Activated Complex

An activated complex can be defined as a middle stage that forms due to the change of a reactant into an end product. Activated complexes are a composition that leads to the highest energy juncture along the reaction path. An activated complex is usually confused with a transition state. However, it is different from the transition state. The transition state represents the highest potential energy configuration of the atoms in a reaction only. In contrast, activated complexes are the range of configurations near the state of transition that the atoms go through during the transformation from products to reactants. 

Activated Complex Meaning

Let’s take an example of a normal chemical reaction between two reactants, X and Y, that creates two products, A and B. These two reactants must mandatorily collide and interact with one another to construct the products. Increased concentration of reactants, increase in temperature or addition of catalyst are many factors that enhance the chances of reactants X and Y encountering each other. Any reaction with activated complexes, reactants X and Y form complex X-Y. Sufficient energy is the most important prerequisite to form the complex. The activated complex is temporary and unstable because the energy is more elevated than the products or reactants. An activated complex breaks down into reactants if there isn’t sufficient energy for the complex to result in products. The products are only formed if there is sufficient energy available.

Transition State and Activated Complex

Activated complex and transition state are the two terms usually used interchangeably but are very different. The atom’s most increased potential energy present in any chemical reaction is what we call the transition state. The various configurations of atoms that it forms through reactants to products is an activated complex. In simple terms, a transition state can be the molecular configuration at the height of the given reaction’s energy diagram. In contrast, an activated complex can occur anywhere near the state of transition.

The main difference remains that the activated complex is all the possible intermediates, and the state of transition is intermediate with the highest potential energy.

Theory of Activated Complex

The activated complex theory is also known as the absolute reaction rate theory. There’s an intermediate configuration where there is a maximum value of potential energy on the path of reaction between the initial arrangement and the final arrangement of molecules and atoms. This configuration corresponds to the maximum value known as the activated complex; the state meanwhile is known as the state of transition. 

In this theory, the activated complex is considered to have been created in an equilibrium state with the molecules and atoms in the initial stages, so its thermodynamics and statistical properties can be specified. The number of activated complexes formed and the frequency at which they tend to go to the end-stage determines the rate at which the end stage is attained.

Example of Activated Complex Reactions

Let us consider chemical reactions. In this chemical reaction, X and Y are reactants, X-Y is an activated complex, A and B are products.

X + Y ⇄ X − Y → A + B

 The reactant molecules should collide for the reaction to be conducted. The presence of a catalyst, concentration of reactant, expansion in temperature facilitates collisions between molecules. Not all collision creates chemical reactions. Once the reactant reaches energy levels, it joins the transition stage and creates activated complexes.

The energy produced by this activated complex will be greater than the products or reactants. But if the energy is not enough to maintain the present chemical reaction, then activated complexes could reform the reactants in a reaction that takes place backwards.

Conclusion 

The collection of intermediate forms in chemical reactions continues while the bonds break and new bonds form. It represents not just one defined state but a range of transient configurations that a group of atoms pass through in between reactants and products. 

Between the initial arrangements and the final arrangements of molecules or atoms on the path of reaction, there is an intermediate configuration with a maximum value in potential energy. This configuration that depicts the maximum value is activated complex, and the state is known as the transition state.