A reaction coordinate is an abstract, one-dimensional axis that represents the continuous progress of a chemical reaction as starting reactants gradually transform into final products. It is universally used as the horizontal axis on a standard potential energy diagram, allowing chemists to easily visualize the intricate energy changes and structural reorganizations occurring during a chemical process.
When analyzing a complex chemical reaction, it is highly helpful to visually trace the exact pathway the molecules take from start to finish. The reaction coordinate essentially acts as a map of this energetic journey.
It is important to note that the reaction coordinate does not represent actual time; rather, it exclusively tracks the physical structural changes within the molecules as older bonds are broken and new ones are successfully formed. As the reaction progresses along this coordinate from left to right, the atoms shift their internal positions, moving rapidly through various intermediate structures until they finally settle into the stable, low-energy configuration of the products.
The highest energy point along the entire reaction coordinate always corresponds to the transition state, which is the most unstable, high-energy arrangement of atoms during the reaction.
The specific energy difference between the starting reactants and this massive peak represents the activation energy required to kickstart the chemical change. Once the molecules manage to pass this energetic peak, they proceed down the remaining path of the reaction coordinate, releasing stored energy as they form the final product molecules.
In more complex, multi-step reactions, the reaction coordinate will feature several distinct peaks and valleys. Each peak represents a unique transition state for that specific step, while each valley represents a reactive intermediate that briefly exists before rapidly moving on to the next chemical stage.