Unit 2 Acids and Bases

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Unit 2 Acids and Bases

• http//www.cdli.ca/courses/
• http//www.cbhs.k12.nf.ca/adrianyoung/

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• Topics
• Properties / Operational Definitions
• Acid-Base Theories
• pH pOH calculations
• Equilibria (Kw, Ka, Kb)
• Indicators
• Titrations
• STSE Acids Around Us

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Operational Definitions

• An Operational Definition is a list of
  properties, or operations that can be performed,
  to identify a substance.
• See p. 550 for operational definitions of acids
  and bases

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Operational Definitions (Properties see p. 550)

• Acids
• pH lt 7
• taste sour
• react with active metals (Mg, Zn) to produce
  hydrogen gas

• Bases
• pH gt 7
• taste bitter
• no reaction with active metals
• feel slippery

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Operational Definitions

• Acids
• blue litmus turns red
• react with carbonates to produce CO2 gas

• Bases
• red litmus turns blue
• no reaction with carbonates

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Operational Definitions

• Acids
• conduct electric current
• neutralize bases to produce water and a salt

• Bases
• conduct electric current
• neutralize acids to produce water and a salt

any ionic compound
any ionic compound
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Acid-Base Theories

• 1. Arrhenius Theory (p. 549 )
• acid any substance that dissociates or IONIZES
  in water to produce H ions
• ie. an acid must contain H ions

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Arrhenius Theory

• eg.
• HCl(aq) ?
• H2SO4(aq) ?
• HSO4-(aq) ?

H(aq) Cl-(aq)
H(aq) HSO4-(aq)
H(aq) SO42-(aq)
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Arrhenius Theory

• base any substance that dissociates in water
  to produce OH- ions
• ie. a base must contain OH- ions

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Arrhenius Theory

• eg. NaOH(aq) ?
• Ca(OH)2(aq) ?

Na(aq) OH-(aq)
Ca(aq) 2 OH-(aq)
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Arrhenius Theory

• Which is an Arrhenius acid?
• a) KOH c) CH4
• b) HCN d) CH3OH

• Which is a Arrhenius base?

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Limitations of Arrhenius theory (p.551)

• H cannot exist as an ion in water.
• The positive H ions are attracted to the polar
  water molecules forming HYDRONIUM ions or
  H3O(aq)
• H(aq) H2O(l) ? H3O(aq)

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Limitations of Arrhenius theory

• CO2 dissolves in water to produce an acid.
• NH3 dissolves in water to produce a base.
• Neither of these observations can be explained
  by Arrhenius theory

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Limitations of Arrhenius theory

• Some acid-base reactions can occur in solvents
  other than water.
• Arrhenius theory can explain only aqueous acids
  or bases.

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Limitations of Arrhenius theory

• Arrhenius theory is not able to predict whether
  certain species are acids or bases.
• eg. NaHSO4 H2PO4- HCO3-

Arrhenius theory needs some work
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Acid-Base Theories
To be used when Arrhenius is inadequate

• 2. Modified Arrhenius Theory (p. 552)
• acid any substance that reacts with water to
  produce H3O ions
• eg.
• HCl(g) H2O(l) ? H3O(aq) Cl-(aq)

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Modified Arrhenius Theory

• base any substance that reacts with water to
  produce OH- ions
• eg.
• NH3(aq) H2O(l) )? NH4(aq) OH-(aq)
• pp. 558, 559 s 1, 3, 8, 9

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Acid-Base Theories

• 3. Brønsted-Lowry Theory (p. 553)
• acid any substance from which a proton (H) may
  be removed
• ie. an acid is a substance that loses a proton
  (H)

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Brønsted-Lowry Theory

• base any substance that can remove a proton
  (H) from an acid.
• ie. a base is a substance that gains a proton
  (H)
• In BLT , an acid-base reaction
• requires the transfer of a proton
• (H) from an acid to a base.

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Brønsted-Lowry Theory
conjugate acid
base

• eg.
• HCN(aq) NH3(aq) ?

?
CN-(aq) NH4(aq)
conjugate base
acid
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Brønsted-Lowry Theory

• What is a conjugate acid-base pair?? (p. 554)
• Two particles (molecules or ions) that differ by
  one proton are called a conjugate acid-base pair.
  
• The conjugate base forms when an acid loses a
  proton.
• The conjugate acid forms when a base gains a
  proton (H).

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Brønsted-Lowry Theory
conjugate acid
base
conjugate base
acid
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Brønsted-Lowry Theory
conjugate acid
base

• eg.
• NH3(aq) H2O(l) ? NH4(aq) OH-(aq)
• H2O(l) H2O(l) ?

?
conjugate base
acid
?
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Brønsted-Lowry Theory

• - an amphoteric substance can be either an acid
  or a base
• these include WATER and negative ions that
  contain at least one hydrogen atom
• eg. H2O, HCO3-(aq), H2PO4-(aq)

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Brønsted-Lowry Theory
p.557 s 1 7 p. 558 s 8, 9 p. 559 s
2, 4-7, 10,11
Quiz - Tuesday Mar. 1
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Strength of Acids and Bases
Strong acids produce more H ions OR more H3O
ions than weak acids with the same molar
concentration

• A strong acid is an acid that ionizes or
  dissociates 100 in water
• eg. HCl(aq)?
• Strong acids react 100 with water (BLT)
• eg. HCl(aq) H2O(l) ?

H(aq) Cl-(aq)
H3O(aq) Cl-(aq)
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Strength of Acids and Bases

• NOTE The equilibrium symbol, º , is NOT used
  for strong acids because there is NO REVERSE
  REACTION.

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Strength of Acids and Bases

• A weak acid is an acid that ionizes or
  dissociates LESS THAN 100
• eg. HF(aq)
• Weak acids react less than 100 with water
• eg. HF(aq) H2O(l)

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Strength of Acids and Bases

• For weak acids, an equilibrium is established
  between the original acid molecule and the ions
  formed.
• DO NOT confuse the terms strong and weak with
  concentrated and dilute.

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Strength of Acids and Bases

• eg. Classify the following acids
• 0.00100 mol/L HCl(aq)
• strong and dilute
• 12.4 mol/L HCl(aq)
• strong and concentrated
• 10.5 mol/L CH3COOH(aq)
• weak and concentrated

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Strength of Acids and Bases

• monoprotic acids that contain or lose one
  proton
• diprotic acids that contain or lose two protons
• polyprotic acids that have more than one proton

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Strength of Acids and Bases

• A strong base is a base that dissociates 100 in
  water, or reacts 100 with water, to produce OH-
  ion.
• The only strong bases are hydroxide compounds of
  most Group 1 and Group 2 elements
• eg. NaOH(s) ?
• Ca(OH)2(s) ?

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Strength of Acids and Bases

• A weak base is a base that reacts less than 100
  in water to produce OH- ion.
• eg. S2-(aq) H2O(l) º HS-(aq) OH-(aq)

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Writing Acid-Base Equations (BLT)

• Step 1 List all the molecules/ions present in
  the solution
• ionic compounds form cations and anions
• strong acids exist as hydronium ion and the anion
  (conjugate base)
• for weak acids use full formula of the compound
  (i.e. un-ionized molecule)
• always include water in the list.

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Writing Acid-Base Equations (BLT)

• Step 2 Identify the strongEST acid and the
  strongEST base from Step 1.
• Step 3 Write the equation for the reaction by
  transferring a proton from the strongest acid to
  the strongest base.

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Writing Acid-Base Equations (BLT)

• Step 4 Determine the type of reaction arrow to
  use in the equation.
• Stoichiometric (100) reactions occur between
• Hydronium (H3O) and bases stronger than nitrite
  (NO2-)
• hydroxide (OH-)and acids stronger than
  hypochlorous acid (HOCl)

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Writing Acid-Base Equations (BLT)

• Step 5 Determine the position of the equilibrium
  by comparing the strengths of both acids in the
  equation.
• The favoured side is the side with the weaker
  acid! 

