M C2H3O2−?
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O
+(aq)
STEP 3 Substitute [HA] and [A−] into the Ka expression.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
The Ka for acetic acid, HC2H3O2, is 1.8 × 10–5. What is the pH
of a buffer prepared with 1.0 M HC2H3O2 and 1.0 M C2H3O2−?
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O
+(aq)
STEP 4 Use [H3O+] to calculate pH.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Calculating the pH of a Buffer
Because Ka is a constant at a given temperature,
• the [H3O+] is determined by the [HC2H3O2]/[C2H3O2
−] ratio.
• the addition of small amounts of either acid or base changes the ratio of [HC2H3O2]/[C2H3O2
−] only slightly.
• the changes in [H3O+] will be small and the pH will be
maintained.
• the addition of a large amount of acid or base may exceed the buffering capacity of the system.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Buffers and pH Changes
Buffers can be prepared from conjugate acid–base
pairs such as H2PO4−/HPO4
2− and HPO42−/PO4
3−,
HCO3−/CO3
2−, or NH4+/NH3.
The pH of the buffer solution will depend on the
conjugate acid–base pair chosen.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Buffers and pH Changes
Using a common phosphate buffer for biological specimens,
we can look at the effect of using different ratios of [H2PO4
−/HPO42−] on the [H3O
+] and pH. The Ka of H2PO4−
is 6.2 × 10−8.
The equation and the [H3O+] are written as follows:
H2PO4−(aq) + H2O(l) H3O
+(aq) + HPO42−(aq)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Buffers and pH Changes
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health: Buffers in Blood Plasma
The arterial blood plasma has a normal pH of 7.35 to 7.45. If
changes in H3O+ lower the pH below 6.8 or raise it above 8.0, cells
cannot function properly and death may result.
In our cells, CO2
• is continually produced as an end product of cellular
metabolism.
• is carried to the lungs for elimination, and the rest dissolves in
body fluids such as plasma and saliva, forming carbonic acid,
H2CO3.
As a weak acid, carbonic acid dissociates to give bicarbonate,
HCO3−, and H3O
+.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Kidneys also supply more of the bicarbonate anion, HCO3−,
setting up an important buffer system in the body fluid:
CO2(g) + H2O(l) H2CO3(aq) HCO3−(aq) + H3O
+(aq)
Excess H3O+ entering the body fluids reacts with the HCO3
−, and
excess OH− reacts with the carbonic acid.
H2CO3(aq) + H2O(l) HCO3−(aq) + H3O
+(aq)
Equilibrium shifts in the direction of the reactants.
Chemistry Link to Health: Buffers in Blood Plasma
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health: Buffers in Blood Plasma
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Kidneys also supply more of the bicarbonate anion, HCO3−,
setting up an important buffer system in the body fluid:
CO2(g) + H2O(l) H2CO3(aq) HCO3−(aq) + H3O
+(aq)
• Excess H3O+ entering the body fluids reacts with the HCO3
−:
H2CO3(aq) + H2O(l) HCO3−(aq) + H3O
+(aq)
Equilibrium shifts in the direction of the reactants.
• Excess OH– entering the body fluids reacts with the H2CO3:
H2CO3(aq) + OH−(aq) H2O(l) + HCO3−(aq)
Equilibrium shifts in the direction of the products.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health: Buffers in Blood Plasma
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
To maintain the normal blood plasma pH (7.35 to 7.45),
• the ratio of [H2CO3]/[HCO3−] needs to be about 1 to 10.
• concentrations of 0.0024 M H2CO3 and 0.024 M HCO3−
work to maintain that pH.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health: Buffers in Blood Plasma
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
In the body, the concentration of carbonic acid is closely associated with the partial pressure of CO2, PCO2
.
• If the CO2 level rises, increasing H2CO3, the equilibrium shifts to produce more H3O
+, which lowers the pH. This condition is called acidosis.
• A lowering of the CO2 level leads to a high blood pH, a condition called alkalosis.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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