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Topic: The story of molecular collsions in a chemical reaction  (Read 7541 times)

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Offline ayslam

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The story of molecular collsions in a chemical reaction
« on: January 17, 2007, 01:45:20 AM »
I am concerning the microscopic view of chemical reactions.

Energy of a molecule is basically classified into 2 types: kinetic energy and potential energy. Kinetic energy represents the energy for motion while the latter describe all energies inside the molecules, governing all the internal states, e.g.the bond lengths, valence angle bends, dihedral angles and etc. In terms of collision theory, reactant molecules must collide for the reaction to take place. Collisions can be effective or ineffective. A collision is effective only if the molecules collide with the right orientation and they have enough "potential" energy (activation energy). Ineffective collision occurs when either one of the criteria mentioned above does not meet.
In a collision of two molecules, energy is transfered from one to another, from kinetic to potential, and vice versa. This obeys the conservation of momentum. The transfer of energy, from one type to another, is only be chance??? Some collisions make the molecules climb up the "potential" barrier but some move away from the intermediate state ( the position at the top of the potential barrier). Once a collision involve molecules with enough potential energy and with the right orientation, the reaction occurs.
Temperature represents the average kinetic energy of the molecules. We can control the kinetic energy but not the potential energy. The reaction can be accelerated by increasing the temperature. The reason is that there will be more molecules having higher kinetic energy and then the probability of having molecules with enough potential energy will be higher, since more kinetic energy is available to be transfered to potential energy.
Energy may be lost during the course of the reaction. Some electrons are excited and release the energy in the form of photons. Energy may also be lost when molecules hit the walls of the container. But this depends on the setting of the environment.

Is this story complete and correct? And is there any stochastic process governing the occurrence of collisions?
???

Thanks you very much for the great help in advance!!!

Offline xiankai

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Re: The story of molecular collsions in a chemical reaction
« Reply #1 on: January 17, 2007, 08:16:05 AM »
Quote
A collision is effective only if the molecules collide with the right orientation and they have enough "potential" energy (activation energy).

i'm not sure about the theory, but can this be represented as a force vector, such that if enough energy is transferred despite being having wrong spatial orientation with respect to the other molecule, the reaction may still take place? of course the atoms have to bond together, but if the energy is great enough... who knows.

also, you left out concentration of molecules as a factor.
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Offline brnlow

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Re: The story of molecular collsions in a chemical reaction
« Reply #2 on: January 17, 2007, 10:44:00 AM »
... any stochastic process governing the occurrence of collisions?
???

Thanks you very much for the great help in advance!!!

You might consider the random walk model.

Offline ghb

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Re: The story of molecular collsions in a chemical reaction
« Reply #3 on: January 17, 2007, 01:45:20 PM »
Tunnelling effects also can not be neglated...

Offline english

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Re: The story of molecular collsions in a chemical reaction
« Reply #4 on: January 17, 2007, 02:12:56 PM »
An easier way to say it is when the potential energy of the collison exceeds the minimum energy of activation, the two particles collide, but not necessarily effectively.


Offline ayslam

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Re: The story of molecular collsions in a chemical reaction
« Reply #5 on: January 18, 2007, 01:23:09 AM »
How about the energy transfer part? Do the kinetic and potential energies transfer back and forth in this way? The activation energy is regarding the potential energy, not not the kinetic energy?

Offline xiankai

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Re: The story of molecular collsions in a chemical reaction
« Reply #6 on: January 18, 2007, 09:16:10 AM »
i think the activation energy also requires some kinetic energy. since activation energy is the energy required to break the bonds first, that implies that after supplying the energy, the atoms/molecules on the other end of the bond are free to move around. put another way, enough kinetic energy must be supplied such that the atom/molecule can escape the electrostatic/covalent bond.

but i'll prefer to think its a mixture of the two.
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Offline ghb

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Re: The story of molecular collsions in a chemical reaction
« Reply #7 on: January 22, 2007, 08:08:23 PM »
The activation energy, firstly proposed by Arrhenius in his study of reaction, that a transition state (TS) exists between the reactant and product. In the classical collison theory, the TS had also been defined, see the book "Dynamics of molecular collision", edited by W.H. Miller (1976). But what the activation energy is, this question puzzled people for a long time, even Erying who was awarded by many as the father of transition state theory, didn't explain it clearly, until David Chandler (J. Chem. Phys. 1978, 68, 2959) showed that the activation energy is the free energy barrier.

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