Kinetics

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Kinetics
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What is Kinetics ?

• Analysis of reaction mechanisms on the molecular
  scale
• Derivation of rate expressions
• Design and analysis of experiments to test rate
  equations and derive kinetic parameters
• Theoretical prediction of rate constants
• How can we improve it?

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Elementary steps

• Discuss examples of elementary steps!

A reaction is elementary if it cannot be
split up in further steps. An elementary
reaction proceeds exactly as expressed by the
reaction equation. A reaction mechanism is a
sequence of elementary steps.
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The Rate Equation
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Free Energy and Entropy

• Equilibrium
• ?G 0
• Free energy minimum
• Entropy maximum

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Free Energy and Entropy

• Reaction ?G ? 0

• free energy gt minimum
• entropy lt maximum

Thermodynamics
Kinetics
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Chemical Equilibrium

The Chemical Potential
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Thermodynamics

• Table 2.2 Thermodynamic Data for Important
  Catalytic Reactions
• Reaction ?Ho (kJ/mol) ?Go
  298(K)
• NH3 ? ½ N2 3/2 H2 45.9 16.4
• ½N2 3/2 H2 ? NH3 - 45.9 - 16.4
• N2 3 H2 ? 2 NH3 - 91.9 - 32.8
• 2 NO ? N2 O2 - 182.6 -175.2
• CH4 H2O ? CO 3 H2 205.9 142.0
• CH4 ½ O2 ? CO 2 H2 - 35.9 - 86.8
• CH4 2 O2 ? CO2 2 H2O - 802.6 - 801.0
• CH4 ½ O2 ? CH3OH - 275.6 - 111.77
• CO2 3H2 ? CH3OH H2O - 49.3 3.5
• CO 2H2 ? CH3OH - 90.5 - 25.1
• CO H2O ? CO2 H2 - 41.2 - 28.6
• this per two mole of NH3 or NO. Data taken from
  the JANAF Thermodynamic Tables.

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EX Ammonia Synthesis

• N2 3 H2 2 NH3
• ?G -32.8 kJ/mol
• ?H -91.9 kJ/mol

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The ammonia reactor
N23 H2 2 NH3
?G -32.8 kJ/mol ?H -91.9 kJ/mol
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Power Rate Laws
Parametrization of the rate
Reaction Order
Example
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Equilibrium Constant and Free Energy

?G standard Gibbs energy of reaction ?H -
T?S
Van 't Hoff equation ?H standard
reaction enthalpy ?S Standard reaction entropy
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Temperature dependence of the rate constant
- Eact / RT

• Arrhenius equation k v e

Eact activation energy v preexponential
factor
E
Eact
reactants
products
reaction parameter
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Potential Energy Representationdissociation -
association
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Temperature Dependence of the Rate
ln k ln v Eact / RT
ln k
1000 / T
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Apparent Activation Energy
Example
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Heterogeneous Catalysis
Adsorption
Reaction
Desorption
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Basic surface interactions

• Reactions take place on the metal surface