In chemical kinetics, another commonly used name for the activated complex is what?

Introduction / Context:
Chemical reactions proceed through a series of steps at the molecular level. During a reaction, reactant molecules must pass through a very high energy configuration before forming products. This configuration is known as the activated complex. In chemical kinetics and transition state theory, another term is widely used to describe this high energy arrangement. The question asks you to identify that alternative name.

Given Data / Assumptions:

• We are dealing with the peak energy configuration in a chemical reaction pathway.
• This configuration is often shown at the top of an energy profile diagram.
• Options include several descriptive terms such as transient complex, energy barrier, collision group, transition state, and intermediate product.
• We assume familiarity with basic reaction coordinate diagrams from school chemistry.

Concept / Approach:
According to transition state theory, the activated complex is the high energy, unstable arrangement of atoms that exists at the maximum of the potential energy barrier along the reaction path. This configuration is called the transition state because it represents the point at which the system is just about to transform from reactants into products. It is not a stable intermediate but a fleeting state. The term energy barrier refers to the energy difference, not the structure itself. Therefore, the correct synonym for activated complex is transition state.

Step-by-Step Solution:
Step 1: Visualise an energy profile diagram where energy is plotted against progress of reaction. Step 2: At the highest point of the curve, there is a maximum known as the activated complex, representing a very unstable arrangement of atoms. Step 3: This configuration separates reactants from products and is called the transition state in modern kinetics language. Step 4: The term energy barrier refers to the height of the energy hill, not to the actual molecular structure at the top. Step 5: Collision group and transient complex are not standard technical names, and intermediate product suggests a more stable species formed between steps, not the peak configuration. Step 6: Therefore, the correct alternative name for the activated complex is transition state.

Verification / Alternative check:
Textbooks that explain transition state theory consistently use the term transition state interchangeably with activated complex. They show energy diagrams where the label transition state appears next to the peak and then discuss how activation energy is the energy needed to reach this state. Experimental and theoretical studies in physical chemistry also talk about locating the transition state on a reaction coordinate surface. This clear and consistent usage confirms that transition state is the accepted synonym for activated complex.

Why Other Options Are Wrong:
Transient complex: Although the activated complex is transient, this phrase is descriptive rather than the standard technical term used in transition state theory.
Energy barrier: Refers to the energy difference between reactants and the transition state, not the structural configuration itself.
Collision group: Colliding molecules may form an activated complex, but this term is not used as a synonym in textbooks.
Intermediate product: A reaction intermediate is usually more stable and has a finite lifetime, whereas the transition state exists only at the instant of highest energy and cannot be isolated.

Common Pitfalls:
Students sometimes confuse the energy barrier with the structure at the top of the barrier. Others think that any short lived species is an intermediate, not realising that transition states are conceptually different from isolable intermediates. To avoid confusion, remember that the transition state or activated complex is represented by the peak on the energy diagram and that activation energy is the energy required to reach that state from reactants.

Final Answer:
Another name for the activated complex is the transition state.