The rate law expresses the rate of a reaction in terms of the concentrations of which species?

• A. Catalysts
• B. Reactants
• C. By-products
• D. Products

Answer: (B)

The rate law describes how fast a reaction goes by tying the rate to the concentrations of the species that actually participate in the reaction. In most cases, that means the reactants (and any intermediates involved in the rate-determining step). The idea is that how much of the starting materials is present directly controls how often collisions occur and how quickly products can form. Catalysts can speed things up by providing a different pathway, but their concentration is usually treated as constant in the simple rate law. Products and by-products are the outcomes, not the factors that set the pace, so they don’t determine the rate in the standard expression. For a simple reaction, the rate often depends on the concentrations of the reactants raised to certain powers, showing how changes in those amounts alter the speed.