Acids and Bases

1
Acids and Bases
Operational definitions are based on observed
properties. Compounds can be Classified as acid
or base by observing these sets of properties.
2
Properties of Acids

• Taste sour (acere Latin for sour) (Lemons,
  vinegar)
• Cause certain organic dyes to change colour
  (Turns blue litmus paper to red BAR)
• Acid properties are destroyed by Bases (React
  with bases to form a salt and water)
• Acid solutions are Electrolytes (substance in
  solution that conduct an electric current Acids
  can be strong or weak electrolytes)
• Acids react (corrode) with active metals (Group I
  and II as well as Zn and Aluminum) (Zn(s)
  2HCl(aq) ? ZnCl2(aq) H2(g))
• Acids react with carbonates (CO32-) and hydrogen
  carbonates (HCO31-) to produce carbon dioxide gas
  2HCl(aq) Na2CO3(s) ? 2NaCl(aq) H2O(l)
  CO2(g)
• Certain nonmetal oxides will dissolve to produce
  acid solutions. (SO3(g) H2O ? H2SO4(aq)
  (SO3(g) is the acid anhydride without water)

3
Properties of Bases

• Bases taste bitter mustard and soap
• Bases cause weak organic acids (dyes) to change
  colour (red litmus paper to blue BB Basic Blue
• Acids destroy base properties - react with acids
  to form salts and water
• Bases are electrolytes strong or weak
• Feel soapy, slippery
• Bases are formed when the oxide of some metals
  dissolve in water (CaO(s) H2O ? Ca(OH)2(aq)
  CaO is the base anhydride

4
Acid/Base definitions

• Definition 1 Arrhenius (traditional)
• Acids are compounds with ionizable hydrogen
  produce H ions (or hydronium ions H3O) in
  solution
• Bases are compounds that produce OH- ions in
  solution (problem some bases dont have
  hydroxide ions!)
• The reaction between an acid and a baseH(aq)
  OH-(aq) ? H2O (l)

5
Arrhenius acid is a substance that produces H
(H3O) in water. The HCl molecule is ionized.
(ionization)
Arrhenius base is a substance that produces OH-
in water. The ions are dissociated.
(dissociation)
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• Some acids have more than one ionizable hydrogen
• H2SO4 ? H(aq) HSO41-(aq
• HSO41- ? H(aq) SO42-(aq)
  H2SO4 is diprotic
• H3PO4(aq) ? H(aq) H2PO41-(aq)
• H2PO41-(aq) ? H(aq) HPO42-(aq)
• HPO42-(aq) ? H(aq) PO43-(aq)
• Phosphoric acid is a triprotic acid.

7
Water self-ionization
H2O ? H(aq) OH-(aq)
H OH- 10-7M at SATP
Keq H OH- H2O(l) Kw H
OH- 10-7 x 10-7 (at 25ºC) Kw 10-14 at SATP
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H2O ? H(aq) OH-(aq)

• What happens to this equilibrium if HCl(g)
  dissolves in the water?
• HCl(g) H2O(l) ? H3O(aq) Cl-(aq)
• Increasing Decreasing
• H2O ? H3O(aq) OH-(aq)
• H gt OH- acidic
• What happens when sodium hydroxide dissolves?
  NaOH(s) H2O ? Na(aq) OH-(aq)
• Decreasing Increasing
• H2O ? H3O(aq) OH-(aq)
• H lt OH- basic (alkaline solution)
• If H 10-7 then OH- 10-7 solution is
  neutral (SATP)

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pH and logs

• H is important in the study of acid-base
  chemistry. pH is the widely used scale to show
  H.
• pH -logH or pH 1 .
• logH
• H 10 pH (the antilog)
• A logarithm is the power to which ten must be
  raised to get a number.
• log1000 log(103) 3

10
pH calculations

• For a neutral solution
• pH -logH
• pH -log 10-7
• pH - -7
• pH 7 at SATP
• Example
• H 5 x 10-3
• pH -log 5 x 10-3
• pH -log 0.005
• pH - (-2.3) 2.3

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pH and pOH

• pOH - log OH- or OH- 10 - pOH
• Kw H x OH- 1 x 10-14 (at 25ºC)
• pKw pH pOH
• 14 pH pOH
• Example
• If pH (2.3) what is the OH-?
• pH pOH 14
• pOH 14 pH
• pOH 14 2.3
• pOH 11.7
• pOH -log OH-
• OH- inverse log -11.7 or (10 - 11.7)
• OH- 2.0 x 10-12

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H3O, OH- and pH

• What is the pH of the 0.0010 M NaOH solution?
• OH- 0.0010 (or 1.0 X 10-3 M)
• pOH - log 0.0010
• pOH 3
• pH pOH 14
• pH 14 3 11
• OR Kw H3O OH-
• 1.0 x10-14 H3O x 1.0 X 10-3
• H3O 1.0 x 10-11 M
• pH - log (1.0 x 10-11) 11.00

