Activated complex

Activated Complex is a concept in chemical kinetics that refers to the temporary and transitional structure formed when molecules collide during a chemical reaction. It is also known as the transition state. The activated complex is in a higher energy state than either the reactants or the products, which is why energy, or an activation energy, is required to form it.

Formation of the Activated Complex

The formation of the activated complex occurs during the collision of reactant molecules. This collision must occur with sufficient energy, known as the activation energy, and in the correct orientation for the reaction to proceed. The activated complex is not a stable molecule and cannot be isolated or stored; it exists for only a brief moment in time before it either falls apart to reform the reactants or goes on to form the products of the reaction.

Energy Profile of a Reaction

The energy profile of a reaction, represented by a potential energy diagram, shows the energy of the reactants, the activation energy, the energy of the activated complex, and the energy of the products. The difference in energy between the reactants and the activated complex is the activation energy. The difference in energy between the products and the activated complex is sometimes called the reaction energy.

Role in Reaction Rate

The concept of the activated complex is crucial in understanding the rate of a chemical reaction. According to the Arrhenius equation, the rate of a reaction is directly proportional to the number of molecules with energy equal to or greater than the activation energy. Thus, the higher the activation energy, the fewer the number of molecules that can form the activated complex, and the slower the reaction rate.

Transition State Theory

The transition state theory, also known as the theory of absolute reaction rates, was developed to explain the role of the activated complex in chemical reactions. According to this theory, the rate of a reaction is determined by the energy of the activated complex and the frequency of its formation.

See Also

• Chemical kinetics
• Arrhenius equation
• Potential energy diagram
• Transition state theory