CH160 General Chemistry II Lecture Presentation Applications of AcidBase Equilibria

1
CH160 General Chemistry IILecture
PresentationApplications of Acid-Base Equilibria

• Chapter 17
• Sections 1-4

2
Common Ion Effect

• Consider the ionization of weak acid HA
• HA H2O ltgt H3O A-
• What affect will adding salt NaA to the solution
  have on the acid ionization and solution pH?
• NaA ? Na A-

3
Common Ion Effect

• Consider the ionization of weak acid HA
• HA H2O ltgt H3O A-

A- has 2 sources HA and NaA. Adding NaA
increases A-.
4
Common Ion Effect

• Consider the ionization of weak acid HA
• HA H2O ltgt H3O A-

A- is a common ion
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Example 1(1a on the Example Problem Handout)

• Calculate the percent ionization of the acid and
  the pH of the solution that contains 0.500M
  HC2H3O2 (Ka 1.8 x 10-5) and 0.200 M NaC2H3O2.
  
• (ans. pH 4.34, 0.0090)

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Calculations Using Ka

• Basic Steps for Weak Acid Calculations Using Ka
• Write balanced chemical equation and the
  expression for Ka
• Look up value for Ka
• For each chemical species involved in the
  equilibrium (except H2O), write
• Initial concentration
• Equilibrium concentration
• Let the change in the H3O be the variable x
• Substitute the equilibrium concentrations into Ka
  and solve for x using either
• quadratic approach
• simplified approach
• Calculate pH, equilibrium concentrations,
  ionization, etc., as specified in the problem.

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0.5 M CH3COOH 0.200 M CH3COONa
(problem 1a)
0.5 M CH3COOH
(problem 8a)
CH3COONa
0.009
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Common Ion Effect

• What affect does NaA have on weak acid HA
  ionization?
• HA H2O ltgt H3O A-

Which way does equilibrium shift? What do the
results of problems 8a and 1a tell us? Does this
agree with LeChateliers principle?
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Common Ion Effect

• What affect does NaA have on weak acid HA
  ionization?
• HA H2O ltgt H3O A-

In presence of NaA, HA ionization shifts
left. common-ion effect
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Common Ion Effect

• Common Ion Effect
• shift in ionic equilibrium resulting from adding
  a substance containing an ion in common with the
  equilibrium system
• Understood in terms of LeChateliers principle

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Buffer Solutions

• What is a buffer solution?
• solution with ability to resist pH changes upon
  addition of small amounts of either acid or base
• Requirements
• must contain an acid to neutralize added OH- ions
  
• must contain a base to neutralize added H3O ions
  
• acidic and basic species in buffer must not
  neutralize each other.

12
Buffer Action

• How do buffers work? Consider buffer with weak
  acid HA and salt NaA
• Addition of acid
• A- H ? HA

13
Buffer Action

• How do buffers work? Consider buffer with weak
  acid HA and salt NaA
• Addition of acid (small amount)
• A- H ? HA

A- decreases slightly
HA increases slightly
Added acid is neutralized.
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Buffer Action

• How do buffers work? Consider buffer with weak
  acid HA and salt NaA
• Addition of base
• HA OH- ? H2O A-

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Buffer Action

• How do buffers work? Consider buffer with weak
  acid HA and salt NaA
• Addition of base
• HA OH- ? H2O A-

HA decreases slightly
A- increases slightly
Added base is neutralized.
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Buffer Action

• Two important properties of buffer solutions
• Buffer capacity
• Amount of acid or base the buffer can react with
  before giving a significant pH change (1 pH unit)
  
• Determined by how much buffer acid and base are
  used to make buffer
• pH
• Determined by Ka and relative amounts of buffer
  acid and base present

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Calculation of Buffer pH

• Calculating pH for buffer containing both weak
  acid HA and salt NaA. The major equilibrium is
• HA H2O ltgt H3O A- Ka H3OA-/HA

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Calculation of Buffer pH

• Calculating pH for buffer containing both weak
  acid HA and salt NaA.
• HA H2O ltgt H3O A- Ka H3OA-/HA

A- has 2 sources HA and NaA (This seems
familiar! Didnt we just do this?)
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Calculation of Buffer pH

• Calculating pH for buffer containing both weak
  acid HA and salt NaA.
• HA H2O ltgt H3O A- Ka H3OA-/HA

Neither CHA nor CNaA change much since only a
very small amount of HA ionizes.
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Calculation of Buffer pH

• Calculating pH for buffer containing both weak
  acid HA and salt NaA.
• HA H2O ltgt H3O A- Ka H3OA-/HA
• Since ionization is small CA- ? A- and CHA
  ? HA

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Calculation of Buffer pH

• Calculating pH for buffer containing both weak
  acid HA and salt NaA.
• HA H2O ltgt H3O A- Ka H3OA-/HA
• Since ionization is small CA- ? A- and CHA
  ? HA
• Solving for H3O gives
• H3O KaCHA/CA-
• pH -logH3O
• (This is just a common ion effect problem.)

