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Chapter 17Chapter 17Chapter 17Chapter 17Complex Aqueous EquilibriaComplex Aqueous Equilibria
Common Ion Effect
Buffers
Titrations
Predict using LeChatelier’s PrinciplePredict using LeChatelier’s Principle
CH3COOH(aq) H+(aq) + CH3COO−(aq)
Consider the equilibrium involving the weak acid at 298K shown above. What will happen to the pH if some solid CH COONa is added while the temperature is heldCH3COONa is added while the temperature is held constant?
1. The pH will increasep
2. The pH will decrease
3 The pH will not be effected3. The pH will not be effected.
4. There is not enough information to determine the effect on the pHon the pH.
Chem 112 2MJ Bojan
Test the prediction by doing an experiment.
DEMO:CH3COOH(aq) H+(aq) + CH3COO−(aq)
pH =
Add CH3COONa(s): CH COONa (aq) Na+(aq) + CH COO−(aq)CH3COONa (aq) → Na+(aq) + CH3COO−(aq)
pH =
What happened to[CH3COO−]?
Common Ion Effect: if a system is at equilibrium and it is disturbed by adding a strong electrolyte with
[CH3COOH]?
[H+]?
by adding a strong electrolyte with a common ion, the ionization of the weak electrolyte will be
d d[H+]? reduced.Chem 112 3MJ Bojan
What happens when a strong acid is added to a weak acid and to a mixture of the acid and its conjugate base?
CH COOH( ) H+( ) CH COO ( )CH3COOH(aq) H+(aq) + CH3COO−(aq)
pH0 1M CH COOH(aq) Mixture pH0.1M CH3COOH(aq) Mixture pH
0.1M CH3COOH0.1M CH3COONa
Add HClAdd HCl
Add NaOH Add NaOH
How does the addition of strong acid or base effect the pH of the weak acid?
Example of the Common Ion effect
of the mixture?
A Buffer is a solution that “resists” a change in pHg p
E g blood contains substancesE.g. blood contains substances that keep its pH fixed at 7.3 -important for life functions
Buffer solutions consist of either:
Chem 112 5MJ Bojan
http://chemcollective.org/activities/tutorials/buffers/buffers3
How do Buffers work? When strong acid or base is added, the ratio of weak acid: weak base does not change much.
If a small amount of hydroxide is
Buffer with[HA] = [A]
Buffer afteradding OH
Buffer afteradding H+
yadded to an equimolarsolution of HA in NaA, the HF
Similarly, if acid is added, the A− reacts with it to form HA and water
OH H+
,reacts with the OH− to make A− and water.
HA and water.
[HA][HA] [A] [HA] [A][A]
6http://chemcollective.org/activities/tutorials/buffers/buffers3
QuantitativelyC l l t th H f l ti th t i 0 1M CH COOHCalculate the pH of a solution that is 0.1M CH3COOH
and 0.1M CH3COONa
Chem 112 7MJ Bojan
We can derive an expression that will enable us to calculate the pH of a buffer solution.p
Consider the equilibrium constant expression for the dissociation of a generic acid, HX:
HX(aq) H+(aq) + X−(aq)
This is the Henderson Hasselbalch equationThis is the Henderson–Hasselbalch equation.Chem 112 8MJ Bojan
Calculating the pH of a BufferWhat is the pH of a buffer that is 0.12 M in lactic acid,
CH3CH(OH)COOH, and 0.10 M in sodium lactate?
K f l ti id i 1 4 × 10 4Ka for lactic acid is 1.4 × 10−4.
Chem 112 9MJ Bojan
Calculating the pH of a Buffer
Calculate the pH of a solution that is 0.1M NH3 and 0.2M NH4Cl.
Kb for NH3 is 1.8 × 10−5.b 3
Chem 112 10MJ Bojan
When Strong Acids or Bases Are Added to a Buffer reaction occursto a Buffer… reaction occurs.
Follow a 2-step process to determine pH1. neutralization2. pH determination
neutralizationA(aq) + H+(aq) → HA(aq)
BufferHA + A
Calculate[HA] and [A]
pH determination
neutralizationHA(aq) + OH(aq) → A(aq) + H2O()
Stoichiometry calculationEquilibriumcalculation
Chem 112 MJ Bojan 11
Stoichiometry calculation calculation
Finding the pH after adding Strong Acid or Base to a Bufferg
1. Determine how the neutralization reaction affects the amounts of the weak acid and its conjugate base in solution.
2. Use the Henderson–Hasselbalch equation to determine the new pH of the solution.
Add 0 020 mol OH Add 0 020 mol H+Add 0.020 mol OH Add 0.020 mol H
Buffer: 0.300 M CH3COOH0.300 M CH3COONa
pH =??? pH =???
