A mechanism is the series of steps that substances go through while changing from reactants to products. Each of the steps is a reaction. The step with the highest activation energy will normally be the slowest step, or rate‐determining step. When a mechanism has been determined and proven to be correct, it allows a scientist to explain how a reaction works and to make predictions.

An illustration of a mechanism is the production of cakes at a bakery. An equation for this operation simply shows the type and amount of ingredients that are delivered to the bakery and the number and kinds of cakes that leave the bakery.

The mechanism tracks each step in transforming the original ingredients into the final cakes.

  • flour, eggs, and water are mixed (10 minutes)
  • butter, sugar, and flavorings are added (1 minute)
  • batter is mixed to uniform consistency (5 minutes)
  • batter is put in pans and set in oven (20 minutes)
  • cakes are baked in oven (45 minutes)
  • cakes are removed from oven and cooled (15 minutes)
  • frosting is mixed (5 minutes)
  • cakes are hand frosted and decorated (60 minutes)

These eight steps compose the mechanism for the production of the finished cakes. The slowest step, the frosting and decorating of the cakes, is the rate‐determining step. If this frosting step could be modified to take less time, say, by machine decorating the cakes in thirty minutes, cake production would increase and a new rate‐determining step would be in operation. In this illustration, the baking process, which requires 45 minutes, would become the rate‐determining step.