Acids and Bases

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Acids and Bases Basic Concepts
Topic 18
Litmus Test and Other Color Changes

• Acids and bases cause certain colored dyes to
  change color. The most common of these dyes is
  litmus.

• When mixed with an acid, litmus is red.

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Acids and Bases Basic Concepts
Topic 18
Litmus Test and Other Color Changes

• When added to a base, litmus is blue.

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AcidBase Concepts - Chapter 19

• Arrhenius Acid A substance which dissociates to
  form hydrogen ions (H) in solution.
  HA(aq) ? H(aq) A(aq)
• Arrhenius Base A substance that dissociates in,
  or reacts with water to form hydroxide ions (OH)
• MOH(aq) ? M(aq) OH(aq)

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AcidBase Concepts 02

• Brønsted-Lowry Acid Substance that can donate H
• Brønsted-Lowry Base Substance that can accept H
• Chemical species whose formulas differ only by
  one proton are said to be conjugate acidbase
  pairs.

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Acids and Bases Basic Concepts
Topic 18
Submicroscopic Behavior of Acids

• When HCl dissolves in water, it produces
  hydronium ions by the reaction shown below.

• HCl is definitely an acid it produces H3O when
  dissolved in water.

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AcidBase Concepts 03
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AcidBase Concepts 04
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AcidBase Concepts 07

• Write balanced equations for the dissociation of
  each of the following BrønstedLowry acids in
  water. What are their conjugate bases?
• H2SO4
• (b) HSO4
• (c) H3O

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Dissociation of Water 01

• Water can act as an acid, or as a base. H2O(l)
  ? H(aq) OH(aq)
• This is called the autoionization of water.
  H2O(l) H2O(l) ? H3O(aq) OH(aq)

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Acids and Bases Basic Concepts
Topic 18
The pH Scale

• pH is a mathematical scale in which the
  concentration of hydronium ions in a solution is
  expressed as a number from 0 to 14.

• A scale of 0 to 14 is much easier to work with
  than a range from 1 to 1014 (100 to 1014).

• The pH scale is a convenient way to describe the
  concentration of hydronium ions in acidic
  solutions, as well as the hydroxide ions in basic
  solutions.

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Dissociation of Water 02

• This equilibrium gives us the ion product of
  water. Kw Kc H OH 1.0 x1014
• If we know either H or OH and we can
  determine the unknown.

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Dissociation of Water 03

• The concentration of OH ions in a certain
  household ammonia cleaning solution is 0.0025M.
  Calculate the concentration of H ions.
• Calculate the concentration of OH ions in a HCl
  solution whose hydrogen ion concentration is 1.3M

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Acids and Bases Basic Concepts
Topic 18
Interpreting the pH Scale

• pH of 7 is neutral. A pH less than 7 is acidic,
  and a pH greater than 7 is basic.

• As the pH drops from 7, the solution becomes more
  acidic.

• As pH increases from 7, the solution becomes more
  basic.

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Acids and Bases Basic Concepts
Topic 18
pH of Common Materials

• In a neutral solution, the concentration of
  hydroxide ions and the concentration of hydronium
  ions are equal.

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pH A measure of Acidity 01

• The pH of a solution is defined as the negative
  logarithm of the hydrogen ion concentration (in
  mol/L). pH log H pH pOH
  14Acidic solutions H gt 1.0x107 M, pH lt
  7.00Basic solutions H lt 1.0x107 M, pH gt
  7.00Neutral solutions H 1.0x107 M, pH
  7.00

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pH A measure of Acidity 02

• Nitric acid (HNO3) is used in the production of
  fertilizer, dyes, drugs, and explosives.
  Calculate the pH of a HNO3 solution having a
  hydrogen ion concentration of 0.76M.
• The pH of a certain orange juice is 3.33.
  Calculate the H ion concentration.
• The OH ion concentration of a blood sample is
  2.5x107 M. What is the pH of the Blood?

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pH A measure of Acidity 03
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Strength of Acids and Bases 01

• Strong acids and bases are strong electrolytes
  that are assumed to ionize completely in water.
• Weak acids and bases are weak electrolytes that
  ionize only to a limited extent in water.
• Solutions of weak acids and bases contain ionized
  and nonionized species.

