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Chapter 20Chapter 20Acids and BasesAcids and Bases
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 = HA, where Thus, general formula = HA, where A is a monatomic or polyatomic A is a monatomic or polyatomic anionanion
HClHCl(g)(g) is hydrogen monochloride is hydrogen monochloride 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. BINARY - When anion ends with -1. BINARY - When anion ends with -
ide, the acid starts with ide, the acid starts with hydro-,hydro-, and and the stem of the anion has the suffix the stem of the anion has the suffix -ic-ic followed by the word followed by the word acidacid
2. TERNARY - When anion ends with -2. TERNARY - When anion ends with -ite, the anion has the suffix ite, the anion has the suffix -ous-ous, , then then acidacid
3. TERNARY - When anion ends with -3. TERNARY - When anion ends with -ate, the anion suffix is ate, the anion suffix is -ic-ic and then and then acidacid
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 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
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 pH = -log[HpH = -log[H++]] pOH = -log [OHpOH = -log [OH--] ] KKww = [H = [H++] x [OH] x [OH--] = 1 x 10] = 1 x 10-14-14
pH + pOH = 14pH + pOH = 14 Thus, a solution with a pH less Thus, a solution with a pH less
than 7 is an than 7 is an acidacid; a pH greater ; a pH greater than 7 is a than 7 is a basebase; 7 is ; 7 is neutralneutral
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.woodrow.org/teachers/ci/1992/Arrhenius.htmlhttp://www.woodrow.org/teachers/ci/1992/Arrhenius.html
Svante Arrhenius (1859-1927)Svante Arrhenius (1859-1927)
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.
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.
Section 20.4Section 20.4Strengths of Acids and BasesStrengths of Acids and Bases
OBJECTIVES:OBJECTIVES:– Calculate an acid dissociation
constant (Ka) from concentration and pH measurements.
Section 20.4Section 20.4Strengths of Acids and BasesStrengths of Acids and Bases
OBJECTIVES:OBJECTIVES:– Arrange acids by strength according
to their acid dissociation constants (Ka).
Section 20.4Section 20.4Strengths of Acids and BasesStrengths of Acids and Bases
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?
PracticePractice Write the expression for HNOWrite the expression for HNO22
Write the KWrite the Kbb for NH for NH33
Sample 20-8, p. 604Sample 20-8, p. 604 Carefully study Key Terms and Carefully study Key Terms and
equations, p. 608equations, p. 608 Be sure to do the ChemASAP Be sure to do the ChemASAP
programs, and take all the self-programs, and take all the self-tests that are available!tests that are available!