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# Collision theory

Collision theory states that molecules must collide to react. For most reactions, however, only a small fraction of collisions produce a reaction. In order for a collision to be successful, the reactant molecules must collide both with sufficient energy to overcome the activation energy barrier and in the proper orientation to form any new bonds in the products. Created by Jay.

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• what decides the correct orientation?
• The molecules can be very large and each have a small portion that can interact. The correct orientation would be the 2 reactive sites exposed to each other. It's based on sterics.
• In the graphs the X-axis is the reaction progress. Does it have some sort of units, e.g. ms, µs or ns?
• The progression of the reaction is basically the same as the time, so it is in some units of time, whether it be minutes, seconds, etc. They aren't included in this graph because it doesn't really matter, since the video is focused on the collision theory and how it affects chemical reactions.
• At doesn't jay mean an exergonic reaction?
• Actually, 'Exergonic' implies that the change in free energy is negative (i.e. energy flows from the system to the surroundings). But here, we can't determine the change in free energy by looking at the energy diagram (the energy diagram is for enthalpy only).
Hence, it is an exothermic reaction (since we can't speculate about the change in free energy).
• In this case if the energy required for the products to form is 40 KJ/mol and the energy of the products is 10 KJ/mol, where did the other 30 KJ/mol go?
• Well, if the system is closed (no particles can come inside or go out) then this energy would be converted into heat or movement.
If we have the inverse process (the product have more internal energy) then it means that the energy from heat, pressure, friction, electricity... put into the system is converted into chemical energy.
• how do the reactants reach the activation energy ?is it by itself or in other case its supplied through heat to the system ?
• They reach the activation energy by means of collisions.
At a given temperature there is always a distribution of speeds.
Some particles are moving fast, others move more slowly.
But there will always be a few collisions with enough speed that the collision energy is enough to get over the activation energy barrier·
As you supply heat and increase the temperature, a greater percentage of the collisions will have enough energy to get over the barrier and react, so the reaction will be faster.
• Whats the difference between activated complex and transition state if any?
• Transition state theory describes a hypothetical transition state that exists between reactants and products during a chemical reaction. The species formed in this hypothetical transition state is called the activated complex. They are practically the same, but note that there is a key difference.
• Why can't activation energy be negative?
(1 vote)
• If activation energy were negative, it would completely invalidate the theory of collisions. Let's say that 2 molecules of different elements collide to form 2 molecules of a compound. Let's talk about this, first analytically and then mathematically.

First, analytically. The idea of an activated complex is a highly energetic state where bonds are being broken and formed simultaneously. This makes sense if the activation energy is positive. But if the activation energy is negative, the activated complex is MORE stable than both the reactants and the products! If that were the case, then the reaction should stop at the activated complex, because that is apparently the most stable configuration of the system. If this were the case, no molecules of any element/compound should be stable because every atom is in limbo - breaking and reforming bonds with other atoms every instant.

Next, mathematically.
If activation energy is negative, that means the reaction keeps on proceeding faster and faster with time. (by Arrhenius equation, k = Ae^(-Ea/RT))
But we know for a fact that a reaction either slows or attains a constant rate as it goes to completion.

I hope, by now, it is clear why activation energy being negative is absurd.
• Why is the energy of products less than the energy of the reactants?
(1 vote)
• That is because this example is an exothermic reaction where the products are thermodynamically more stable than the reactants. It could just as easily be the other way around. If the products are thermodynamically less stable than the reactants, the reaction is endothermic.

Please note: A reaction may proceed on its own even if the products are less thermodynamically stable than the reactants.
Reaction mechanics depend on both thermodynamics and kinetics.