Chapter 20 Acids and Bases. Items from Chapter 19... n Reversible Reactions - p. 539 –In a...

Chapter 20Chapter 20Acids and BasesAcids and Bases

Items from Chapter 19...Items from Chapter 19... Reversible ReactionsReversible Reactions - p. 539 - p. 539

– In a reversible reaction, the reactions occur simultaneously in both directions

– double arrows used to indicate this

2SO2(g) + O2(g) 2SO3(g)

– In principle, almost all reactions are reversible to some extent

Items from Chapter 19...Items from Chapter 19... Le Chatelier’s PrincipleLe Chatelier’s Principle - p.541 - p.541

– If a stress is applied to a system in dynamic equilibrium, the system changes to relieve the stress.

– Stresses that upset the equilibrium in a chemical system include: changes in concentration, changes in temperature, and changes in pressure

Items from Chapter 19...Items from Chapter 19... Equilibrium ConstantsEquilibrium Constants (K (Keqeq) - p. ) - p.

545545– Chemists generally express the

position of equilibrium in terms of numerical values

– These values relate to the amounts of reactants and products at equilibrium

Items from Chapter 19...Items from Chapter 19... Equilibrium ConstantsEquilibrium Constants - p. 545 - p. 545

– consider this reaction:

aA + bB cC + dD

– The equilibrium constant (Keq) is the ratio of product concentration to the reactant concentration at equilibrium, with each concentration raised to a power (= the coefficient)


– consider this reaction:

aA + bB cC + dD

– Thus, the “equilibrium constant expression” has the general form:

[C]c x [D]d

[A]a x [B]b

( [ ] = molarity )

Keq =


– the equilibrium constants provide valuable information, such as whether products or reactants are favored:

Keq > 1, products favored at equilibrium

Keq < 1, reactants favored at equilibrium

Sample Problem 19-2, p. 545Sample Problem 19-2, p. 545

Section 20.1Section 20.1Describing Acids and BasesDescribing Acids and Bases

OBJECTIVES:OBJECTIVES:– List the properties of acids and

bases.

Section 20.1Section 20.1Describing Acids and BasesDescribing Acids and Bases

OBJECTIVES:OBJECTIVES:– Name an acid or base, when given

the formula.

Properties of acidsProperties of acids Taste sour (don’t try this at home).Taste sour (don’t try this at home). Conduct electricity.Conduct electricity.

– Some are strong, others are weak electrolytes. React with metals to form hydrogen React with metals to form hydrogen

gas.gas. Change indicators (blue litmus to red).Change indicators (blue litmus to red). React with hydroxides to form water React with hydroxides to form water

and a salt.and a salt.

Properties of basesProperties of bases React with acids to form water React with acids to form water

and a salt.and a salt. Taste bitter.Taste bitter. Feel slippery (don’t try this Feel slippery (don’t try this

either).either). Can be strong or weak Can be strong or weak

electrolytes.electrolytes. Change indicators (red litmus Change indicators (red litmus

turns blue).turns blue).

Names and Formulas of AcidsNames and Formulas of Acids An acid is a chemical that produces An acid is a chemical that produces

hydrogen ions (Hhydrogen ions (H1+1+) when dissolved ) when dissolved in waterin water

Thus, general formula = HX, where Thus, general formula = HX, where X is a monatomic or polyatomic X is a monatomic or polyatomic anionanion

HClHCl(g)(g) named hydrogen chloride named hydrogen chloride

HClHCl(aq)(aq) is named as an acid is named as an acid Name focuses on the anion presentName focuses on the anion present

Names and Formulas of AcidsNames and Formulas of Acids1. When anion ends with -ide, the 1. When anion ends with -ide, the

acid starts with acid starts with hydro-,hydro-, and the and the stem of the anion has the suffix stem of the anion has the suffix -ic-ic followed by the word followed by the word acidacid

2. When anion ends with -ite, the 2. When anion ends with -ite, the anion has the suffix anion has the suffix -ous-ous, then , then acidacid

3. When anion ends with -ate, the 3. When anion ends with -ate, the anion suffix is anion suffix is -ic-ic and then and then acidacid

Table 20.1, page 578 for examplesTable 20.1, page 578 for examples

Names and Formulas of BasesNames and Formulas of Bases A base produces hydroxide ions A base produces hydroxide ions

(OH(OH1-1-) when dissolved ) when dissolved in waterin water.. Named the same way as any other Named the same way as any other

ionic compoundionic compound– name the cation, followed by anion

To write the formula: write To write the formula: write symbols; write charges; then cross symbols; write charges; then cross (if needed)(if needed)

Sample Problem 20-1, p. 579Sample Problem 20-1, p. 579

Section 20.2Section 20.2Hydrogen Ions and AcidityHydrogen Ions and Acidity

OBJECTIVES:OBJECTIVES:– Given the hydrogen-ion or

hydroxide-ion concentration, classify a solution as neutral, acidic, or basic.

