We have studied the energy level diagrams for exothermic and endothermic reactions drawn on a vertical scale. But enthalpy change of a reaction does not tell the whole story about the reaction process. Many reactions which are energetically favourable, i.e., have -ve value of ΔrHe do not occur unless and until they are given some amount of energy. For example, combustion of carbon is exothermic reaction.
C(s) + O2(g) CO2(g); MI = – 393.5 kJ mol-1
But this reaction does not occur of its own inspite of the abundant availability of carbon and oxygen. For this reaction to occur we have to add some heat energy to the carbon and oxygen mixture in the form of burning. This means that the reactants do not directly change into products but instead, there exists some barrier of energy between the reactants and products. In fact, some additional energy is required by the reactants to commence the cleavage of old bonds.
CONCEPT OF ACTIVATION ENERGY
Let us consider the exothermic reaction
A2 + B2 à 2AB;ΔrH = – ve
The simple energy level diagram is represented in Fig. 16.9. The reaction involves the breaking of bonds between A – A, B – B molecules and formation of A – B bonds. The net energy released is more than that required. Therefore, Δ r H i.e., enthalpy of reaction = – ve. Now in going from A2 and B2 to A – B, the reactants form a complex structure of high energy which represents partial elongation of A – A and B – B bonds and commencement of the formation of A – B bonds.
This arrangement is more energetic than the reactants as well as products and is called activated complex.
Fig. 16.9. Energy eve1 diagram for reaction progress
The additional energy required by the reacting species over and above their average energy is called activation energy represented by E a The values of E a is different for different reactions.
It can be noted that higher the value of E a for a reaction, slower will be the chemical process.
Function of Catalyst
A catalyst provides an alternative path of lower activation energy to the reactants. Since activation energy gets lowered, more number of. reactants molecules can change into the products per unit time. Hence the reaction rate is accelerated.
The energy level diagram for catalysed and uncatalysed reaction is shown in Fig. 16.10.
It can be easily understood from the diagram shown in Fig. 16.10. That catalyst does change the individual energy of reactants and products. Hence, the value of Δ r H for uncatalysed and catalysed reaction remains same.
Fig. 16.10. Energy level diagram for uncatalysed and catalysed reaction.