Acid-Base Equilibria: Acids and Bases

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• Acid-Base Equilibria Acids and Bases
• What makes an Acid an Acid?
• An acid possess a sour taste
• An acid dissolves active metals magnesium
• An acid causes certain vegetable dyes to
  turn characteristic colors
• What makes a Base a Base?
• A bases possess a bitter taste
• A base feels slippery to the touch
• A base causes certain vegetable dues to turn a
  characteristic color

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7 strong acids and 8 strong bases

• Acids - HI, HBr, HCl, HClO3, HClO4, H2SO4, HNO3
• Bases LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2,
  Sr(OH)2, Ba(OH)2

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• The Arrhenius Definition of
• an Acid and a Base
• An acid is a substance that produces H ions in
  water solutions
• HCl ? H Cl-
• A base is a substance that produces OH- ions in a
  water solution
• NaOH ? Na OH-

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acid dissociation equations

• HC6H5O3 ? C6H5O31- H
• Fe(H2O)63 ? Fe(H2O)5(OH)2 H
• CH3CH2NH31 ? CH3CH2NH2 H

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• The Proton in Water
• When HCl dissolves in water we write
• HCl(g) ? H(aq) Cl-(aq)

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Reality for the Hydronium ion
H5O2
H9O4
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Acidic solutions are formed by a chemical
reaction in whichand acid transfers a proton
(H) to water, so we can write them either way.

• HCl(aq) H2O(aq) ?? H3O(aq) Cl-(aq)
• or
• HCl(aq) ? ? H(aq) Cl-(aq)

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Nitrogen compounds are Bronsted acids when they
are protonated.

• NH4Cl
• NH4 ? NH3 H
• CH3)2NH2 ? (CH3)2NH H

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The Bronsted-Lowry definition for Acids and Bases

• Acids may be defined as a substance that is
  capable of donating protons
• Bases may be defined as substance that accepts
  protons.
• HCl NH3 ? NH4 Cl-

acid base conjugate conjugate acid
base
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Is Water an Acid?

• NH3(aq) H2O(aq) ? NH4(aq) OH-(aq)

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Is Water a Base?

• HC2H3O2(aq) H2O(aq) ? H3O(aq) C2H3O21-(aq)

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The auto ionization of water

• The reaction occurs to a very small extent about
  1 in 108 molecules is ionized at any given
  moment

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Dissociation of Water, pH Scale

• H2O(l) ? H(aq) OH-(aq)
• K H OH-
• H2O
• since water is a liquid and its concentration is
  therefore constant, this expression may be
  written as
• Kw H OH- 1.0 x 10-14
• H OH- 1.0 x 10-7 M

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• Sample exercise Indicate whether each of the
  following solutions is neutral, acidic, or basic
  
• H 2 x 10-5 M
• OH- 0.010 M
• OH- 1.0 x 10-7 M

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• Calculate the concentration of H(aq) in
• a solution in which the OH- is 0.020M
• a solution in which the
• OH- 2.5 x 10-6 M.
• Indicate whether the solution is acidic or basic

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The pH Scale

• pH -log H
• If H 2. 5 x 105 the pH is?
• pH -log 2. 5 x 10-5 4.6
• If pH is 3.8 the H concentration is
• Antilog -3.8 1.58 x 10-4 M

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• In a sample of lemon juice, H 3.8 x 10-4 M.
  What is the pH?
• A commonly available window cleaner has a H
  5.3 x 10-9 M. What is the pH?
• In a sample of freshly pressed apple juice has a
  pH of 3.76. Calculate the H

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What if we took the log of the Kw expression

• Kw H OH- 1.0 x 10-14
• pKw pH pOH 14

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• What is the pH, H, OH-, of a solution with a
  pOH of 2.5?
• Is the solution acidic or basic?

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Major species

• HCl(aq) H2O(aq) ?? H3O(aq) Cl-(aq)
• or
• HCl(aq) ? ? H(aq) Cl-(aq)
• HC2H3O2(aq) H2O(aq) ? H3O(aq) C2H3O21-(aq)
• Pb(NO3)2 NaCl ?NaNO3 PbCl2

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Indicators
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What is the pH of 0.010 M solution of HCl?

• If it ionizes completely which is what strong
  means then take the negative log of the
  concentration.
• HCl(aq) ? H(aq) Cl-(aq)
• .01M .01M .01M
• pH 2

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What is the pH of a solution made from 20mL of
2.0M HCl and 35mL of 3.2M HNO3?
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What about H2SO4

• H2SO4 ? H HSO41- (Strong)
• HSO41- ? H SO42- (Weak)

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What about weak acids

• HX(aq) ? H(aq) X-(aq), then
• Ka HX-
• HX
• The smaller the value of the acid dissociation
  constant Ka, the weaker the acid

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What is the Ka of a 0.10 M solution of formic
acid (HCHO2) which has a pH 2.38?

• HCHO2 ? H CHO21-
• I 0.10 0 0
• C .00417 .00417 .00417
• E .0958 .00417 .00417
• Ka (.00417)2 1.8 x 10-4
• .0958

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What is the concentration of H ions in a 0.10 M
solution of HC2H3O2 (Ka 1.8 x 10-5)? pH?
ionization?

