Reaction Mechanisms

1
Reaction Mechanisms Arrhenius Equation
2
Reaction Mechanisms

• Some reactions are not represented in the
  reaction equation such as,
• Absorbing light energy
• Colliding to the walls of the container
• Many reactions proceed through a sequence of
  steps to arrive at the products
• Each step is called an elementary reaction and
  occurs through collision of atoms, ions and
  molecules.

3
Reaction Mechanisms

• The slowest step in a reaction mechanism is
  called the Rate-determining Step
• In multistep reactions, increasing the
  concentration of reactants at any steps other
  than the rate-determining step will not increase
  the rate of reaction
• A chemical species that form and then are
  consumed in the reaction is called Reaction
  Intermediates

4
Rate Equation

• mX nY -----gt products of reaction or
  intermediates
• then r a Xm Yn
• r kXm Yn
• When writing the reaction mechanism, there are
  three rules
• each step must be elementary, with no more than
  three reactants
• The rate-determining step must be consistent with
  the rate equation
• The elementary steps must all add up to be the
  overall equation

5

• Elementary reactions (Molecularity)
• Unimolecular Decomposition (ex. N2O5(g) ?
  NO2(g) NO3(g))rate kN2O5
• Bimolecular Collision of 2 atoms, ions or
  molecules(ex. NO(g) O3(g) ? NO2(g)
  O2(g))rate kNoO3
• Termolecular Simultaneous collision of 3
  molecules(ex. 2I(g) Ar(g) ? I2(g) Ar(g)
  )rate kIIAr kI2Ar

6

• Reaction mechanism is a detailed sequence of
  elementary reactions, with their rate, that are
  combined to yield the overall reaction.
• Consider the mechanism1) Cl2(g) ? 2Cl(g) 2)
  Cl(g) CHCl3(g) ? HCl(g) CCl3 (g) 3) CCl3 (g)
  Cl (g) ? CCl4(g)
• Some of the possible questions are

7

• a) What's the molecularity of each step?1)
  Unimolecular2) Bimolecular3) Bimolecular
• b) Write the overall equation for the
  reactionCl2(g) CHCl3(g) ?HCl(g) CCl4(g)
• c) Identify the reaction intermediate(s)Cl(g)
  CCl3

8
The Arrhenius Equation

• To explain the large effect of temperature and
  catalysis, the Arrhenius Equation is used to
  account for their effects
• k Ae-Ea/RT
• Ea is the activation energy (J)
• A is constant related to the geometry of
  molecule
• R is the gas constant (8.314 J/(molK))
• T is the temperature (K)
• the answer k is the rate constant for the rate
  law equation r kAnBm

9

• By taking ln (natural log) to each side,ln k
  -Ea/RT ln A
• Arrhenius equation is written for the rate
  constant determined at each temperature giving,
  ln k1 -Ea/RT1 ln A ln k2 -Ea/RT2
  ln A
• Subtracting second equation from the
  first,ln(k1/k2) -Ea/R(1/T1 1/T2)
• The rate and the rate constant are directly
  proportional to each other as long as the
  concentrations are held constant.

10
Examples

• At 200K the rate constant for a reaction is 3.5 X
  10-3s-1 , and at 250K the rate constant is 4.0X
  10-3s-1 . What is the activation energy?Ea
  1.11KJ/mol
• What is the rate of reaction at 450oC if the
  reaction rate is 6.75 X 10-6mol/(Ls) at 25oC?
  The activation energy was previously determined
  to be 35.5kJ/mol.The rate of reaction at 450oC
  is 3.05 X 10-2 mol/(Ls)

11
The Arrhenius Equation

• To test the answer for reasonableness, two
  principles must be remembered1) The larger rate
  constant (or rate) will always be associated with
  the higher temperature2) The activation energy
  always has a positive sign.

12
Summary

• Reactions that contain more than 3 molecules
  colliding occurs in multi-steps
• Each elementary step should only have 3 or less
  reactants
• r kAnBm
• k Ae-Ea/RT
• Temperature and activation energy affects the
  value of k exponentially