A chemical reaction converts one set of molecular species into another by rearranging atoms and chemical bonds.

Balanced reaction notation records the net transformation, but it does not show the microscopic sequence by which bonds break and form.

Reaction pathways

A reaction usually proceeds through intermediate molecular configurations rather than jumping directly from reactants to products.

A reaction coordinate is an abstract variable used to represent progress along such a pathway.

energy
  ^              barrier
  |                /\
  | reactants ____/  \____ products
  +--------------------------> reaction coordinate

The high-energy region separating reactants from products creates an activation barrier. Molecules must reach suitable configurations and energies before the transformation can proceed rapidly.

Thermodynamics and kinetics answer different questions

Thermodynamics compares the relative favorability of states and equilibrium. Kinetics describes how quickly a system moves between states.

A reaction can be thermodynamically favorable yet extremely slow because its activation barrier is large.

Conversely, making a reaction faster does not by itself make its products more favorable at equilibrium.

Catalysts

A catalyst increases reaction rate by providing an alternative mechanism with a lower activation barrier.

The catalyst participates in intermediate steps but is regenerated overall, so it is not consumed stoichiometrically by the net reaction.

A catalyst accelerates both forward and reverse processes that use the catalyzed pathway. It therefore changes how quickly equilibrium is approached, not the thermodynamic equilibrium position itself.

Multistep reactions

Many reactions occur through several elementary steps and transient intermediates. The overall rate can depend strongly on the slowest or most weakly populated parts of the mechanism.

This makes chemistry naturally network-like: one molecular species can participate in several competing reactions, and products from one reaction can become reactants for another.

Biochemistry builds on exactly this structure. Cells use specialized catalysts to make selected reaction pathways fast enough to operate under biological conditions.