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Problem 1 The pH of rainwater collected in a
certain region of the northeastern New Brunswick
on a particular day was 4.82. What is the H ion
concentration of the rainwater?
H 1.51 x 10-5
Problem 2 The OH- ion concentration of a blood
sample is 2.5 x 10-7M. What is the pH of the
blood?
pOH 6.6 pH 7.4
Problem 3 A chemist dilutes concentrated
hydrochloric acid to make two solutions (a) 3.0
M and (b) 0.0024 M. Calculate the H3O, pH,
OH-, and pOH of the two solutions at 25C.
b) H3O 2.4x10-3, pH 2.62, pOH 11.38,
OH- 4.2 x 10-12
a) H3O 3.0, pH - 0.48, pOH 14.48,
OH- 3.3 x 10-15
Problem 4 What is the H3O, OH-, and pOH of
a solution with pH 3.67? Is this an acid, base,
or neutral?
H3O 2.14 x10-4, pOH 10.33, OH- 4.68x
10-11 It is an acid.
Problem 5 Problem 4 with pH 8.05?
H3O 8.92 x10-9, pOH 5.95, OH- 1.12x
10-6 It is an base.
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Acid/Base Definitions

• Definition 2 Brønsted Lowry
• Acids proton donor A proton is a hydrogen
  ion (the atom lost its electron)
• Bases proton acceptor (accepts a hydrogen ion)
• No longer needs to contain the OH- ion

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A Brønsted-Lowry acid is a proton donor A
Brønsted-Lowry base is a proton acceptor
acid
conjugate base
conjugate acid
base
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The Bronsted-Lowry concept
base
conjugate acid
conjugate base
acid
conjugate acid-base pairs

• Acids and bases are identified based on whether
  they donate or accept H.
• Conjugate acids and bases are found on the
  products side of the equation. A conjugate base
  is the same as the starting acid minus H.

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Practice problems
Identify the acid, base, conjugate acid,
conjugate base, and conjugate acid-base pairs
CH3COOH(aq) H2O(l) ? CH3COO(aq) H3O(aq)
acid
base
conjugate acid
conjugate base
conjugate acid-base pairs
OH (aq) HCO3(aq) ? CO32(aq) H2O(l)
acid
base
conjugate acid
conjugate base
conjugate acid-base pairs
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• Base Conjugate acid
• \ \
• NH3(g) H2O(l) ? NH4(aq) OH-(aq)
• /
  /
• Acid
  Conjugate Base
• HCl(aq) H2O(l) ? H3O(aq) Cl-(aq)
• Acid Base Conjugate Conjugate
• Acid Base
• The water has acted as both an acid and a base,
  depending on what it is mixed with. Substances
  that can act as both an acid and a base are
  amphoteric (also called amphiproteric).

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• Strong acid and base
• HA(aq) H2O(l) ? H3O(aq) A-(aq)
• B(aq) H2O(l) ? BH(aq) OH-(aq)
• At equilibrium the ionic form is favored
• Weak acid and base
• HA(aq) H2O(l) ? H3O(aq) A-(aq)
• B(aq) H2O(l) ? BH(aq) OH-(aq)
• At equilibrium the molecular form is favored

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CH3COOH(aq) H2O(l) ? H(aq) CH3COO-(aq)

• Keq H CH3COO- .
• CH3COOH H2O(l)
• H2O is a constant, so collect the constants
• (Keq)H2O(l) H CH3COO-
• CH3COOH
•  (Keq)H2O is represented Ka(ionization constant
  for an acid)
• Ka He CH3COO-e 1.8 x 10-5
• CH3COOHe
• Ka lt 1 weak acid
• General Formula for the ionization constant of a
  weak acid.

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• a) What is the pH of an ethanoic acid solution
  with a concentration of 0.100M?
• Ka CH3COOH 1.82 x 10-5
• CH3COOH(aq) ? H(aq) CH3COO -(aq)
• H x
• CH3COOHi 0.100M
• Ka He CH3COO-e He
  CH3COO-e 11 ratio
• CH3COOHe
• He x CH3COOHR (CH3COOHR
  ionized)
• Ka x2e CH3COOHe
  CH3COOHi - He 0.100M
• CH3COOHe Because it is a very weak
  acid CH3COOHR 0
• x2 Ka x CH3COOHe
• x2 1.82 x 10-5 x 0.100
• x2 1.82 x 10-6
• H x 0.00135M
• pH -logH pH -log0.00135 pH 2.87

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• b) What is the percent ionization of this acetic
  (ethanoic) acid solution?
• ionization H x 100
• CH3COOH
• 0.00135 x 100 1.35
• 0.100
• (very low degree of ionization)

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• The value of Ka for phosphoric acid, H3PO4(aq),
  is 7.0 x 10-3 at 25?C. a) Calculate the
  H3O in a 0.10 M solution of H3PO4.
• H3PO4 ? H(aq) H2PO4-(aq)
• Ka H H2PO4- H H2PO4- x
  H3PO4e H3PO4i - x
• H3PO4 
• Ka x2 .
• 0.10 x 
• 7.0 x 10-3 x2 .
• 0.10 x
• x2 -7.0 x 10-3x 7.0 x 10-4
• x2 7.0 x 10-3 x 7.0 x 10-4 0
• x -b v b2 4ac
• 2a
• x -7.0 x 10-3 v (7.0 x 10-3)2 4 x 1 x 7.0
  x 10-4
• 2 x 1
  

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• x -7.0 x 10-3 v 4.9 x 10-5 -2.8 x 10-3
• 2
• x -7.0 x 10-3 v 2.85 x 10-3
• 2
•  
• x -7.0 x 10-3 5.34 x 10-2
• 2
•  
• x 4.64 x 10-2
• 2
•  
• x 2.32 x 10-2 H
• pH - log2.32x10-2 1.63