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Writing Acid-Base Equations (BLT)

• Sample problems
• Write the net ionic equation for the acid-base
  reaction between
• - aqueous sodium hydroxide (NaOH(aq)) and
  hydrochloric acid (HCl(aq)).

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• species present

Na(aq)
OH-(aq)
H3O(aq)
Cl-(aq)
H2O(l)
strongest acid
strongest base
H3O(aq) OH-(aq)
H2O(l) H2O(l)
OR
H3O(aq) OH-(aq) ? 2 H2O(l)
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Writing Acid-Base Equations (BLT)

• Sample problems
• Write an equation for the acid-base reaction
  between nitrous acid (HNO2(aq)) and aqueous
  sodium sulfite (Na2SO3(aq)).

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• species present

Na(aq)
SO32-(aq)
HNO2(aq)
H2O(l)
SB
SA
º
HNO2(aq) SO32-(aq)
NO2-(aq) HSO3 - (aq)
Weaker Acid
Stronger Acid
Products favored
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Write the Net Ionic Equation for each aqueous
reaction below

• Na2CO3(aq) and CH3COOH(aq)
• NH3(aq) and HNO2(aq)
• HNO3(aq) and RbOH
• H2SO4(aq) and K3PO4(aq)
• HF(aq) and NH4CH3COO(aq)
• CaCl2(aq) and PbSO4(aq)
• p. 564 s 10 11

Acids Bases 3
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• species present

H2O(l)
Na(aq)
CH3COOH(aq)
CO32-(aq)
SB
SA
º
CH3COOH(aq) CO32-(aq)
CH3COO-(aq) HCO3-(aq)
Weaker Acid
Stronger Acid
Products Favoured
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• species present

HNO2(aq)
NH3(aq)
H2O(l)
SA
SB
º
HNO2(aq) NH3(aq)
NO2-(aq) NH4 (aq)
Weaker Acid Products favored
Stronger Acid
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• species present

Rb(aq)
NO3-(aq)
OH-(aq)
H3O(aq)
H2O(l)
strongest acid
strongest base
H3O(aq) OH-(aq)
H2O(l) H2O(l)
OR
H3O(aq) OH-(aq) ? 2 H2O(l)
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• species present

K(aq)
HSO4-(aq)
PO43-(aq)
H2O(l)
H3O(aq)
SA
SB
H3O(aq) PO43-(aq)
H2O(l) HPO42-(aq)
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• species present

HF(aq)
H2O(l)
NH4(aq)
CH3COO-(aq)
SA
SB
º
HF(aq) CH3COO-(aq)
F-(aq) CH3COOH(aq)
Weaker Acid Products favored
Stronger Acid
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strongest acid

• species present

Pb2(aq)
SO42-(aq)
Ca2(aq)
Cl-(aq)
H2O(l)
strongest base
º
H2O(l) SO42-(aq)
HSO4-(aq) OH-(l)
Weaker Acid Reactants favored
Stronger Acid
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NO!! Products are NOT always favoured

• Try these
• CH3COOH(aq) NH4F(aq)
• HCN(aq) NaHS(aq)

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Acid-Base Calculations

• Kw
• Ka Kb
• H3O OH-
• pH pOH

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Kw (Ionization Constant for water)

• With very sensitive conductivity testers, pure
  water shows slight electrical conductivity.
• PURE WATER MUST HAVE
• A SMALL CONCENTRATION OF
• DISSOLVED IONS

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Kw
Auto-Ionization of water

• H2O(l) H2O(l)

º H3O(aq) OH-(aq)
K
H3O OH-
Kw

• H2O H2O

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Kw

• In pure water at 25 C
• H3O 1.00 x 10-7 mol/L
• OH- 1.00 x 10-7 mol/L
• Calculate Kw at 25 C.

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H2O(l) H2O(l) º H3O(aq) OH-

• LCP What happens if we add OH- ions
  (NaOH(aq)) to water?
• shift to the left
• H3O ?
• OH- ?
• Does Kw change?

