chemical equilibria, principle of ph, buffers
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Chemical equilibria, principle of pH, buffers. Basics of Medical Chemistry Course L ászló Csanády Department of Medical Biochemistry. v 1. C+D. A+B. v -1. Chemical equilibria. Reaction rate (v): Amount of product formed per unit time (mol/s, M/s). Rate law: - PowerPoint PPT PresentationTRANSCRIPT
Chemical equilibria,principle of pH, buffers
Basics of Medical Chemistry Course
László Csanády
Department of Medical Biochemistry
Chemical equilibria
A+B C+Dv1
v-1
Reaction rate (v):
Amount of product formed per unit time (mol/s, M/s).
Rate law:
For elementary (single-step) reactions the reaction rate is proportional to the concentrations of each reactant.
v1=k1.|A|.|B| k1, k-1: reaction rate constants (M-1s-1)
v-1=k-1.|C|.|D| |X|: instantaneous concentration (M)
Chemical equilibria
A+B C+Dv1
v-1
At equilibriumthe forward and the reverse rates are equal.
Chemical equilibria
A+B C+Dv1
v-1
At equilibriumthe forward and the reverse rates are equal.
k1
k-1
[C].[D][A].[B]=K =
equilibrium constant
v1=v-1
k1.[A].[B]=k-1
.[C].[D]
[X]: equilibriumconcentration (M)
Chemical equilibria
All chemical reactions can be broken down into a set of elementary steps. Equilibrium is achieved when all steps are at equilibrium:
A+B C* 2D
[C*][A].[B]K1 =
[D].[D] [C*]K2 =
[C*][A].[B]K = K1
. K2 = [D]2
[C*].
[D]2
[A].[B]K =overall equilibrium constant
"Law ofMass Action"
Chemical equilibria
Predicting the direction of a reaction:
|C|.|D||A|.|B|Q =
A+B C+D
reaction quotient
Q<K reaction goes forwardQ=K reaction is at equilibriumQ>K reaction goes backward
Le Chatelier principleWhen a system at equilibrium is perturbed, it will reach a new equilibrium by counteracting the perturbation.
1. Initial state (equilibrium):|C|.|D||A|.|B|Q = = K
2. Perturbation: remove product: |C||C|* (|C|*<|C|)
|C|*.|D| |A|.|B|Q = < K3. reaction goes forward
4. Final state (new equilibrium):(|C|*+x).(|D|+x) (|A|-x).(|B|-x)Q = = K
I. Adding/removing reactants/products
A+B C+D
Le Chatelier principleWhen a system at equilibrium is perturbed, it will reach a new equilibrium by counteracting the perturbation.
1. Initial state (equilibrium):|C|.|D||A|.|B|Q = = K
2. Perturbation: add reactant: |A||A|* (|A|*>|A|)
|C|.|D||A|*.|B|Q = < K3. reaction goes forward
4. Final state (new equilibrium):(|C|+x).(|D|+x)(|A|*-x).(|B|-x)Q = = K
I. Adding/removing reactants/products
A+B C+D
Le Chatelier principleWhen a system at equilibrium is perturbed, it will reach a new equilibrium by counteracting the perturbation.
1. Initial state (equilibrium):|C|.|D||A|.|B|Q = = K
2. Perturbation: add product: |C||C|* (|C|*>|C|)
|C|*.|D| |A|.|B|Q = > K3. reaction goes backward
4. Final state (new equilibrium):(|C|*-x).(|D|-x)(|A|+x).(|B|+x)Q = = K
I. Adding/removing reactants/products
A+B C+D
Le Chatelier principleWhen a system at equilibrium is perturbed, it will reach a new equilibrium by counteracting the perturbation.
1. Initial state (equilibrium):|C|.|D| |A|Q = = K
2. Perturbation: increase volume by 2x: |X||X|/2
reaction goes forward
4. Final state (new eq.):((|C|/2)+x).((|D|/2)+x) (|A|/2)-xQ = = K
II. Effect of dilution
A C+D
3.(|C|/2).(|D|/2) |A|/2Q = = < K
K2
Le Chatelier principleWhen a system at equilibrium is perturbed, it will reach a new equilibrium by counteracting the perturbation.
