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Catalysis as Phenomenon Catalysis and Chemical Engineering / L2
Part 4. Formal Kinetics of
Complex Reactions
Complex Reaction Catalysis and Chemical Engineering / L2
What means ‘complex’ ?
Reactions of ‘Simple Types’ – rate can be described by
the Law of Mass Action in itsbasic form
In the framework of the Formal Kinetics
Complex reaction – combination of several reactions of ‘simple types’:
1. parallel
2. consecutive A → В → Р
3. reversible (two-side)
Further Complication – combination of more than two ‘simple’ reactionsin general – x(1) + y(2) + z(3)
Complex Reaction Catalysis and Chemical Engineering / L2
What means ‘complex’ ?
CH4 + O2 → <products>
Complex Reaction Catalysis and Chemical Engineering / L2
What means ‘complex’ ?
Chemical Kinetics – the doctrine of the Chemical Processes,
their mechanisms and developmentin time and space
Mechanism of reaction:
for complex (multi-step) reaction –
the sequence of chemical steps and intermediate products(intermediates) that leads from the initial reactant(s) to the finalproduct(s);
for elementary reactions –
trajectories of motions of atomic nuclei, variations of electrondensity and energetic state of the system during its transition from reactant(s) to product(s)
Reverse Kinetic Problem (Task) –
to determine the type of kinetic model (to chose among
possible types of description, kinetic equations, etc.) and/orits parameters based on the existing data on the system
behaviour (experimental data)
– as a rule, incorrectly determined task; has no rigorous andunambiguous solution exclusively based on the kinetic data;
Direct Kinetic Problem (Task) –
to predict (simulate) the system behaviour based on the
existing kinetic model (reaction scheme, kinetic equations) and
its parameters
– requires repeated solution of Direct Kinetic Problem and selectionof ‘optimal’ description of experimental data
Complex Reaction Catalysis and Chemical Engineering / L2
Complex Reaction Catalysis and Chemical Engineering / L2
What means ‘complex’ ?
Reactions of ‘Simple Types’ – rate can be described by
the Law of Mass Action in itsbasic form
Laws and Principles Catalysis and Chemical Engineering / L2
Scientific Laws – models of Objects and Phenomena that reflect:
(i) the current level of their comprehension;
(ii) stable (repeatable, reproducible) relation(ship)sbetween them ⇒
based on practice (observations, experiment, etc.)
Principles – also based on practice ‘tools of knowledge’,
but sublimed to the level of ‘common sense’, i.e.
higher degree of generalization than laws.
Any scientific discipline (including Chemical Kinetics) operates
both laws and principles
Complex Reaction Catalysis and Chemical Engineering / L2
What means ‘complex’ ?
Reactions of ‘Simple Types’ – rate can be described by
the Law of Mass Action in itsbasic form
Complex reactions –
Independence Principle (W.F. Ostwald):
the Law of Mass Action can be used for each ‘simple’ reaction
occurring in the system as if it is the only reaction in given
conditions
Complex Reaction Catalysis and Chemical Engineering / L2
What means ‘complex’ ?
Reactions of ‘Simple Types’ – rate can be described by
the Law of Mass Action in itsbasic form
-d[A]/dt = k1[A] + k2[A] = k’[A]
-d[A]/dt = k1[A];
d[B]/dt = k1[A] – k2[B]
-d[A]/dt = k1[A] – k2[B]
+ mass-balance equations, i.e. [A] + [B] + [D] + [P] = [A]0
Complex Reaction Catalysis and Chemical Engineering / L2
if n1 = n2 = 1:
X = 1- e-(k1+k2)t; Yj = X kj/(Σki); Sj = kj/(Σki)
n1 A → P1
n2 A → P2
if n1 = n2: S1 = S2 @ any X
if n1 < n2: S1/S2 ↑ @ X ↑
‘Simple Complexity’ – some features of
parallel reactions:
Complex Reaction Catalysis and Chemical Engineering / L2
if n1 = n-1 = 1:
-d(1-X)/dt = k1(1-X) – k-1X; Y = 1 – X
and
‘Simple Complexity’ – some features of
reversible (two-side) reactions:
< 1
t → ∞ ⇒
k1C(A) = k-1C(P) ⇒ X → X∞ =
EQUILIBRIUM: rates of all reciprocally reverse processes
(i.e., chemical reactions) are equal
Complex Reaction Catalysis and Chemical Engineering / L2
REVERSIBILITY and EQUILIBRIUM
(L. Botzman, J.C. Maxwell, R. Wegscheider, R.C. Tolman,
A. Einstein, G.N. Lewis, L. Onzager, …)
Complex Reaction Catalysis and Chemical Engineering / L2
Microscopic Reversibility Principle (for Chemistry):
any ‘simple’ (elementary) chemical reaction can proceed in
both reciprocally reverse directions
REVERSIBILITY and EQUILIBRIUM
(L. Botzman, J.C. Maxwell, R. Wegscheider, R.C. Tolman,
A. Einstein, G.N. Lewis, L. Onzager, …)
Detailed Equilibrium (or Balance) Principle:
if the state of global equilibrium is reached in some system, any
partial equilibria are also achievedor
at equilibrium each elementary process should be balanced
by its reverse process
Complex Reaction Catalysis and Chemical Engineering / L2
Microscopic Reversibility Principle (for Chemistry):
any ‘simple’ (elementary) chemical reaction can proceed in
both reciprocally reverse directions
‘thermodynamic consistency’ in microkinetic analysis –
ensures that complex system reaches global equilibrium
at t → ∞
Complex Reaction Catalysis and Chemical Engineering / L2
Detailed Equilibrium (or Balance) Principle:
if the state of global equilibrium is reached in some system, any
partial equilibria are also achieved
Example: Selection of catalysts
СO + H2O СO2 + H2
СH4 + H2O СO + 3H2
! ! ! !
СH4 + CO2 2СO + 2H2
‘basic’ reactions:
‘target’ reaction:
Complex Reaction Catalysis and Chemical Engineering / L2
Detailed Equilibrium (or Balance) Principle:
if the state of global equilibrium is reached in some system, any
partial equilibria are also achieved
Example: Selection of catalysts
СH4 + H2O СO + 3H2
С2H6 + 2H2O 2СO + 5H2
! ! ! !
С2H6 + H2 2СH4
‘basic’ reactions:
‘target’ reaction: