Energy barrier is the difference of energy between a reactive and the transition state that leads to a product. Molecules have a Boltzmann distribution of energies according to temperature, and some of them will have enough to become a product.

When you consider a molecule to be adsorbed in a surface, for example, chemisorption of oxygen: when the O2 molecule is far from the surface, the total energy is low, but when oxygen approaches near the surface, the total energy starts to increase, up to a point where it reaches a maximum, after this maximum, if the surface can chemisorb, a bond will form, energy will be released and the energy of the oxygen-surface system will decrease and stabilize again.

You can find a lot more information in any undergraduate chemistry textbook.

Energy barrier is the difference of energy between a reactive and the transition state that leads to a product. Molecules have a Boltzmann distribution of energies according to temperature, and some of them will have enough to become a product.

When you consider a molecule to be adsorbed in a surface, for example, chemisorption of oxygen: when the O2 molecule is far from the surface, the total energy is low, but when oxygen approaches near the surface, the total energy starts to increase, up to a point where it reaches a maximum, after this maximum, if the surface can chemisorb, a bond will form, energy will be released and the energy of the oxygen-surface system will decrease and stabilize again.

You can find a lot more information in any undergraduate chemistry textbook.

Salaf, related to your question whether the molecule jump to the transition state after getting some energy and come down to the product.

Physically which jump around are the electrons. Any chemical reaction can be viewed as reorganization of electronic distribution, electrons move from one energy level to the other. When a molecule absorbs energy (activation energy) it will undergo electronic rearrangement that will lead to change in the nature of bonds, some bonds will form and others will break. The transient system  (albeit, molecule) which has partly broken bonds and partly formed bonds are generally termed as transition states. As the electronic arrangement in transition state is unstable, it has higher energy compared to both the reactant and the product.This energy includes both electronic component and steric component. When the reaction is over, the partially broken bonds in transition state are removed and new bonds are fully formed, that is the product molecule. As the product molecule is stable it has lower energy (both in terms of electronic and steric)  than the transition state. Hence, transition state is an unstable electronic arrangement which can proceed in either direction leading to the formation of the product or back to the reactant.

Let's talk about two atoms, but the idea can be extended to molecular reactions. 

When two atoms are approaching, at long distance the electrostatic repulsion between the electron clouds is dominant, therefore the potential energy is growing as they are approaching. The distance is high so that the perturbation of the atomic wave function is weak. But if the kinetic energy is su efficient to get the atoms closer, the electronic wave function is adapting (you can apply the le Chatelier principle also to this system) to reduce the effect of the perturbation and the gradient of the potential surface (the force) is decreasing. If they continue to approach, the electronic clouds are overlapped (this is a simplification) and the attraction between the electronic cloud and the atomic cores becomes effective. Therefore, the net force is positive and you have negative slope in the potential. Getting closer and closer, the the attraction between electrons and cores is contrasted by the repulsion between cores (both positively charged) and the minimum of the potential is obtained by the balance of the attraction between electrons and cores and repulsion between cores. 

This is only a simplified view. 

The potential energy is calculated determining the energy of the two particle system calculating the electronic wave function of the two atom system at different nuclear distance. In this case the Born-Oppenheimer approximations is considered, supposing that the electron clouds adapt rapidly to the geometrical configuration of nucleus. 

The detailed explanations of our colleagues look quite complete.

However, I guess that at least part of your question rises from the general confusion of terms: activation energy and energy barrier.

Historically, activation energy refers to an experimental parameter in the exponential factor of the Arrhenius equation. From the temperature dependence of the rate coefficient, one may extract the Ea, if ln k shows a linear dependence on 1/T.

However, when the Activated Complex Theory arises, then the expression activation energy corresponds to a theoretical concept.

More recently in the modern formulation of Transition State Theory (TST), associated to the concept of Potential Energy surface (PES), the energy barrier is defined as the difference between the  TS ground energy and the energy of the reactants.

Please have a look to Don Truhlar´s papers.

Thank you very much dear , Fernando Vallejos-Burgos , Debashree Bandyopadhyay , Gianpiero Colonna and Adela E. Croce for answering . It help me to define terms like Activated complex, transition state , potential energy surface , activation energy ,reaction coordinate etc in some more advanced and manner .