1. Initial state (equilibrium):|C|.|D| |A|Q = = K
2. Perturbation: increase pressure by 2x volume decreases by 2x (p.V=const.): |X|2|X|
4. Final state (new eq.):(2|C|-x).(2|D|-x) 2|A|+xQ = = K
II. Effect of pressure on gaseous reactions
A C+D
reaction goes backward3.(2|C|).(2|D|) 2|A|Q = = 2K > K
Le Chatelier principleWhen a system at equilibrium is perturbed, it will reach a new equilibrium by counteracting the perturbation.
1. Initial state (equilibrium):|C||A|Q = = K
2. Perturbation: add heat (increase temperature)
4. Final state (new eq.):|C|-x|A|+xQ = = K
II. Effect of temperature
A C+ heat
reaction goes backward3.
Equilibria in aqueous solutionsThe water ion-product
[H3O+][OH-] [H2O]2
K =
● [H2O]=(1000 g/l)/(18 g/mol)=55.5 mol/l=55.5 M● in dilute solutions [H2O] constant
Kw=K.[H2O]2=[H3O+][OH-]
H2O + H2O OH- + H3O+
H+
[H3O]+ sometimes referred to as [H+]:
Kw=[H+].[OH-]=10-14 M2 H2O H+ + OH-
Equilibria in aqueous solutionsThe principle of pH
Definition: pH=-lg[H+] pOH=-lg[OH-]
H2O H+ + OH-
"Neutral" pH (pure water):
[H+].[OH-]=10-14 M2 pH+pOH=14
[H+].[OH-]=10-14 M2
[H+]=[OH-] } [H+]=10-7M [OH-]=10-7M
pH=7 pOH=7
H2O H+ + OH-H2O H+ + OH-
Equilibria in aqueous solutionsThe principle of pH
Definition: pH=-lg[H+] pOH=-lg[OH-]
Acidic pH (solution of an acid):
[H+].[OH-]=10-14 M2 pH+pOH=14
[H+].[OH-]=10-14 M2
[H+]>[OH-] } [H+]>10-7M [OH-]<10-7M
pH<7 pOH>7
H2O H+ + OH-
H2O H+ + OH-H2O H+ + OH-
Equilibria in aqueous solutionsThe principle of pH
Definition: pH=-lg[H+] pOH=-lg[OH-]
Basic pH (solution of a base):
[H+].[OH-]=10-14 M2 pH+pOH=14
[H+].[OH-]=10-14 M2
[H+]<[OH-] } [H+]<10-7M [OH-]>10-7M
pH>7 pOH<7
H2O H+ + OH-
Equilibria in aqueous solutionsConjugate pairs of weak acids and bases
Solution of a weak acid:
Solution of a salt of a weak acid:
NaAc Na+ + Ac- (strong electrolyte)
HAcconjugate
acidconjugate
base
H+ + Ac- [H+][Ac-] [HAc]Ka =
(pKa=-lgKa)
[HAc][OH-] [Ac-]Kb =
(pKb=-lgKb)
HAc + OH-
conjugatebase
conjugateacid
Ac- + H2O
anion hydrolysis
[HAc][OH-] [Ac-]Kb =
Equilibria in aqueous solutionsConjugate pairs of weak acids and bases
[H+][Ac-] [HAc]Ka =
(pKa=-lgKa)
(pKb=-lgKb)
[H+][Ac-] [HAc][OH-] [HAc] [Ac-]KaKb = .
