Chemistry 223 Chapter 21:
Download
1 / 53

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


  • 157 Views
  • Uploaded on

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.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Chemistry 223 Chapter 21: Buffers and the Titration of Acids and Bases PART 2' - adair


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
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 Is Added?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 Is Added?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 HC Is Added?2H3O2 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

HC Is Added?2H3O2 + OH− C2H3O2 + H2O


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

HC Is Added?2H3O2 + 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 HC Is Added?2H3O2 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 HC Is Added?2H3O2 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

HC Is Added?2H3O2 + H2O  C2H3O2 + H3O+qq


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

HC Is Added?2H3O2 + 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

K Is Added?a 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

K Is Added?a 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  Is Added?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

K Is Added?a 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

HC 0.100 mol 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

HC 0.100 mol 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 0.100 mol HCmolNaOH 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

Clicker question 0.100 mol HC: 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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?− + H3O+  HF + H2O


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

F moles KF in 1.00 L of solution when 0.020 moles of HCl is added?− + H3O+  HF + H2O


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

F moles KF in 1.00 L of solution when 0.020 moles of HCl is added?− + H3O+  HF + H2O

−0.020

−0.020

+0.020


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

HF + H moles KF in 1.00 L of solution when 0.020 moles of HCl is added?2O  F + H3O+


Henderson hasselbalch equation for basic buffers
Henderson-Hasselbalch Equation for moles KF in 1.00 L of solution when 0.020 moles of HCl is added?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: + H moles KF in 1.00 L of solution when 0.020 moles of HCl is added?2O  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 p moles KF in 1.00 L of solution when 0.020 moles of HCl is added?Ka 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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?

  • 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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?

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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?

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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?

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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?

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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?

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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?D

at pH corresponding to ________________ of titration


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

I moles KF in 1.00 L of solution when 0.020 moles of HCl is added?ndicators

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

I moles KF in 1.00 L of solution when 0.020 moles of HCl is added?ndicators

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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?

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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?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 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?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.