Its really sorry that any of your answers are exactly not touching my point . It might be because of my low level of understanding in any basics concepts. What am expecting that how you will explain the reaction profile by looking on a conical flask?

My doubt is to reach the "activated complex" stage reactant molecule cluster (or only electron) will jump (like a Grasshoppers ) or not ? :D

Here am picturing my view of a chemical reaction. Kindly find the attachment . I hope you people may develop proper solution after looking on that

salaf

Looking at your picture, there is something that it is not clear. What is the reaction coordinate (x axis)? In my description, I was thinking about an elementary process, considering classical motion of the interacting particles, therefore the x coordinate is the inter-particle distance. In this view, your question should be translated as: can two particles jump instantaneously from a relative distance x to x'? The answer is obviously not. Do you have a different view of the dynamic of a reaction? You can also have tunneling of the barrier, but this is another story and also in this case there is a continuous dynamics. The time scale of elementary process is femtoseconds. 

Therefore, to an higher level, considering the kinetics, the reaction dynamics is too fast and it is considered to happen in a time interval dt=0.

Gianpiero Colonna ,  X-axis is obviously  reaction co-ordinate and Y is potential energy. It is fine that  X to XI not possible but any kind of X to Y is possible (movement by height i mean) ?

One thing i understood from your explanation is if we setting reaction coordinate in X-axis(as usual) there is no question of dynamics (if its horizontal or vertical) because reaction happening in electronic particle/wave level and it mean bond-length or bond angle may change during reaction . The height and breadth(Y and X axis) of diagram simply indicate the potential energy/reaction co-ordinate variation .

But now tunneling in chemical kinetics confusing me . We studied some kind of barrier height (infinite potential wall) and thickness(less thick more tunneling probability) in basic quantum mechanics . Here how we will define tunneling ?

I did not understand your first question. A jump in y means a change in potential energy curve, i.e. and electronic transition. Y (potential energy) is a function of X which is a generalized position coordinate. If I chose X, Y is automatically determined. The motion of the particle are supposed to follow the equations of motion in classical mechanics.   

For an infinite high barrier there is no possibility of tunneling except if it is a Dirac delta.

If you want to describe the reaction using the Quantum point of view, you have to consider wave functions.

To make a parallel, the potential curve (or surface) represents an approximation (Born-Oppenheimer), which consider the force on the nucleus, due to the rearrangements of the electro cloud. 

Consider two atoms far apart. You should have a wave function that is the composition of single atom. It is very complex function because, in general, should contain also the description of the electrons. Because they are far, the potential energy is constant and therefore there is no perturbation of the single atom, which propagates as independent particle. When they are close enough the wave functions are mixed and you cannot distinguish the two atoms any more. After a time sufficiently long you will have  again two wave packets corresponding to the independent atoms, but you will have another packet that corresponds to the molecule. If you have more open reaction channels, you will have a packet for each reaction path. Now the probability of the reaction is the amplitude of third wave packet. The wave function is not an eigenfunction, but is a combination of them, which evolution can be calculated as 

sum Fs exp(i/h E_s t+phase_s)

Now the eigenfunctions, which represents at the same time the separate atoms and the molecule, has non-null values on both side of the barrier, even if E_s is lower than the top of the barrier. 

Therefore, if you calculate the probability of reaction considering only the states with E_s> barrier this is lower than the total probability. That is the tunneling effect. This description is schematic, but vary close to what is done in the reality when calculating the reaction probability. Obviously the full quantum problem is too complex to be solved, and approximate methods are used, but this is the idea. 

Moreover there is another imprecision in your picture. Two atoms cannot react to form a molecule: this process is prohibited because you cannot conserve  at the same time momentum and energy. As a consequence you need a third body for this kind of reactions, such as another atom or molecule, a photon, an electron and so on, to allow energy/momentum conservation.

I hope it is clear, it is a complex problem to be discussed in few words. Give a look to books about molecular dynamics or cross section calculation to have more details.

Nicely explained dear Gianpiero Colonna

Your explanation provide me more insight on this .

Thank you very much for your time

salaf