• HC2H3O2 ? H C2H3O21-
• I .10 0 0
• C -X X X
• E .10 X X X
• 1.8 x 10-5 X2 X 1.3 x 10-3
• .10 X pH 2.87

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percent dissociation

• 1.3 X 10-3 x 100 1.3
• .10

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What is the pH and percent ionization of a 0.20 M
solution of HCN? Ka 4.9 x 10-10
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Acid-Base Equilibria Strong Bases
The most common soluble strong Bases are the
hydroxides of group IA and Ca, Ba and Sr
What is the pH of a 0.010 M solution of Ba(OH)2?
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Amines
Anions of Weak Acids
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Dealing with Weak Bases
The base dissociation constant Kb refers to the
equilibrium in which a base reacts with H2O
to form the conjugate acid and OH-
Weak base H2O ? conjugate acid OH-
NH3 (aq) H2O (l) ? NH4 (aq) OH-(aq)
NH4 OH-
Kb
NH3
Calculate the OH- in a 0.15 M solution of NH3.
NH3 H2O ? NH4 OH-
I
C
E
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Polyprotic Acids

• H2SO3(aq) ? H(aq) HSO3-(aq)
• Ka1 1.7 x 10-2
• HSO3-(aq) ? H(aq) SO32-(aq)
• Ka2 6.4 x 10-8
• Calculate the pH of a .1M solution

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Anions of Weak Acids
HC2H3O2(aq) H2O(aq) ?? H3O(aq) C2H3O2- (aq)
Bronsted base
Bronsted acid
Conjugate acid
Conjugate base
A second class of weak base is composed of the
anions of weak acids Anions of weak acids can be
incorporated into salts.
NaC2H3O2 ?? Na(aq) C2H3O2- (aq)
C2H3O2- H2O ? HC2H3O2 OH- Kb 5.6 x 1010
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• NaOH(aq)HC2H3O2(aq?H2ONaC2H3O2
• NaC2H3O2 ?? Na(aq) C2H3O2-(aq)
• Na(aq) H2O ? NaOH(aq) H(aq)
• C2H3O2-(aq)H2O ?HC2H3O2 (aq)OH- (aq)

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• NH4Cl ? NH4 Cl-
• NH4 ? NH3 H

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• NH4Cl
• NaC2H3O2
• NH4C2H3O2

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Anions of Weak Acids
Calculate the pH of a 0.01 M solution of sodium
hypochlorite (NaClO)
ClO- H2O ? HClO OH-
I
C
E
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Now its you turn the Kb for BrO- is 5.0 x
10-6. Calculate the pH of a 0.050 M solution of
NaBrO
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Ka and Kb
NH3(aq) H2O? NH4 (aq) OH-(aq)
NH4(aq) ? NH3(aq) H(aq)
NH4OH-
HNH3
Ka
Kb
NH4
NH3
NH4(aq) ? NH3(aq) H (aq)
NH3(aq) H2O(l)? NH4(aq) OH- (aq)
H2O ? H(aq) OH-(aq)
When two reactions are added to give a third
reaction, the equilibrium constant for the third
reaction reaction is given by the product of the
equilibrium constants for the two added reactions
pKa pKb pKw
Ka x Kb Kw
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Calculate the (a) base-dissociation constant, Kb,
for the fluoride ion, is the pKa of HF 3.17
pKa -log Ka
3.17 -log Ka
Antilog -3.17 6.76 x 10-4
Since
Ka x Kb Kw
(6.76 x 10-4)x Kb 1.0 x 10-14
Kb 1.0 x 10-14/ 6.76 x 10-4 1.5 x 10-11
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Calculate the pKb for carbonic acid (Ka 4.3 x
10-7)
Now its your turn
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Acid-Base Properties of Salt Solutions

• Anions of weak acids, HX, are basic and will
  react with
• H2O to produce OH-

X- (aq) H2O (l) ? HX(aq) OH-(aq)

• Anions of strong acids, such as NO3-, exhibit no
  basicitiy,
• these ions do not react with water and
  consequently do not
• influence the pH

• Anions of polyprotic acids, such as HCO3-, that
  still have ionizable protons are capable of
  acting as either proton donors or acceptors
  depending upon the magnitudes of the Ka or Kb

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• Anions of polyprotic acids, such as HCO3-, that
  still have ionizable protons are capable of
  acting as either proton donors or acceptors
  depending upon the magnitudes of the Ka or Kb

Predict whether the salt Na2HPO4 will form an
acidic or basic solution on dissolvingin water.
Na2HPO4 ? 2Na (aq) HPO4-
HPO4- acting like an acid
K3 4.2 x 10-13
HPO4-(aq) H2O ? H3O PO43-(aq)
HPO4- acting like an base
1.0 x 10-14
HPO4- (aq) H2O ? H2PO42-(aq) OH-(aq)
Kw
Kb

6.2 x 10-8
Ka
So HPO- is the conjugate base of H2PO4-. Since
the K2 of H2PO4- 6.2 x 10-8 then
1.6 x 10-7

Since Kb is larger than Ka, HPO4- will act like a
base
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• Salt derived from a strong base and a strong acid
  
• will have a pH of 7

• Salt derived from a strong base and a weak acid
• will have a pH above 7

• Salt derived from a weak acid and a weak base
• depends upon whether the dissolved ion acts as
• an acid or a base as determined by the size of
• the Ka or Kb

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Acid-Base Character and Chemical Structure
two things to consider polarity difference and
strength of the bond
HF gt HCl gt HBr gt HI (most polar least)
Based on electronegativity difference HF is
the most polar but a weak acid because the bond
is so strong
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Acid strength of oxyacids

• The more oxygen's the stronger the acid because
  of the oxygen pulling the electrons towards
  themselves.
• HClO4gt HClO3gt HClO2gt HOCl