Section 20.2Section 20.2Hydrogen Ions and AcidityHydrogen Ions and Acidity

OBJECTIVES:OBJECTIVES:– Convert hydrogen-ion concentrations

into values of pH, and hydroxide-ion concentrations into values of pOH.

Hydrogen Ions from WaterHydrogen Ions from Water Water ionizes, or falls apart into ions:Water ionizes, or falls apart into ions:

H2O H1+ + OH1-

Called the “self ionization” of waterCalled the “self ionization” of water Occurs to a very small extent:Occurs to a very small extent:

[H1+ ] = [OH1-] = 1 x 10-7 M Since they are equal, a Since they are equal, a neutralneutral solution solution

results from waterresults from water KKww = [H = [H1+1+ ] x [OH ] x [OH1-1-] = 1 x 10] = 1 x 10-14-14 MM22

KKww is called the “ion product constant” is called the “ion product constant”

Ion Product ConstantIon Product Constant HH22O O H H++ + OH + OH--

KKww is constant in every aqueous solution: is constant in every aqueous solution: [H[H++] x [OH] x [OH--] = 1 x 10] = 1 x 10-14 -14 MM22

If [HIf [H++] > 10] > 10-7-7 then [OH then [OH--] < 10] < 10-7-7 If [HIf [H++] < 10] < 10-7-7 then [OH then [OH--] > 10] > 10-7-7

If we know one, other can be determinedIf we know one, other can be determined If [HIf [H++] > 10] > 10-7-7 , it is , it is acidicacidic and [OH and [OH--] < 10] < 10-7-7 If [HIf [H++] < 10] < 10-7-7 , it is , it is basicbasic and [OH and [OH--] > 10] > 10-7-7

Basic solutions also called “alkaline”Basic solutions also called “alkaline” Sample problem 20-2, p. 582Sample problem 20-2, p. 582

Logarithms and the pH conceptLogarithms and the pH concept Logarithms are powers of ten.Logarithms are powers of ten.

– Review from earlier lessons, and p. 585 definition: pH = definition: pH = -log[H-log[H++]] in neutral pH = in neutral pH = -log(1 x 10-log(1 x 10-7-7) = 7) = 7 in acidic solution [Hin acidic solution [H++] > 10] > 10-7-7

pH pH < -log(10< -log(10-7-7)) pH < 7 (from 0 to 7 is the acid range)pH < 7 (from 0 to 7 is the acid range) in base, pH > 7 (7 to 14 is base in base, pH > 7 (7 to 14 is base

range)range)

pH and pOHpH and pOH pOH = -log [OHpOH = -log [OH--] ] [H[H++] x [OH] x [OH--] = 1 x 10] = 1 x 10-14 -14 MM22

pH + pOH = 14pH + pOH = 14 Thus, a solution with a pOH Thus, a solution with a pOH

less than 7 is basic; with a pOH less than 7 is basic; with a pOH greater than 7 is an acidgreater than 7 is an acid

0 1 3 5 7 9 11

13

140135791

113

14

Basic

100

10-

1

10-

3

10-

5

10-

7

10-

9

10-

11

10-

13

10-

14

Basic

100

10-

1

10-

3

10-

5

10-

7

10-

9

10-

11

10-

13

10-

14

Acidic Neutral

[OH-]

pH

[H+]

pOH

Examples:Examples: Sample 20-3, p.586Sample 20-3, p.586 Sample 20-4, p.586Sample 20-4, p.586 Sample 20-5, p.587Sample 20-5, p.587 Sample 20-6, p.588Sample 20-6, p.588

Measuring pHMeasuring pH Why measure pH?Why measure pH?

– Everything from swimming pools, soil conditions for plants, medical diagnosis, soaps and shampoos, etc.

Sometimes we can use Sometimes we can use indicators, other times we indicators, other times we might need a pH metermight need a pH meter

Acid-Base IndicatorsAcid-Base Indicators An indicator is an acid or base An indicator is an acid or base

that undergoes dissociation in that undergoes dissociation in a known pH range, and has a known pH range, and has different colors in solution different colors in solution (more later in chapter)(more later in chapter)

Examples: litmus, Examples: litmus, phenolphthalein, bromthymol phenolphthalein, bromthymol blue: Fig 20.8, p.590blue: Fig 20.8, p.590

Acid-Base IndicatorsAcid-Base Indicators Although useful, there are Although useful, there are

limitations to indicators:limitations to indicators:– usually given for a certain temperature

(25 oC), thus may change at different temperatures

– what if the solution already has color?