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Calculation of Buffer pH

• We can also take the log of our equation
• H3O KaCHA/CA-
• -logH3O -log(KaCHA/CA-)
• -logH3O -logKa - log CHA/CA-
• pH pKa - log CHA/CA-

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Example 2(2a on Example Problem Handout)

• Calculate the pH of a buffer solution that
  contains 0.25 M sodium acetate, NaC2H3O2, and
  0.35 M acetic acid, HC2H3O2 (Ka 1.8 x 10-5).
• (ans. pH 4.60)

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Calculating pH Changes in Buffers

• How do we calculate buffer pH after adding acid
  (H3O) or base (OH-)?

H3O A- ?? HA H2O or OH- HA ? A- H2O
H3O or OH-
Buffer
Neutralization Rxn
Calculate new H3O from H3O KaCHA/CA-
Calculate new HA A-
pH
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Calculating pH Changes in Buffers

• How do we calculate buffer pH after adding acid
  (H3O) or base (OH-)?

H3O A- ?? HA H2O or OH- HA ? A- H2O
H3O or OH-
Buffer
Neutralization Rxn
This much is a stoichiometry problem. (Oh, oh!
General Chemistry I stuff here.).
Calculate new HA A-
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Calculating pH Changes in Buffers

• How do we calculate buffer pH after adding acid
  (H3O) or base (OH-)?

This part is an equilibrium calculation.
Calculate new H3O from H3O KaCHA/CA-
pH
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Example 3(3a on Example Problem Handout)

• Calculate the pH of the solution formed and the
  change in pH observed when (a) 0.050 moles of HCl
  and (b) 0.025 moles of NaOH are added to 500 mL
  of the buffer in example (2a). (c) Calculate
  the change in pH that occurs when 0.050 moles HCl
  are added to 500 mL H2O.
• (ans. (a) pH 4.27, ?pH -0.33, (b) pH 4.74,
  ?pH 0.14, (c) ?pH -6)

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Buffer Preparation

• What if I need to make a buffer solution of known
  pH? Select
• Buffer system.
• Often pKa of buffer acid is close to desired pH.
• Relative amounts of buffer acid and base.
• Buffer capacity increases with concentrations.
• Buffer effectiveness best with concentrations on
  same order of magnitude.

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Example 4 (4 on Example Problem Handout)

• Starting with 1.0L of 0.100 M CH3COOH (Ka 1.8 x
  10-5), how many grams of sodium acetate, CH3COONa
  (FW 82.034 g/mol), to give a buffer with a pH
  of 4.40? (Assume no volume change.)
• (ans. 3.7 g)

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Quantitative Acid-Base Chemistry

• How do we calculate the pH of a solution formed
  by mixing an acid solution with a base solution?
• Consider addition of 0.1 M strong base, MOH, to
  0.1 M strong acid, HX
• pH changes can be observed from titration curve
• pH vs. volume standard

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Strong Acid-Strong Base Titration HX NaOH ? NaX
H2O
pH
mL NaOH
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Calculating pH in Acid-Base Reactions

• How do we calculate pH after adding a strong base
  to a strong acid?

MOH
HX MOH ? MX H2O
Strong acid
Neutralization Rxn
Calculate HX or MOH left (ignore neutral MX)
Calculate new H3O from HX or MOH
pH
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Calculating pH in Acid-Base Reactions

• How do we calculate pH after adding a strong base
  to a strong acid? Considerations
• Stoichiometry
• Limiting reagent
• At what point in rxn does calculation take place?
• Initial
• Pre-equivalence
• Equivalence
• Post-equivalence
• Does dilution occur?
• If mixing 2 solutions Vtotal V1 V2

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Strong Acid-Strong Base Titration HX NaOH ? NaX
H2O
-NaOH in excess -NaOH/NaX left -OH- CMOH
-All HX MOH consumed -NaX left (neutral) -pH
7.0
pH
-HX in excess -HX/NaX left -H3O CHX
Only HX H3O CHX
mL NaOH
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Example 5(5 of Example Problem Handout)

• Calculate the pH for a solution prepared by
  mixing 25.00 mL of 0.100 M HCl(aq) with a) 10.00
  mL b) 25.00 mL and c) 35.00 mL of 0.100 M
  NaOH(aq). (ans. a) 1.37, b) 7.0, c) 12.22)

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Quantitative Acid-Base Chemistry

• Acid-Base reactions can be used for quantitative
  measurements
• Acid-base titration
• Volume of standard base (acid) solution (known
  concentration) required to react with unknown
  acid (base) solution is measured
• Amount of acid (base) in unknown can be calculated

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Example 6(6 on Example Problem Handout)

• Calculate the concentration of an HCl solution if
  25.0 mL of this solution requires 18.0 mL of
  0.150 M NaOH to reach the equivalence point.
• (ans. 0.108 M)

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Weak Acid-Strong Base Titration HA NaOH ? NaA
H2O
pH
mL NaOH
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Weak Acid-Strong Base Titration HA NaOH ? NaA
H2O
-NaOH in excess -NaOH/NaA left -OH- ? CMOH
-HA in excess -HA/NaA left buffer -H3O
Ka(CHA/CNaA)
-All HA MOH consumed -NaA left (weak
base) -OH- ? (KbCNaA)1/2 -pH gt 7.0
pH
-only HA H3O ? (KaCHA)1/2
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Strong Acid-Weak Base Titration HX B ? HB X-
pH