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GAS PHASE REACTION
The Collision Theory and Activation Energy
Collision Theory Of Bimolecular Reaction
An effective (successful) collision is a collision that occurs with enough energy
and with particles aligned correctly
In an ineffective (unsuccessful) collision, the colliding particles remain unchanged
• For the reaction A + B C, A and B must collide Assumptions• Molecules are hard spheres• For reactions to occur, molecules must collide• Reactions occur only if molecules are energetic• Collisions should transfer certain minimum
energy
Collision Theory
Reaction Rate depends on:
1) The number of collision per unit time between the reacting species.
2) The fraction of these collisions that are successful in producing a new molecule.
Factors affecting rate of reaction
1. Concentration
2. Temperature
3. Particle size
4. Nature of reactants
5. Presence of a catalyst
Effect of Concentration
• Concentration reactants increases ↑• Frequency of collision increases ↑• Frequency of effective collision increases
Collision Theory
Particle size affects rate
• The smaller the particle size the faster the rate.
The Collision Theory: why higher temperatures helps
-Reactions result when atoms/molecules collide with sufficient energy to break bonds
- Molecules at a higher temperature move faster— have a greater energy (energy distribution increases)
The Collision Theory: why higher temperatures help
Nature of reactants And catalyst
We shall consider a bimolecular reaction A + B → P
ZAB =NA NB AB2 ට8𝜋𝑅𝑇𝑀𝐴+𝑀𝐵𝑀𝐴 𝑀𝐵 ….(1)
ZAB = collision frequency
molecular diameter
AB = 𝐴 +𝐵
AB = mean molecular diameter
MA and MB are molecular weights
Rate of reaction = no of molecules colliding per unit time × fraction of effective collision
= NA NB AB2ට8𝜋𝑅𝑇𝑀𝐴+𝑀𝐵𝑀𝐴 𝑀𝐵 e-Ea/RT
… .(2)
but rate =kሾ𝐴ሿሾ𝐵ሿ =k NA NB …(3)
from 2 and 3
k NA NB = NA NBAB2ට8𝜋𝑅𝑇𝑀𝐴+𝑀𝐵𝑀𝐴 𝑀𝐵 e-Ea/RT
k= AB2ට8𝜋𝑅𝑇𝑀𝐴+𝑀𝐵𝑀𝐴 𝑀𝐵 e-Ea/RT (dm3/s)
k= AB2ට8𝜋𝐾𝑁𝑇𝑀𝐴+𝑀𝐵𝑀𝐴 𝑀𝐵 e-Ea/RT (dm3mol-1/s-1)
….(K=R/N)
• During a collision, the sum of PE + KE remains constant; that is, as particles collide they slow down and KE decreases but PE increases
• Activation Energy: the minimum energy with which reactant particles must collide to produce an effective collision
The sparks generated by striking steel against a flint provide the activation energy to initiate combustion in this Bunsen burner. The blue flame sustains itself after the sparks stop because the continued combustion of the flame is now energetically favorable.
Activation Energy
Activation Energy: the minimum energy with which reactant particles must collide to produce an effective collision
Activated complex: an unstable chemical species containing partially broken and partially formed bonds
•The activation energy can be thought of as a barrier to energy transfer (from the kinetic energy to the potential energy) between reacting molecules that must be overcome.
Steric factor or orientation
• If the values of the predicted rate constants are compared with the values of known rate constants
• collision theory fails to estimate the constants correctly and the more complex the molecules are, the more it fails.
• particles have been supposed to be spherical and able to react in all directions; that is not true, as the orientation of the collisions is not always the right one.
H2O2 I-
Bad orientation: no productive reaction occurs
I-O-HOH-
If collision orientation is favorable, a reaction occurs