Unit 18 Acid-Base Equilibria: Buffers

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Unit 18Acid-Base Equilibria Buffers Hydrolysis
CHM 1046 General Chemistry and Qualitative
Analysis

• Textbook Reference
• Chapter 19 (sec. 1-4)
• Modules 7-8

• Dr. Jorge L. Alonso
• Miami-Dade College Kendall Campus
• Miami, FL

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The Common-Ion Effect
A solution composed of two substances, each
containing a same ion in common. One is a weak
electrolyte (equilibrium) the other strong(not
equil). Examples (1) HF(aq) NaF(s)
(2) HC2H3O2(aq) NaC2H3O2 (s)
(3) HF(aq) HCl (aq)
Add strong electrolyte
NaC2H3O2
Consider a solution of acetic acid, a weak
electrolyte
What will Le Châtelier predict will happen to the
equilibrium?
It will shift to the left.

• The extent of ionization of a weak electrolyte
  is decreased by adding to the solution a strong
  electrolyte that has an ion in common with the
  weak electrolyte.

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The Common-Ion Effect
Problem

• Calculate the fluoride ion concentration and pH
  of a solution that is 0.20 M in HF and 0.10 M in
  HCl. Ka for HF is 6.8 ? 10-4.

Because HCl, a strong acid, is also present, the
initial H3O is not 0, but rather 0.10 M.
HF, M H3O, M F-, M
Initially 0.20 0.10 0
Change -x x x
At Equilibrium 0.20 - x ? 0.20 0.10 x ? 0.10 x
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The Common-Ion Effect

• X
• x 1.4 ? 10-3

What is F- ? And pH?
Therefore, F- x 1.4 ? 10-3 H3O 0.10
x 0.10 1.4 ? 10-3 0.10 M

• So, pH -log (0.10)
• pH 1.00

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Buffers

• Solutions that are particularly resistant to pH
  changes, even when strong acid or base is added.

• Buffers are solutions of a weak acid mixed with a
  salt of its conjugate base (a weak conjugate
  acid-base pair solution).

i.e., acetic acid sodium acetate


Which is less acidic, buffer (WA Salt of
c.base) or same conc. of weak acid alone?
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Buffers
Add acid (H3O)
Add base (OH-)
HF H2O H3O F-

OH- HF ? H2O F-
HF ? H3O F-
If a hydroxide is added to an equimolar solution
of HF in NaF, for example, the HF reacts with the
OH- to make F- and water.

• If an acid is added, the F- reacts with H to
  form HF and water.

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pH of Buffer Calculations The
HendersonHasselbalch Equation
For the dissociation of a generic acid, HA

• Rearranging slightly, this becomes

Taking the negative log of both side, we get
Rearranging, this becomes
?
HendersonHasselbalch Equation.
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HendersonHasselbalch Equation
Problem

• What is the pH of a buffer that is 0.12 M in
  lactic acid, HC3H5O3, and 0.10 M in sodium
  lactate? Ka 1.4 ? 10-4.

pH 3.85 (-0.08) pH 3.77
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pH Range of Buffers Systems
pKa 3. 3. 4. 4. 7. 9. 9.

• The pH range is the range of pH values over which
  a buffer system works effectively.
• It is best to choose an acid with a pKa close to
  the desired pH.

pH pKa
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base acid
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When Strong Acids or Bases Are Added to a Buffer

• You end up with two problems (1) an acid base
  neutralization, and (2) an equilibrium problem

Strong base (OH-)
Strong acid (H3O)
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Addition of Strong Acid or Base to a Buffer
Add base (OH-)
Add acid (H3O)
Add some OH-
(HA OH- ? A- H2O)
(H3O A- ? HA H2O)

1. Neutralization is NOT an equilibrium reaction.
2. However, it affects the amounts of the weak acid
   HA and its conjugate base A- left over, and a
   new equilibrium will be established.

What are the new equilibrium conc. after acid or
base is added?
(1) Use Mole ICEnd Table used for
neutralization Rx
Mole (?) HA A- H3O or OH-
Initially ? ? ?
Change (H3O or OH-) ? ? - ?
End (after)reaction ? ? 0 ?
Add some H3O
Moles (?) ? Molarity (?/L)
For a buffer
(2)
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Calculating pH Changes in Buffers
Problem

• A buffer is made by adding 0.300 mol
  HC2H3O2 and 0.300 mol NaC2H3O2 to enough water to
  make 1.00 L of solution. The pH of the buffer is
  4.74. Calculate the pH of this solution after
  0.020 mol of NaOH is added.

HC2H3O2(aq) OH-(aq) ? C2H3O2-(aq) H2O(l)
Before the reaction, since mol HC2H3O2 mol
C2H3O2- pH pKa -log (1.8 ? 10-5) 4.74
The 0.020 mol NaOH will react with 0.020 mol of
the acetic acid
Use Mole ICEnd Table used for neutralization
Rx
Mole HC2H3O2 C2H3O2- OH-
Initially 0.300 ? 0.300 ? 0.020 ?
Change - 0.020 ? 0.020 ? - 0.020 ?
End (after) reaction 0.280 ? 0.320 ? 0.000 ?
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Calculating pH Changes in Buffers
Mole HC2H3O2 C2H3O2- OH-
Initially 0.300 ? 0.300 ? 0.020 ?
Change - 0.020 ? 0.020 ? - 0.020 ?
End (After) reaction 0.280 ? 0.320 ? 0.000 ?
Now use the HendersonHasselbalch equation to
calculate the new pH
pH 4.74 0.058 pH 4.80
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Hydrolysishydro water lysis breaking
down
Module 8
Electrochemistry (also called electrolysis or
electrohydrolysis)
elect.
2 H2O 2 H2 O2
Organic/Biochemistry (enzymatic splitting of
organic molecules by using water)
hydrolysis
dehydration synthesis
Acid Base Chemistry (splitting water by cations
or anions to form acidic or basic solutions.
Cation HOH CationOH H
Anion- HOH HAnion OH-
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The Acid-Base Properties of Salt Solutions and
Hydrolysis
Salt (Ionic Compound) Metal Nonmetal
Cation Anion-
Hydrolysis splitting water by cations or anions
to form acidic or basic solutions
Cation HOH CationOH
H
Cations of Weak Bases
Al 3
Al3
Anion- HOH HAnion
OH-
Anions of Weak Acids
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The Effect of Anions
conjugate