– ability of human eye to distinguish colors

Acid-Base IndicatorsAcid-Base Indicators A A pH meterpH meter may give more may give more

definitive resultsdefinitive results– some are large, others portable

– works by measuring the voltage between two electrodes

– needs to be calibrated

– Fig. 20.10, p.591

Section 20.3Section 20.3Acid-Base TheoriesAcid-Base Theories

OBJECTIVES:OBJECTIVES:– Compare and contrast acids and

bases as defined by the theories of Arrhenius, Brønsted-Lowry, and Lewis

Section 20.3Section 20.3Acid-Base TheoriesAcid-Base Theories

OBJECTIVES:OBJECTIVES:– Identify conjugate acid-base pairs in

acid-base reactions.

Svante ArrheniusSvante Arrhenius Swedish chemist (1859-1927) - Swedish chemist (1859-1927) -

Nobel prize winner in chemistry Nobel prize winner in chemistry (1903)(1903)

one of the first chemists to one of the first chemists to explain the chemical theory of explain the chemical theory of the behavior of acids and basesthe behavior of acids and bases

Dr. Hubert Alyea-last graduate Dr. Hubert Alyea-last graduate student of Arrhenius. (link below)student of Arrhenius. (link below)

http://www.http://www.woodrowwoodrow.org/teachers/.org/teachers/cici/1992//1992/ArrheniusArrhenius.html.html

Hubert N. Alyea (1903-1996)Hubert N. Alyea (1903-1996)

1. Arrhenius Definition1. Arrhenius Definition AcidsAcids produce hydrogen ions produce hydrogen ions

(H(H1+1+) in aqueous solution.) in aqueous solution. BasesBases produce hydroxide ions produce hydroxide ions

(OH(OH1-1-) when dissolved in water.) when dissolved in water. Limited to aqueous solutions.Limited to aqueous solutions. Only one kind of base Only one kind of base

(hydroxides)(hydroxides) NHNH33 (ammonia) could not be an (ammonia) could not be an

Arrhenius base.Arrhenius base.

Svante Arrhenius (1859-1927)Svante Arrhenius (1859-1927)

Polyprotic AcidsPolyprotic Acids Some compounds have more than Some compounds have more than

1 ionizable hydrogen.1 ionizable hydrogen. HNOHNO3 3 nitric acid - monoproticnitric acid - monoprotic

HH22SOSO44 sulfuric acid - diprotic - 2 H sulfuric acid - diprotic - 2 H++

HH33POPO44 phosphoric acid - triprotic - 3 phosphoric acid - triprotic - 3 HH++

Having more than one ionizable Having more than one ionizable hydrogen does not mean stronger!hydrogen does not mean stronger!

Polyprotic AcidsPolyprotic Acids However, not all compounds However, not all compounds

that have hydrogen are acidsthat have hydrogen are acids Also, not all the hydrogen in an Also, not all the hydrogen in an

acid may be released as ionsacid may be released as ions– only those that have very polar

bonds are ionizable - this is when the hydrogen is joined to a very electronegative element

Arrhenius examples...Arrhenius examples... Consider HClConsider HCl What about CHWhat about CH44 (methane)? (methane)?

CHCH33COOH (ethanoic acid, or COOH (ethanoic acid, or acetic acid) - it has 4 acetic acid) - it has 4 hydrogens like methane hydrogens like methane does…?does…?

Table 20.4, p. 595 for basesTable 20.4, p. 595 for bases

2. Brønsted-Lowry Definitions2. Brønsted-Lowry Definitions Broader definition than ArrheniusBroader definition than Arrhenius Acid is hydrogen-ion donor (HAcid is hydrogen-ion donor (H+ + or or

proton); base is hydrogen-ion acceptor.proton); base is hydrogen-ion acceptor. Acids and bases always come in pairs.Acids and bases always come in pairs. HCl is an acid.HCl is an acid.

– When it dissolves in water, it gives it’s proton to water.

HCl(g) + HHCl(g) + H22O(l) O(l) HH33OO++ + Cl + Cl--

Water is a base; makes hydronium ion.Water is a base; makes hydronium ion.

