A reaction always proceeds by breaking of bonds in the reacting molecules therefore, whether a reaction is exothermic or endothermic, to start with energy must come from some source to break the bonds.
We know from our knowledge of physical chemistry that molecules of the reactants are in a state of rapid motion and possess kinetic energy. The reaction occurs when the responding molecules approach in proper alignment and collide. On such collisions, the kinetic energy possessed by the molecules is transformed into potential energy of the system. Thus to start a reaction, the required energy is supplied by the collisions of the reacting molecules, where rate could be enhanced if necessary, say by heating.
Not all collisions between the reacting molecules are fruitful. It is only the collisions of molecules which possess certain minimum energy that can bring about the reaction. The molecules that have come to possess higher potential energy through collisions are said to have been activated to enter into the reaction.
The minimum amount of potential energy that must be provided by collisions of the reacting molecules for the reaction to occur is known as the Activation energy (Ea).
It is true that for an exothermic reaction, the collisions of the reactant molecules readily supply the required initial energy and the reaction takes place spontaneously. On the other hand, for an endothermic reaction the molecules need to be activated by supplying energy, say in the form of heat to make the reaction go.
Imagine some balls attempting to climb over a hill from one valley to another (Fig. 1.1). Only those balls will cross over which possess a certain minimum of energy to reach the hilltop from where they would get over the barrier. Similarly, only such reacting molecules that possess certain minimum energy could change into products but before doing so, these would have to cross over the Activation energy barrier.
Consider the energy changes during the course of the reaction,
In the beginning, both C and A-B possess certain potential energy indicated by the point (a) on the curve (Fig. 1.2). These responding molecules also possess Kinetic power which on collisions is transformed into potential energy. This result in the increase of potential energy and the system moves up along the curve till the cliff (b) is reached. The energy of cliff state is a sort of temporary phase and leads to products C-A + B (c) when the potential energy of the system is again changed into kinetic energy and then into heat or any other form of energy.
Fig. 6.7 shows that for an exothermic reaction, the system originally possesses more potential energy than the product and the excess energy (ΔH) is liberated as heat. For an endothermic reaction, the system to start with has less potential energy than the products (-ΔH) and, therefore, it absorbs heat from the surroundings.
The reaction C + A-B C-A +B could be visualized to take place by the following steps. The molecule C approaches A-B from a direction remote from B (proper alignment). While C draws nearer to A, B starts being repelled from A until a stage is reached when C and B are rather loosely attached to A and are approximately equidistant from it. This is the least stable arrangement and is called the transition state (TS) or activated complex. The sequence of events may be represented as:
The conversion state is not a real molecule, the bonds being partial. In this condition the system possesses maximum energy and is most unstable. Hence the transition state of a system could be described as an extremely transitory specific arrangement of atoms and groups through which a reaction system must pass on its way to the products. In other words, the transition state (activated complex) has extremely short life-time and at once decomposes to give the products.
A transition state refers to an imaginary molecule and cannot be isolated. On the other hand , an intermediate is a stable entity and can be isolated under appropriate conditions. A response which proceeds through an intermediate has to surmount two power barriers. One for the conversion of the reactants to the intermediate (Ea) and the other for the conversion of the intermediate into products (Ea) as shown in Fig 1.3.