• An anion that is the conjugate base of a strong
  acid will not affect the pH.

• An anion that is the conjugate base of a weak
  acid will increase the pH (behave as bases).

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Effect of Cations

• Cations of the strong bases will not affect the pH

SOLUBILITY RULES for Ionic Compounds (Salts) All
OH- are insoluble, except for IA metals, NH4,
Ca2, Ba2 , and Sr2 (heavy IIA).

• Cations of a weak bases will lower the pH (behave
  as acids)

• Greater charge and smaller size make a cation
  more acidic.

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Ka of Weak Acid and Kb of its Conjugate Base

• The Ka of an acid and Kb of its conjugate base
  are related in this way Ka ? Kb Kw
• Therefore, if you know one of them, you can
  calculate the other.

Problem Is NaC2H3O2 acidic or basic? Calculate
its Ka or Kb.
5.6 x 10-10
Ka ? Kb Kw
(1.0 x 10-14)
Kb
(1.8 x 10-5) ? Kb (1.0 x 10-14)
(1.8 x 10-5)
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Effect of Cations and Anions
ANION
CATION

• Cations of the strong bases will not affect the pH

• When a solution contains both the conjugate base
  of a weak acid and the conjugate acid of a weak
  base, the affect on pH depends on the Ka and Kb
  values.

SOLUBILITY RULES for Ionic Compounds (Salts) All
OH- are insoluble, except for IA metals, NH4,
Ca2, Ba2 , and Sr2 (heavy IIA).

• Cations of a weak bases will lower the pH (behave
  as acids)

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Acidic and Basic Salts
Acid Salts salts of weak polyprotic acids. Are
not necessarily acidic, but do neutralize bases.
Examples Acid Salt Acid Salt(s) HNO3 NaNO3 --
--------- H2CO3 Na2CO3 NaHCO3 H3PO4 Na3PO4 Na2
HPO4 NaH2PO4
Basic Salts salts of weak polyhydroxy bases. Are
not necessarily basic, but do neutralize acids.
Mg(OH) 2(s) Fe(OH)3(s) Cr(OH) 3(s)
Milk of Magnesia

• Solubility Rule
• All OH- are insoluble except for IA metals, NH4
  slightly soluble Ca 2 Ba2 Sr2

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Buffers can act as eitheracids or bases

• ...they are amphiprotic
• .
• HCO3-(aq)
• H2PO4-(aq)
• H2O(l)

Add acid
Add base
OH-
H3O
? CO32- H2O
H2O H2CO3 ?
H2O H3PO4 ?
? HPO42- H2O
? OH- H2O
H2O H3O ?
Not a buffer
accepts protons /
donates protons
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2007 (B)
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Making Solution Molarity
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2007B Q5
Carboxylic acid
Organic Acids carboxylic acid functional group
-COOH ? -COO- H
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2005A Q1
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Review of Strong Acid Bases
Strong acids have very weak conjugate bases HX
H2O ? H3O X- acid
base conj. a conj. b
Has no affinity for H
Example HCl
Strong bases have very weak conjugate acids MOH
? M OH- base conj. a
conj. b
Has no affinity for OH-
Example NaOH
Salts of Strong acids bases have very weak
conjugate acids bases MX H2O ?
M(aq) X- (aq) salt
conj. a conj. b
Example NaCl
Have no affinity for H or OH-
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Review of Weak Acid Bases
Weak acids (HA) have very strong conjugate bases
(A-) HA H2O ? H3O
A- acid base conj. a
conj. b
Has affinity for H
Example HF
Weak bases (B) have very strong conjugate acids
(BH) B HOH ? BH
OH- base acids conj. a
conj. b
Example NH3
Has affinity for OH-

• Salts (1) of strong base weak acid
• MA ? M(aq) A-(aq)
• A- HOH ? HA OH- (hydrolysis)
• (2) of a weak base strong acid
• BHX ? BH(aq) X-(aq)
• BH HOH ? B H3O (hydrolysis)
• (3) of a weak base weak acid
• BHA ? BH(aq) A-(aq)

Example NaF
Example NH4Cl
Example NH4F
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Reactions of Cations with Water

• Cations with acidic protons (like NH4) will
  lower the pH of a solution NH4 H2O ? H3O
  NH3

H
O

• Most metal cations that are hydrated in solution
  also lower the pH of the solution, acting as
  Lewis acids
• The attraction between nonbonding electrons on
  waters oxygen and the metal cation causes a
  shift of the electron density in water.
• This makes the O-H bond more polar and the water
  more acidic.

H
H
O
H
Greater charge and smaller size make a cation
more acidic.
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The Effect of Cations
conjugate