Chemistry 223 Chapter 21:
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Chemistry 223 Chapter 21: Buffers and the Titration of Acids and Bases PART 2 PowerPoint PPT Presentation


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Chemistry 223 Chapter 21: Buffers and the Titration of Acids and Bases PART 2. Generally, “ x is small” will work when both of these are true: initial [ ]’s of acid & salt are K a is _______________ Most problems: initial acid & salt [ ]’s should be _____ to ______ > value of K a.

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Chemistry 223 Chapter 21: Buffers and the Titration of Acids and Bases PART 2

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Chemistry 223 Chapter 21:

Buffers and the Titration of

Acids and Bases PART 2


  • Generally, “x is small” will work when both of these are true:

    • initial [ ]’s of acid & salt are

    • Ka is _______________

  • Most problems: initial acid & salt [ ]’s should be _____ to ______ > value of Ka


How Much Does the pH of a Buffer Change When an Acid or Base Is Added?

  • Calculating new pH after adding acid or base requires doing 2 calculations:


  • stoichiometry calculation for added chemical with HA or A- of buffer to reduce initial [ ] and increase [ ] of the other

    • added acid reacts with

    • added base reacts with


2. an equilibrium calculation (I.C.E.)

of [H3O+] using


What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it?

HC2H3O2 + OH− C2H3O2 + H2O


HC2H3O2 + OH− C2H3O2 + H2O


HC2H3O2 + OH− C2H3O2 + H2O


0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L with 0.010 mol NaOH added.

HC2H3O2 + OH− C2H3O2 + H2O


0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L with 0.010 mol NaOH added.

HC2H3O2 + OH− C2H3O2 + H2O

−0.010

+0.010

−0.010

0.110

0

0.090

0.090

0.110

0


HC2H3O2 + H2O  C2H3O2 + H3O+qq


HC2H3O2 + H2O  C2H3O2 + H3O+

x

+x

+x

x

0.090 x

0.110 + x


Ka for HC2H3O2 = 1.8 x 10−5

0.110 +x

0.090 x


Ka for HC2H3O2 = 1.8 x 10−5

x = 1.47 x 10−5


0.090 x

x

0.110 + x

x = 1.47 x 10−5


Ka for HC2H3O2 = 1.8 x 10−5


Example: What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it?

HC2H3O2 + H2O  C2H3O2 + H3O+


HC2H3O2 + H2O  C2H3O2 + H3O+

Ka for HC2H3O2 = 1.8 x 10−5

Tro: Chemistry: A Molecular Approach, 2/e


HC2H3O2 + H2O  C2H3O2 + H3O+

pKa for HC2H3O2 = 4.745


Compare effect on pH of adding 0.010 molNaOH to a 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 buffer in 1.00 L to adding 0.010 molNaOH to 1.00 L of pure water

HC2H3O2 + H2O  C2H3O2 + H3O+

pKa for HC2H3O2 = 4.745


adding 0.010 mol NaOH to 1.00 L of pure water


Clicker question: What is the pH of a buffer that has 0.140 moles HF (pKa = 3.15) and 0.071 moles KF in 1.00 L of solution when 0.020 moles of HCl is added? (The “x is small” approximation is valid)


What is the pH of a buffer that has 0.140 moles HF and 0.071 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?


F− + H3O+  HF + H2O


F− + H3O+  HF + H2O


F− + H3O+  HF + H2O

−0.020

−0.020

+0.020


HF + H2O  F + H3O+


Henderson-Hasselbalch Equation for Basic Buffers

  • chemical equation of a basic buffer is written with a weak base as a reactant and its conjugate acid as a product


B: + H2O  H:B+ + OH−

chemical eqtn of basic buffer must be looked at like an acid reaction

this does not affect [ ]’s,

just the way we are looking at the rxtn


Relationship between pKa and pKb

  • relationship btwn Ka of a weak acid and Kb of its conjugate base,

  • also a relationship btwn pKa of a weak acid and pKb of its conjugate base


What is pH of a buffer that is 0.50 M NH3 (pKb = 4.75) and 0.20 M NH4Cl?

NH3 + H2O  NH4+ + OH−


Henderson-Hasselbalch Equation for Basic Buffers

  • chemeqtn of basic buffer is written with weak base as a reactant and its conjugate acid as a product

  • B: + H2O  H:B+ + OH−


Henderson-Hasselbalch Equation for Basic Buffers

  • We can rewrite Henderson-Hasselbalch eqtn for chem eqtn of basic buffer in terms of __________


What is pH of a buffer that is 0.50 M NH3 (pKb = 4.75) and 0.20 M NH4Cl?

NH3 + H2O  NH4+ + OH−


Polyprotic Acids & Bases

Polyprotic acids contain > one ionizable p+

and the protons are lost in a stepwise manner.

Fully protonated species is always the strongest acid.

Why?


Strong acids have

very weak conjugate bases

Strong bases have

very weak conjugate acids


Titrations of Polyprotic Acids or Bases

When strong base is added to soltn of a polyprotic acid –

neutralization rxtn occurs in stages.

Most acidic group titrated 1st,

followed by next most acidic group.


Titrations of Polyprotic Acids or Bases

If pKa values are separated by at least 3 pKa units,

then overall titration curve shows

well-resolved “steps”

corresponding to titration of each p+.


Indicators

Most acid-base titrations are not monitored by recording pH

as a function of

amount of strong acid or base soltn

used as a titrant


Indicators

Instead, acid-base indicator is used,

Compounds that D color at a particular pH


If carefully selected, undergo dramatic color D

at pH corresponding to ________________ of titration


Indicators

Acid-base indicators are typically weak acids or bases.

Color D’scorrespond to

_____________________________

of the indicator itself.


Indicators

Chemistry of indicators general equation:

Protonated form =

conjugate base (deprotonated) =


Indicators

ionization constant for deprotonation of indicator Hn:

Kin = [H+] [n–] / [Hn]

pKindetermines pH at which indicator D’s color


Properties of Good Indicators

  • Color change must be easily detected

  • Color change must be rapid

  • Indicator must not react with substance being titrated

  • Indicator should have pKin within


Choosing a Good Indicator

for Acid/Base Titration

For titrations of

strong acids and strong bases

(and vice versa),

any indicator with a pKin


Choosing a Good Indicator

for Acid/Base Titration

For titration of

a weak acid,

pH at equivalence pt whichis >7,

indicator such as

phenolphthalein or thymol blue,

(pKin > 7)

should be used.


Choosing a Good Indicator

for Acid/Base Titration

For titration of

a weak base,

pH at equivalence pt is < 7,

an indicator like methyl red or bromcresol blue,

(pKin < 7),

should be used.


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