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Strength of Acids and Bases 02

• If an acid is strong, its conjugate base has no
  measurable strength.
• H3O is the strongest acid that can exist in
  aqueous solution.
• OH ion is the strongest base that can exist in
  aqueous solution.

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Strength of Acids and Bases 03
ACID CONJ. BASE
ACID CONJ. BASE

• HClO4
• HI
• HBr
• HCl
• H2SO4
• HNO3
• H3O
• HSO4

HSO4 HF HNO2 HCOOH NH4 HCN H2O NH3
ClO4 I Br Cl HSO4 NO3 H2O SO42
SO42 F NO2 HCOO NH3 CN OH NH2
Increasing Acid Strength
Increasing Acid Strength
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Strength of Acids and Bases 04

• Calculate the pH of (a) a 1.0 x 103M HCl
  solution and (b) a 0.020 M Ba(OH)2 solution.
• Calculate the pH of a 1.8 x 102 M Ba(OH)2
  solution.
• Predict the direction of the following
• HNO2(aq) CN(aq) ? HCN(aq) NO2(aq)
• CH3CO2H(aq) HCO2(aq) ? CH3CO2(aq) HCO2H(aq)

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Strength of Acids and Bases 05

• (a) Arrange the three acids in order of
  increasing value of Ka.
• (b) Which acid, if any, is a strong acid?
• (c) Which solution has the highest pH, and which
  has the lowest?

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Strength of Acids and Bases 06

• (a) Which is the stronger acid, HX or HY?
• (b) Which is the stronger base, X or Y?
• (c) If you mix equal concentrations of reactants
  and products, will the following reaction proceed
  to the right or to the left? HX Y ? HY X

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Acid Ionization Constants 01

• Acid Ionization Constant the equilibrium
  constant for the ionization of an acid. HA(aq)
  H2O(l) ? H3O(aq) A(aq)
• Or simply HA(aq) ? H(aq) A(aq)

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Acid Ionization Constants 02
ACID Ka
CONJ. BASE Kb
7.1 x 10 4 4.5 x 10 4 3.0 x 10 4 1.7 x 10
4 8.0 x 10 5 6.5 x 10 5 1.8 x 10 5 4.9 x 10
10 1.3 x 10 10
HF HNO2 C9H8O4 (aspirin) HCO2H (formic) C6H8O6
(ascorbic) C6H5CO2H (benzoic) CH3CO2H
(acetic) HCN C6H5OH (Phenol)
F NO2 C9H7O4 HCO2 C6H7O6 C6H5CO2
CH3CO2 CN C6H5O
1.4 x 10 11 2.2 x 10 11 3.3 x 10 11 5.9 x 10
11 1.3 x 10 10 1.5 x 10 10 5.6 x 10 10 2.0 x
10 5 7.7 x 10 5
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Acid Ionization Constants 03

• pH of a Weak Acid
• Write a balanced equation equilibrium
  expression.
• Identify all species present that may affect pH.
• Major species will be HA, H, A. We are not
  concerned with H2O or OH at this point.
• Summarize the changes in concentration of HA, H,
  A using an I. C. E table.

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Acid Ionization Constants 04

• Initial Change Equilibrium Table Determine the
  pH of 0.50 M HA solution at 25C. Ka7.1 x 104.

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Acid Ionization Constants 05

• pH of a Weak Acid (Contd)
• Substitute new values into equilibrium
  expression.
• If Ka is significantly (gt1000 x) smaller than
  HA the expression (0.50 x) approximates to
  (0.50).
• The equation can now be solved for x and pH.
• If Ka is not significantly smaller than HA the
  quadratic equation must be used to solve for x
  and pH.

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Acid Ionization Constants 06

• The Quadratic Equation
• The expression must first be rearranged to
• The values are substituted into the quadratic and
  solved for a positive solution to x and pH.

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Acid Ionization Constants 07

• Calculate the pH of a 0.036M nitrous acid (HNO2)
  solution.
• What is the pH of a 0.122M monoprotic acid whose
  Ka is 5.7 x 104?
• The pH of a 0.10M solution of formic acid (HCO2H)
  is 2.39. What is the Ka of the acid?
• The pH of a 0.060M weak monoprotic acid is 3.44.
  Calculate the Ka of the acid.

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Acid Ionization Constants 08

• Percent Dissociation A measure of the strength
  of an acid.
• Stronger acids have higher percent dissociation.
• Percent dissociation of a weak acid decreases as
  its concentration increases.

