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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

Chemistry 223 Chapter 21:

Buffers and the Titration of

Acids and Bases PART 2


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

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:


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

  • 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


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

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

of [H3O+] using


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

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


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

HC2H3O2 + OH− C2H3O2 + H2O


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

HC2H3O2 + OH− C2H3O2 + H2O


0 100 mol hc 2 h 3 o 2 and 0 100 mol nac 2 h 3 o 2 in 1 00 l with 0 010 mol naoh added

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 hc 2 h 3 o 2 and 0 100 mol nac 2 h 3 o 2 in 1 00 l with 0 010 mol naoh added1

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


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

HC2H3O2 + H2O  C2H3O2 + H3O+qq


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

HC2H3O2 + H2O  C2H3O2 + H3O+

x

+x

+x

x

0.090 x

0.110 + x


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

Ka for HC2H3O2 = 1.8 x 10−5

0.110 +x

0.090 x


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

Ka for HC2H3O2 = 1.8 x 10−5

x = 1.47 x 10−5


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

0.090 x

x

0.110 + x

x = 1.47 x 10−5


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

Ka for HC2H3O2 = 1.8 x 10−5


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

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+


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

HC2H3O2 + H2O  C2H3O2 + H3O+

Ka for HC2H3O2 = 1.8 x 10−5

Tro: Chemistry: A Molecular Approach, 2/e


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

HC2H3O2 + H2O  C2H3O2 + H3O+

pKa for HC2H3O2 = 4.745


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

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


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

adding 0.010 mol NaOH to 1.00 L of pure water


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

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)


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

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?


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

F− + H3O+  HF + H2O


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

F− + H3O+  HF + H2O


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

F− + H3O+  HF + H2O

−0.020

−0.020

+0.020


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

HF + H2O  F + H3O+


Henderson hasselbalch equation for basic buffers

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


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

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 p k a and p k b

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 nh 3 p k b 4 75 and 0 20 m nh 4 cl

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 buffers1

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 buffers2

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 nh 3 p k b 4 75 and 0 20 m nh 4 cl1

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

NH3 + H2O  NH4+ + OH−


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

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?


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

Strong acids have

very weak conjugate bases

Strong bases have

very weak conjugate acids


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

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.


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

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+.


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

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


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

Indicators

Instead, acid-base indicator is used,

Compounds that D color at a particular pH


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

If carefully selected, undergo dramatic color D

at pH corresponding to ________________ of titration


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

Indicators

Acid-base indicators are typically weak acids or bases.

Color D’scorrespond to

_____________________________

of the indicator itself.


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

Indicators

Chemistry of indicators general equation:

Protonated form =

conjugate base (deprotonated) =


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

Indicators

ionization constant for deprotonation of indicator Hn:

Kin = [H+] [n–] / [Hn]

pKindetermines pH at which indicator D’s color


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

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


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

Choosing a Good Indicator

for Acid/Base Titration

For titrations of

strong acids and strong bases

(and vice versa),

any indicator with a pKin


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

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.


Chemistry 223 chapter 21 buffers and the titration of acids and bases part 2

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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