Acid and Base - University of Massachusetts Bostonalpha.chem.umb.edu/chemistry/ch311/week4.pdf ·...
Transcript of Acid and Base - University of Massachusetts Bostonalpha.chem.umb.edu/chemistry/ch311/week4.pdf ·...
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Properties of Umass Boston
Acid and Base
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Properties of Umass Boston
Dissociation of diprotic acid• For the solution of H2L+
H2L+ HL + H+ Ka1 = 4.69 x 10-3
HL L- + H+ Ka2 =1.79 x 10-10
Assumption:since Ka1>>Ka2so the dissociation of HL is insignificant comparing with the dissociation of H2L+, which can be treated as monoprotic acid.
• For the solution of L-
L- + H2O ⇄ HL + OH- Kb1=Kw/Ka2=5.48x10-5
HL + H2O H2L+ + OH- Kb2=Kw/Ka1=2.13x10-12
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Properties of Umass Boston
Treatment of the intermediates
• Systematic treatment:HL H+ + L- Ka2HL + H2O H2L+ + OH- Kb2
Charge balance: [H+]+[H2L+]=[L-]+[OH-]Ka2 = [H+][L-]/[HL] [L-] = [HL]Ka2/[H+]Kb2 = [H2L+][OH-]/[HL] [H2L+]= [HL]Kb2/[OH-]
=[HL][H+]Kb2/KwH2L+ HL + H+ Ka1
Ka1Kb2=Kw [H2L+]=[HL][H+]/Ka1
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Properties of Umass Boston
Approximations
• Since HL is week acid/base, the [HL]≈F (added)then
• Assume, Ka2[HL]>>Kw, then
• Assume, F>>Ka1
FKKKFKKH
a
waaa
++
=+
1
121][
][][][
1
121
HLKKKHLKKH
a
waaa
++
=+
FKFKKH
a
aa
+=+
1
21][
21][ aa KKH =+ pH=1/2(pKa1+pKa2)
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Properties of Umass Boston
Polyprotic acids and bases • Treatments of polyprotic acid and base are
similar to that of diprotic acid and baseH3A H2A- + H+ Ka1H2A- HA2- + H+ Ka2HA2- A3- + H+ Ka3A3- + H2O HA2-+OH- Kb1=Kw/Ka3HA2-+H2O H2A-+OH- Kb2=Kw/Ka2H2A-+H2O H3A + OH- Kb3=Kw/Ka1
• Only deal with immediate neighbors
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Properties of Umass Boston
Only deal with immediate neighbors
• H3A is treated as monoprotic acid in H3A solution, e.g. dissolving in water, Ka=Ka1
H3A H2A- + H+ Ka1H2A- HA2- + H+ Ka2HA2- A3- + H+ Ka3
H3A H2A- + H+ Ka1begin F 0 0end F-x x x
xFxKK a −
=≈2
1
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Properties of Umass Boston
Only deal with immediate neighbors• H2A- and HA2- are treated as the intermediate form for
a diprotic acid.H3A H2A- + H+ Ka1H2A- HA2- + H+ Ka2HA2- A3- + H+ Ka3
• For H2A-
H3A H2A- + H+ Ka1H2A- HA2- + H+ Ka2
• For HA2-
H2A- HA2- + H+ Ka2HA2- A3- + H+ Ka3
−+
++
≈ 2
2
232][ HAforFK
KKFKKHa
waaa
−+
++
≈ AHforFK
KKFKKH waa2
1
121][
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Properties of Umass Boston
Only deal with immediate neighbors• A3- is treated as monobasic, Kb=Kb1
A3- + H2O HA2-+OH- Kb1=Kw/Ka3HA2-+H2O H2A-+OH- Kb2=Kw/Ka2H2A-+H2O H3A + OH- Kb3=Kw/Ka1
A3- + H2O HA2-+OH- Kb1begin F 0 0end F-x x x
xFxKK b −
=≈2
1
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Properties of Umass Boston
Principal species – dominate type of species at certain pH
At the time [H3A]=[H2A-]= 1/2F
H3A H2A- + H+ Ka1begin F 0 0end 0.5F 0.5F x
pH<pK H3A is the dominate species; pH>pK H2A- is the dominate species.
pKpHHKF
FHKK a
=⇒=
=≈
+
+
][5.0
5.0][1
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Properties of Umass Boston
Principal species – dominate type of species at certain pH
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Properties of Umass Boston
Composition of solution• General form of fraction for the polyprotic acid HnA:
nnnn
nj
AH
n
AH
n
AH
KKKKHKKHKHDD
HKKKDHK
DH
jn
n
n
⋅⋅⋅⋅⋅+⋅⋅⋅⋅⋅+++=
⋅⋅⋅⋅=
=
=
−+−++
−
−+
+
−
−
3212
211
1
121
11
][][][
][
][
][
1
α
α
α
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Properties of Umass Boston
Titration
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Properties of Umass Boston
Titration • Increments of reagent solution (titrant) are
added to analyte until the reaction is completed– Titrant (know concentration)– Analyte (unknown)– End point: equivalence point – the quantity of added
titrant is the exact amount necessary for stoichiometric reaction with the analyte
Example: using known H2SO4 to titrate unknown KOH -H2SO4 +2 KOH → 2H2O + K2SO4at end point one mole of H2SO4 reacts with 2 mole of KOH
.