Johannes Bronsted / Thomas LowryJohannes Bronsted / Thomas Lowry (1879-1947) (1874-1936) (1879-1947) (1874-1936)

Acids and bases come in pairs...Acids and bases come in pairs... A A conjugate baseconjugate base is the is the

remainder of the original acid, remainder of the original acid, after it donates it’s hydrogen ionafter it donates it’s hydrogen ion

A A conjugate acidconjugate acid is the particle is the particle formed when the original base formed when the original base gains a hydrogen iongains a hydrogen ion

Indicators are weak acids or Indicators are weak acids or bases that have a different color bases that have a different color from their original acid and basefrom their original acid and base

Acids and bases come in pairs...Acids and bases come in pairs... General equation is: General equation is: HA(aq) + HHA(aq) + H22O(l) O(l) H H33OO++(aq) + A(aq) + A--(aq)(aq)

Acid + Base Acid + Base Conjugate acid +Conjugate acid + Conjugate base Conjugate base

NHNH33 + H + H22O NHO NH441+1+ + OH + OH1-1-

base acid c.a. c.b.base acid c.a. c.b. HCl + HHCl + H22O HO H33OO1+1+ + Cl+ Cl1-1-

acid base c.a. c.b.acid base c.a. c.b. Amphoteric - acts as acid or baseAmphoteric - acts as acid or base

3. Lewis Acids and Bases3. Lewis Acids and Bases Gilbert Lewis focused on the Gilbert Lewis focused on the

donation or acceptance of a pair donation or acceptance of a pair of electrons during a reactionof electrons during a reaction

Lewis AcidLewis Acid - electron pair - electron pair acceptoracceptor

Lewis BaseLewis Base - electron pair donor - electron pair donor Most general of all 3 definitions; Most general of all 3 definitions;

acids don’t even need hydrogen!acids don’t even need hydrogen! Sample Problem 20-7, p.599Sample Problem 20-7, p.599

Gilbert Lewis (1875-1946)Gilbert Lewis (1875-1946)

Section 20.4Section 20.4Strengths of Acids and BasesStrengths of Acids and Bases

OBJECTIVES:OBJECTIVES:– Define strong acids and weak acids.


OBJECTIVES:OBJECTIVES:– Calculate an acid dissociation

constant (Ka) from concentration and pH measurements.


OBJECTIVES:OBJECTIVES:– Arrange acids by strength according

to their acid dissociation constants (Ka).


OBJECTIVES:OBJECTIVES:– Arrange bases by strength according

to their base dissociation constants (Kb).

StrengthStrength Strong acids and bases are strong Strong acids and bases are strong

electrolyteselectrolytes– They fall apart (ionize) completely.– Weak acids don’t completely ionize.

StrengthStrength different from different from concentrationconcentration Strong-forms many ions when dissolvedStrong-forms many ions when dissolved Mg(OH)Mg(OH)22 is a strong base- it falls is a strong base- it falls

completely apart when dissolved. completely apart when dissolved. – But, not much dissolves- not concentrated

Measuring strengthMeasuring strength Ionization is reversible.Ionization is reversible. HAHA HH++ + A + A- -

This makes an equilibriumThis makes an equilibrium Acid dissociation constant = KAcid dissociation constant = Kaa

KKaa = [H = [H++ ][A ][A- - ] (water is ] (water is

constant)constant) [HA][HA] Stronger acid = more products Stronger acid = more products

(ions), thus a larger K(ions), thus a larger Kaa (Table 20.8, (Table 20.8,

p.602)p.602)

What about bases?What about bases? Strong bases dissociate completely.Strong bases dissociate completely.

B + HB + H22OO BH BH++ + OH + OH--

Base dissociation constant = KBase dissociation constant = Kbb

KKbb = [BH = [BH++ ][OH ][OH--]]

[B] [B] (we ignore the (we ignore the water)water)

Stronger base = more dissociated, Stronger base = more dissociated, thus a larger Kthus a larger Kb.b.

Strength vs. ConcentrationStrength vs. Concentration The words The words concentratedconcentrated and and dilutedilute

tell how much of an acid or base is tell how much of an acid or base is dissolved in solution - refers to the dissolved in solution - refers to the number of moles of acid or base in number of moles of acid or base in a given volumea given volume

The words The words strongstrong and and weakweak refer to refer to the extent of ionization of an acid the extent of ionization of an acid or baseor base

Is concentrated weak acid Is concentrated weak acid possible?possible?

Chapter 20 Acids and Bases. Items from Chapter 19... n Reversible Reactions - p. 539 –In a...

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