KaKb = [H+].[OH-]=Kw=10-14 M2
pKa+pKb = 14
for a conjugate acid-base pair
BuffersMixing a weak acid with its salt
HAcconjugate
acidconjugate
base
H+ + Ac-
NaAc Na+ + Ac-
HAc + OH-
conjugatebase
conjugateacid
Ac- + H2O
anion hydrolysis
Result:[Ac-] cs
[HAc] ca
[H+][Ac-] [HAc]Ka =
[H+]cs
ca
lgKa lg[H+]+lgcs
ca
pH pKa+lgcs
ca
Hendersson-Hasselbalch eq.
common ioneffect
NH4Cl NH4+ + Cl-
NH3 + H+
conjugateacid
conjugatebase
NH4+
cation hydrolysis
NH3 + H2Oconjugate
baseconjugate
acid
NH4+ + OH-
BuffersMixing a weak base with its salt
Result:[NH3] cb
[NH4+] cs
[H+][NH3] [NH4
+]Ka =
[H+]cb
cs
lgKa lg[H+]+lgcb
cs
pH pKa+lgcb
cs
Hendersson-Hasselbalch eq.
common ioneffect
pH pKa+lgcb
cs
Weak base + its salt:
pH pKa+lgcs
ca
Weak acid + its salt:
BuffersSummary: mixture of weak conjugate acid-base pair
conj. base
conj. acid
The general formula: pH pKa+lgcb
ca
acid: conj. acid salt: conj. base
salt: conj. acid base: conj. base
BuffersSummary: mixture of weak conjugate acid-base pair
The general formula: pH pKa+lgcb
ca
Some simple consequences:
pH of a buffer does not depend on the absolute values of ca and cb – only their ratios matter (e.g., dilution does not affect pH)
for a "symmetrical buffer" (ca=cb) pH=pKa
Buffers
H2OHA
A-
H+
OH-
H+
OH-
pH pKa+lgcb
ca
[HA]=ca
[A-] =cb} pH pKa+lg
cb-xca+x
[HA]=ca+x[A-] =cb-x
}
A-
HAH+
OH-
Compensation: A- bindsalmost x mol/l H+
Perturbation: add + x mol/l H+
Buffers stabilize the pH of a solution
Buffers
H2OHA
A-
H+
OH-
pH pKa+lgcb
ca
[HA]=ca
[A-] =cb} pH pKa+lg
cb+xca-x
[HA]=ca-x[A-] =cb+x }
H+
OH-
HA
A-
H+
OH-
Compensation: HA releasesalmost x mol/l H+
Perturbation: add + x mol/l OH-
Buffers stabilize the pH of a solution
BuffersBuffers stabilize the pH of a solution
BuffersThe carbon-dioxide/bicarbonate buffer
H2CO3 HCO3- + H+
CO2 + H2O
[H2CO3] [CO2]
K' =[HCO3
-].[H+] [H2CO3]
Ka =
Ka=1.4.10-7 M(pKa=6.85)
K'=5.6
[H2CO3] [CO2]
K=K'Ka =[HCO3
-].[H+] [H2CO3]
.[HCO3
-].[H+] [CO2]
=
[HCO3-]
[CO2]lgK = lg[H+]+lg
[HCO3-]
[CO2]pH = pK+lg
BuffersThe carbon-dioxide/bicarbonate buffer
H2CO3 HCO3- + H+
CO2 + H2O
Ka=1.4.10-7 M(pKa=6.85)
K'=5.6
[HCO3-]
[CO2]pH = pK+lg
K=K'Ka =7.8.10-7 M pK=6.1
[HCO3-]
[CO2]pH = 6.1+lg
[HCO3-]blood=24 mM
[CO2]blood =1.2 mM } pHblood=7.4
blood capillarylung alveolus
BuffersThe carbon-dioxide/bicarbonate buffer
H2CO3 HCO3- + H+
CO2 + H2O
[HCO3-]
[CO2]pH = 6.1+lg
[HCO3-]blood=24 mM
[CO2]blood =1.2 mM } pHblood=7.4
pCO2= const.CO2(g)
Henry's law: [CO]2(aq)=.pCO2(g) [CO2(aq)] =const.
A buffer in which one component is kept at constant cc.!
BuffersThe carbon-dioxide/bicarbonate buffer
[HCO3-]
[CO2]pH = 6.1+lg
A buffer in which one component is kept at constant cc.!
acid challenge: + x M H+
[HCO3-]-x
[CO2]+xpH = 6.1+lg
exhale excess CO2
[HCO3-]-x
[CO2]pH = 6.1+lg