Chem 112 12
MJ Bojan
pH = 4.74
Calculating pH changes in Buffers
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 thisThe pH of the buffer is 4.74. Calculate the pH of this solution after 0.020 mol of NaOH is added.
Chem 112 13MJ Bojan
Finding the pH after adding Strong Acid or Base to a Bufferg
1. Determine how the neutralization reaction affects the amounts of the weak acid and its conjugate base in solution.
2. Use the Henderson–Hasselbalch equation to determine the new pH of the solution.
Add 0 020 mol OH Add 0 020 mol H+Add 0.020 mol OH Add 0.020 mol H
Buffer: 0.300 M CH3COOH0.300 M CH3COONapH = 4 74
Buffer: 0.280 M CH3COOH0.320 M CH3COONapH = 4 80
Buffer: 0.320 M CH3COOH0.280 M CH3COONapH = 4 68
Chem 112 14MJ Bojan
pH = 4.74pH = 4.80 pH = 4.68
Choosing a BufferChoosing a Buffer
pH range of a buffer: range of pH over which the buffer acts effectivelybuffer acts effectively.
optimal pH: Buffer region
Chem 112 15MJ Bojan
Buffer Capacity: volume of acid or base that can be added before the pH of the buffercan be added before the pH of the buffer
changes appreciably.
Whi h l ti ill h th t t b ff it ?Which solution will have the greatest buffer capacity?
1M CH COOH/CH COONa1M CH3COOH/CH3COONa
0.1M CH3COOH/CH3COONa0.1M CH3COOH/CH3COONa
Chem 112 16MJ Bojan
Titration
A pH meter or indicators are used to determine when the solution has reached the equivalence point:
In this technique a known concentration of base (or acid) is slowly added to a solution of acid (or base).
Chem 112 17MJ Bojan
The titration of a strong acid with a strong base can be described by a plot of pH vs. amount of base added.y p p
Chem 112 18MJ Bojan
Quantitative determination of the pH for an acid Quantitative determination of the pH for an acid AcidAcid BaseBase Titration is a two step processTitration is a two step processAcid Acid Base Base Titration is a two step processTitration is a two step process
1. Neutralization
What is in solution after neutralization?
2. Find the pHA) strong acid/baseB) weak/acid baseB) weak/acid base
equilibrium calculationbuffer solutionbuffer solution
Chem 112 19MJ Bojan
Sample calculation of pH during the titration of a Strong Acid with a Strong Baseof a Strong Acid with a Strong Base
Titrate 0.1M HCl with 0.2M NaOH starting with 50 mL of HCl solution. Calculate pH when the following quantities of NaOH are added:
a. 0 mL NaOH, b. 20 mL NaOH, c. 25 mL NaOH, d. 30 mL NaOH
a. Initial concentration of strong acid: (0mL of NaOH added)
Chem 112 20MJ Bojan
Sample calculation of pH during the titration of a Strong Acid with a Strong Baseof a Strong Acid with a Strong Base
Titrate 0.1M HCl with 0.2M NaOH starting with 50 mL of HCl solution. Calculate pH when the following quantities of NaOH are added:
a. 0 mL NaOH, b. 20 mL NaOH, c. 25 mL NaOH, d. 30 mL NaOH
b. concentration of solution after adding 20mL of NaOH:
Chem 112 21MJ Bojan
Sample calculation of pH during the titration of a Strong Acid with a Strong Baseof a Strong Acid with a Strong Base
Titrate 0.1M HCl with 0.2M NaOH starting with 50 mL of HCl solution. Calculate pH when 25 mL of NaOH is added.
c. concentration of solution after adding 25mL of NaOH :
Equivalence point is where moles of acid = moles of base
Chem 112 22MJ Bojan
Sample calculation of pH during the titration of a Strong Acid with a Strong Baseof a Strong Acid with a Strong Base
Titrate 0.1M HCl with 0.2M NaOH starting with 50 mL of HCl solution. Calculate pH when the following quantities of NaOH are added:
a. 0 mL NaOH, b. 20 mL NaOH, c. 25 mL NaOH, d. 30 mL NaOH
d. concentration of solution after adding 30mL of NaOH:
Chem 112 23MJ Bojan
The titration of a weak acid with a strong base can be described by a plot of pH vs. amount of base added.y p p
Chem 112 24MJ Bojan
Calculating the pH during the Titration of a Weak Acid with a Strong Base is still a two step process.g p p
Sol’n withHA and A
neutralizationHA(aq) + OH(aq) → A(aq) + H2O()
Determine [HA], [A]
pH determination (buffer)
Equilibrium calculation
HA and A HA(aq) OH (aq) → A (aq) H2O() [HA], [A ] (buffer)
Stoichiometry calculation
At each point below the equivalence point, the pH of the solution during titration is determined from thethe solution during titration is determined from the amounts of the acid and its conjugate base present at that particular time.
