Transcript Lecture 11

Kinetics and Equilibria
By definition, kinetic processes are not equilibrium processes.
In fact, we may think of kinetic processes as the mechanism
that nature uses to reach the equlibrium state.
Binary Collision rate
in forward direction.
Binary Collision rate
in reverse direction.
has 2 rate constants, we can write,
assuming these are ELEMENTARY reaction steps:
(Equilibrium condition)
Where [A]e etc. are the equilibrium concentrations
of [A] etc.
Using the Arrhenius form for the rate constants kf and kr
k f = Af e
-E Af / RT
k r = Ar e
- EAr / RT
But later we will learn (or you already know from high school):
G0= H0 - T S0
ln[Keq]= -G0/RT
Where H0 is the enthalpy change for the reaction and S0 is the
entropy change for the reaction. G0 is called the Free Energy
Equating these two forms for the equilibrium constant allows us
to connect thermodynamics and kinetics!
 Kinetic form of Keq
 Thermodynamic form of Keq
“Identify” Af / Ar with {e (S/R)} (T “independent” assuming ∆Sº
indep of T).
Act State
EAf
EAr
A+B
+∆Ho = EAf - EAr
C+D
(∆Ho = Enthalpy
change for
A+B C+D)
Acid-Base Equilibria
Several ways to define acid, base:
1)
HCl  H+ + ClCH3COOH  CH3COO- + H+
NaOH  Na+ (aq) + OH- (aq)
Oversimplified:
H+ ~ 10-13 cm in diameter because is a free proton
(unique in + charged species)
Evidence exists for presence of H3O+ in solution. Small size of H+
allows it to be incorporated into the structure of the solvent.
H3O+ is called the hydronium ion-is particularly stable.
Less evidence for species like H9O4+ (4H2O+H+)
More accurate view:
Some non-OH- species can neutralize acids:
HCl(aq)+NH3  NH4+ +Cl2)
HCl (aq) + H2O  H3O+ (aq) + Cl- (aq)
acid
base acid
base
Note: H2O is a
base here.
CO32- + H2O  HCO3- + OHConjugate pairs: CO32-, HCO3- and H2O, OH-
Strengths of Acids and Bases:
Equate to tendency to transfer a proton to H2O
HCl + H2O  H3O+ + ClHSO4- + H2O  H3O+ + SO42-
Need a quantitative measure of acidity or H+ donating power.
HA + H2O  H3O+ + ANote that large K is associated with strong acid since it means
numerator is large compared to denominator. Large KHA is
a good proton donor to H2O.
If acid is strong, e.g., HCl, then conjugate base is weak (Cl-)
Will prove this latter in a quantitative fashion.
3) Lewis Concept (most general): Acid is any substance
that can accept electrons and a base is any
substance that can donate electrons.
Acid-Base Equilibria Considerations
[H 3O  ][SO 4 2- ]
K =
[HSO 4 - ][H2 O]
(K’ is a true equilibrium constant)
[H 3 O ][SO4 2- ]
K [H 2O] = K a =
[HSO 4 - ]
Acid ionization constant
NH3 + H2O  NH4+ + OH -
Ka is called the acid
ionization constant
pH=
-log10[H3O+]
QuickTime™ and a
Video decompressor
are needed to see this picture.
Bonus * Bonus * Bonus * Bonus * Bonus * Bonus
Suppose you have [H3O+] from added HCl = 0.1 M = 10-1 M
Then [10 -1][OH-] = 10-14
In pure H2O, [OH-] = [H3O+] = 10-7, but
Weak Acids and Bases
Add acetic acid to H2O:
K=Ka
What are [CH3COOH], [H3O+], [CH3COO-]?