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Properties of Umass Boston
Detection end point• Detection of end point: indication –
chemical, electrochemical, spectroscopic –means to mark change in a physical property of the solution
Example:HOOC-COOH+ 2 MnO4
- + 6 H+ → 10 CO2 + 2 Mn2+ + H2O(oxalic acid, analyte) (titrant, pink)
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Properties of Umass Boston
Terminology• Titrant• Analyte• End point• Indicator: compound with a physical property (color) that changes
abruptly near the equivalence point.• Titration error• Blank titration• Primary standard: pure and stable reagent (>99.9%) which can be
accurately weighted to make titrant.• Standardization: use primary standard solution to determine the
non primary standard – standard solution.• Direct titration: titrate the analyte until end point.• Back titration: add excess amount of one standard solution and
use the second standard solution to titrate the excess amount of the first standard solution – slow reaction with analyte or no clear indicator
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Properties of Umass Boston
Titration calculation• the quantity of added titrant is the exact amount necessary for
stoichiometric reaction with the analyte – relate the mole of titrate to the mole of analyteExample: use 0.7344 M HCl to titrate 1.372 g of the mixture of Na2CO3 and NaHCO3, 29.11 ml of HCl was consumed. Na2CO3 + 2HCl 2 NaCl + H2O + CO2↑NaHCO3 + HCl NaCl + H2O + CO2↑at the end point, all Na become NaCl1 mole Na2CO3 reacts with 2 mole HCl1 mole NaHCO3 reacts with 1 mole HClTotal HCl consumed 0.7344 x 29.11x10-3=0.02138 MAssume x gram of Na2CO3 in the mixture, then there is1.372-x gram of NaHCO3Mole of Na2CO3: x/105.99(molecular weight)Mole of NaHCO3: (1.372-x)/84.01thus: 2 x mole of Na2CO3 + mole of NaHCO3 = mole of HCl2 x (x/105.99) + (1.372-x)/84.01 = 0.02138x =0.724 g 0.724 g Na2CO3 and 1.372-0.724 = 0.648 g NaHCO3
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Properties of Umass Boston
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Properties of Umass Boston
Titration curve• Change of the indication
signal – the concentration of one reactant (potential, pH, optical absorption..) with the amount of titrant– Understand the chemistry
during the titration– The influence of the
conditions to the titration e.g. pH to the sharp end point.
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Properties of Umass Boston
Types of Titration
• Acid-base• Redox (reduction – oxidation,
electrochemical)• Complex formation (EDTA)• Precipitation
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Properties of Umass Boston
Precipitation Titration• Ksp influence the
sharpness of the end point and accuracy of precipitation titration, the larger Ksp the better.
• Ion selective electrodes are usually used to detect the change of one reactant
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Properties of Umass Boston
Example of Precipitation Titration
• Using AgNO3 solution to titrate the solution containing KI and KCl
• Titration of a mixture, less soluble precipitate (larger Ksp) forms first.
• Ksp1 has to be sufficiently different in order to separate the two different precipitates
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Properties of Umass Boston
Example of Precipitation Titration• Ag+ + I-→ AgI
KSP(AgI)=[Ag+][I-] [Ag+]=Ksp(AgI)/[I-][I-] = initial concentration (unknown) – [Ag] added
• Ag+ + Cl-→ AgClKSP(AgCl)=[Ag+][Cl-] [Ag+]=Ksp(AgI)/[Cl][Cl] = initial concentration (unknown) – [Ag] added-[I-]
• Near the end point of I-, co precipitation may happen
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Properties of Umass Boston
Titration Curve
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Properties of Umass Boston
Buffer
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Properties of Umass Boston
Buffered Solution
• A buffered solution resists changes in pH when acids or bases are added or when dilution occurs.
• The buffer is made of the mixture of acid and its conjugate base.
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Properties of Umass Boston
What happen if a weak acid is mixed with its conjugate base ?
• the moles of acid and conjugate base in the solution will remain close to the amounts added, because of Le Châtelier’s principal.
HA H+ + A-
A- + H2O HA + OH-
• HA dissociates very little and adding A- will make the equilibrium move to right side.
• A- does not react with water much and adding HA will make the equilibrium move to right side
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Properties of Umass Boston
Henderson-Hasselbalch Equation
• The pH of the buffer - the central equation to treat buffer
HA H+ + A- KaKa = [H+][A-]/[HA]logKa=log[H+]+log{[A-]/[HA]}
pH = pKa + log{[A-]/[HA]}
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Properties of Umass Boston
The same relation, different expression• A- + H2O HA + OH- Kb
Kb = [HA][OH-]/[A-]log Kb = log [OH-] + log [HA]/[A-][OH-]=Kw/[H+]log Kb = log Kw – log[H+] + log [HA]/[A-]-log[H+]=log Kb – log Kw - log [HA]/[A-]pH = -logKw/Kb + log [A-]/[HA]
pH = pKa + log [A-]/[HA]
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Properties of Umass Boston
Diprotic Buffers• A buffer made from a diprotic (H2L) or
polyprotic (HnL) acid
H2L HL- + H+ Ka1HL- L2- + H+ Ka2Two Henderson-Hasselbalch equations, they are both true in the same solution
pH=pKa1+log([HL-]/[H2L])pH=pKa2+log([L2-]/[HL-])
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Properties of Umass Boston
The pH change in buffer•
pH = pKa + log{[A-]/[HA]}