Buffer solutions are encountered during titrations!titrations!
Chem 112 25
MJ Bojan
Sample calculation of pH during the titration of a Weak Acid with a Strong Base
Titrate 0.1M CH3COOH (Ka = 1.8 x 10−5) with 0.2M NaOH starting with 50 mL of CH3COOH solution and calculate pH when the following quantities of NaOH are added: a. 0 mL NaOH, b. 12.5 mL NaOH, c. 25mL d. 30 mL NaOH
a. Initial concentration of weak acid: (0mL of NaOH added)
Chem 112 26MJ Bojan
Sample calculation of pH during the titration of a Weak Acid with a Strong Base
Titrate 0.1M CH3COOH (Ka = 1.8 x 10−5) with 0.2M NaOH starting with 50 mL of CH3COOH solution and calculate pH when the following quantities of NaOH are added: a. 0 mL NaOH, b. 12.5 mL NaOH, c. 25mL d. 30 mL NaOH
b. Half way to equivalence (12.5mL of NaOH added)
Chem 112 27MJ Bojan
Sample calculation of pH during the titration of a Weak Acid with a Strong Base
Titrate 0.1M CH3COOH (Ka = 1.8 x 10−5) with 0.2M NaOH starting with 50 mL of CH3COOH solution and calculate pH when the following quantities of NaOHare added: a. 0 mL NaOH, b. 12.5 mL NaOH, c. 25 mL NaOH d. 30 mL
c. At the equivalence point (25mL of NaOH added)
Chem 112 28MJ Bojan
Sample calculation of pH during the titration of a Weak Acid with a Strong Base
Titrate 0.1M CH3COOH (Ka = 1.8 x 10−5) with 0.2M NaOH starting with 50 mL of CH3COOH solution and calculate pH when the following quantities of NaOHare added: a. 0 mL NaOH, b. 12.5 mL, c. 25mL d. 30 mL NaOH
d. Beyond the equivalence point; excess strong base (30 mL of NaOH added)
Chem 112 29MJ Bojan
The initial pH of a weak acid and pH at equivalence after titration ith t b d d th l f K f th k idwith a strong base depends on the value of Ka of the weak acid.
Chem 112 30MJ Bojan
INDICATORS are organic molecules that have INDICATORS are organic molecules that have different colors for the acid and conjugate base.different colors for the acid and conjugate base.
InH+(aq) In(aq) + H+(aq)Some of the more common acid-base indicators
Name pH interval forColor change
Acid color
Base color
Methyl violet 1 - 2 Yellow Violety
Methyl yellow 1.5 - 2.5 Red Yellow
Methyl orange 2.5 - 4.0 Red Yellow
Methyl red 4 2 6 3 Red YellowMethyl red 4.2 - 6.3 Red Yellow
Bromthymol blue 6.0 - 7.6 Yellow Blue
Thymol blue 8.0 - 9.6 Yellow Blue
Phenolphthalein 8.5 - 10 Colorless Pink
Alizarin yellow G 10.1-12.0 Yellow Red
Chem 112 31MJ Bojan
Choose an indicator that will change in Choose an indicator that will change in the range of the equivalence point.the range of the equivalence point.the range of the equivalence point. the range of the equivalence point.
Chem 112 32MJ Bojan
Titration curve of a Weak Base with a Strong Acid shows how pH changes with bases added.p g
Strong acid Strong acid -- strong base:strong base:
The pH at the equivalenceThe pH at the equivalence point
indicator of choice is:
Strong acid Strong acid -- weak baseweak base::
The pH at the equivalence pointpoint
indicator of choice is:
Chem 112 33MJ Bojan
Summary of expectations for pH at equivalence in acid base titrations. q
Titrating a strong acid (base) with a strong base (acid)pH at equivalencepH at equivalence
Tit ti k id ith t bTitrating a weak acid with a strong base
pH at equivalence
Titrating a weak base with a strong acid
pH at equivalencepH at equivalence
Chem 112 34MJ Bojan
Titrations of Polyprotic Acidsyp
Chem 112 35MJ Bojan
Find the pH of this solution?Find the pH of this solution?Find the pH of this solution?Find the pH of this solution?
A buffer solution is 0.20M in HPO42− and 0.10M in H2PO4
−. What is the [H+] in this solution?
H3PO4: Ka1 = 7.5 10−3
Ka2 = 6.2 10−8
K = 1 10−12Ka3 = 1 10 12
A. 3.7 10−3 MB. 3.7 10−4 MC. 3.1 10−8 MD. 3.1 10−9 ME. 5.0 10−13 M
Chem 112 36MJ Bojan