GET REAL!!
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Kw H3O OH- 1.00 x 10-14 H3O
OH-
H3O OH-
0.00357 M
4.89 x 10-3 mol/L
12.5 M
1.50 mol/L
2.80 x 10-12
2.04 x 10-12
8.00 x 10-16
6.67 x 10-15
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Calculations with Kw (p. 564 566)

• For strong acids and strong bases, the H3O
  and OH- may be calculated using the solute
  concentration.
• eg. What is the H3O in a 2.00 mol/L solution
  of HNO3(aq)?
• Ans 2.00 mol/L
• OH- ???

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Calculations with Kw

• eg. What is the OH- in a 2.00 mol/L solution of
  NaOH(aq)?
• Ans 2.00 mol/L
• eg. What is the OH- in a 2.00 mol/L solution
  of Ca(OH)2(aq)?
• Ans 4.00 mol/L
• H3O ???

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Calculations with Kw

• eg. What molar concentration of Al(OH)3(aq) is
  needed to obtain a OH- 0.450 mol/L?
• Ans 0.150 mol/L

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What is the H3O and OH- in
H3O mol/L OH- mol/L




1.0 x 10-8
1.0 x 10-6
5.00 x 10-14
0.200
1.50
6.67 x 10-15
1.0 x 10-2
1.0 x 10-12
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solute H3O OH-
0.680 mol/L HCl(aq)
1.50 M NaOH
0.0500 M Ca(OH)2(aq)
_____ mol/L HClO4(aq) 0.450 M
____ mol/L Mg(OH)2(aq) 0.500 mol/L
1.47 x 10-14
0.680
6.67 x 10-15
1.50
1.00 x 10-13
0.100
0.450
2.22 x 10-14
0.250
2.00 x 10-14
p. 566 s 12 - 15
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pH and pOH (See p. 568)
By what factor does the H3O change when the pH
value changes by 1? by 2?
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pH and pOH (See p. 568)
The H3O changes by a factor of 10 (10X) for
each pH changes of 1.
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pH and pOH FORMULAS

• pH -log H3O
• H3O 10-pH

pOH -log OH- OH- 10-pOH
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pH and pOH

• eg. What is the pH of a 0.0250 mol/L solution of
  HCl(aq)?
• What is the pOH of a 0.00087 mol/L solution of
  NaOH(aq)?
• What is the pH of a 1.25 mol/L solution of
  KOH(aq)?

H3O 0.0250 mol/L
pH 1.602
OH- 0.00087 mol/L
pOH 3.06
OH- 1.25 mol/L
H3O 8.00 x 10-15 mol/L
pH 14.097
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Significant digits in pH values?
The number of significant digits in a
concentration should be the same as the number of
digits to the right of the decimal point in the
pH value. eg. In a sample of OJ the H3O
2.5 10-4 mol/L pH 3.60 (See p. 568)
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H3O OH- pH pOH
0.0035
1.2 x 10-5
4.68
9.15
8.33 x 10-15
-1.10
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H3O OH- pH pOH
0.0035
1.2 x 10-5
4.68
9.15
8.33 x 10-15
-1.10
2.9 x 10-12
2.46
11.54
8.3 x 10-10
9.08
4.92
2.1 x 10-5
4.8 x 10-10
9.32
4.85
1.4 x 10-5
7.1 x 10-10
14.079
1.20
-0.079
7.9 x 10-16
15.10
13
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pH, pOH and Kw

• p. 569 s 16 19
• p. 572 s 20 25
• Examine 23. Where is the energy term in this
  equation?
• H2O(l) H2O(l) º H3O(aq)
  OH-(aq)

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Dilutions

• When a solution is diluted the number of moles
  does not change.
• OR ninitial nfinal
• CiVi CfVf

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• eg. 400.0 mL of water was added to 25.0 mL of
  HCl(aq) that had a pH of 3.563. Calculate the pH
  of the resulting solution.
• calculate H3O
• dilution formula
• calculate pH

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Before dilution H3O 10-3.563
2.753 x 10-4 After dilution (2.753 x 10-4)
(25.0 mL) (Cf)(425.0 mL) H3O 1.609 x
10-5 pH -log (1.609 x 10-5) 4.793
p.574 s 26 - 29