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Acid Ionization Constants 09

• Concentration Dependence

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Base Ionization Constants 01

• Base Ionization Constant The equilibrium
  constant for the ionization of a base.
• The Ionization of weak bases is treated in the
  same way as the ionization of weak acids.
  B(aq) H2O(l) ? BH(aq) OH(aq)
• Calculations follow the same procedure as used
  for a weak acid but OH is calculated, not H.

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Base Ionization Constants 02
BASE Kb
CONJ. ACID Ka
5.6 x 10 4 4.4 x 10 4 4.1 x 10 4 1.8 x 10
5 1.7 x 10 9 3.8 x 10 10 1.5 x 10 14
C2H5NH2 (ethylamine) CH3NH2 (methylamine) C8H10N4O
2 (caffeine) NH3 (ammonia) C5H5N
(pyridine) C6H5NH2 (Aniline) NH2CONH2 (Urea)
C2H5NH3 CH3NH3 C8H11N4O2 NH4 C5H6N
C6H5NH3 NH2CONH3
1.8 x 10 11 2.3 x 10 11 2.4 x 10 11 5.6 x 10
10 5.9 x 10 6 2.6 x 10 5 0.67
Note that the positive charge sits on the nitrogen
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Base Ionization Constants 03

• Product of Ka and Kb multiplyingout the
  expressions for Ka and Kb leaves Kw.
  Ka ? Kb
  Kw
• What is the pH of a 0.40M ammonia solution?
• Calculate the pH of a 0.26M methylamine solution.

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Diprotic Polyprotic Acids 01

• Diprotic and polyprotic acids yield more than one
  hydrogen ion per molecule.
• One proton is lost at a time. Conjugate base of
  first step is acid of second step.
• Ionization constants decrease as protons are
  removed.

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Diprotic Polyprotic Acids 02
ACID Ka
CONJ. BASE Kb
Very Large 1.3 x 10 2 6.5 x 10 2 6.1 x 10
5 1.3 x 10 2 6.3 x 10 8 4.2 x 10 7 4.8 x 10
11 9.5 x 10 8 1 x 10 19 7.5 x 10 3 6.2 x 10
8 4.8 x 10 13
H2SO4 HSO4 C2H2O4 C2HO4 H2SO3 HSO3 H2CO3 HCO3
H2S HS H3PO4 H2PO4 HPO42
HSO4 SO4 2 C2HO4 C2O42 HSO3 SO3
2 HCO3 CO3 2 HS S 2 H2PO4 HPO42 PO43
Very Small 7.7 x 10 13 1.5 x 10 13 1.6 x 10
10 7.7 x 10 13 1.6 x 10 7 2.4 x 10 8 2.1 x 10
4 1.1 x 10 7 1 x 10 5 1.3 x 10 12 1.6 x 10
7 2.1 x 10 2
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Diprotic Polyprotic Acids 03

• Calculate the concentration of all species
  present in a 0.10M solution of oxalic acid
  (C2H2O4). Determine the pH of the solution.
• Calculate the concentration of all species
  present in a 0.20M solution of phosphoric acid
  (H3PO4). Determine the pH of the solution.

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Molecular Structure and Acid Strength 01

• The strength of an acid depends on its tendency
  to ionize.
• For general acids of the type HX
• The stronger the bond, the weaker the acid.
• The more polar the bond, the stronger the acid.
• For the hydrohalic acids, bond strength plays the
  key roll giving HF lt HCl lt HBr lt HI

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Molecular Structure and Acid Strength 02

• The electrostatic potential maps show all the
  hydrohalic acids are polar. The variation in
  polarity is less significant than the bond
  strength which decreases from 567kJ/mol for HF to
  299kJ/mol for HI.

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Molecular Structure and Acid Strength 03

• For binary acids in the same group, HA bond
  strength decreases with increasing size of A, so
  acidity increases.
• For binary acids in the same row, HA polarity
  increases with increasing electronegativity of A,
  so acidity increases.

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AcidBase Properties of Salts 01

• Salts that produce neutral solutions are those
  formed from strong acids and strong bases.
• Salts that produce basic solutions are those
  formed from weak acids and strong bases.
• Salts that produce acidic solutions are those
  formed from strong acids and weak bases.

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