Which of the following is not a factor that affects the rate of a chemical reaction?

• A. Concentration
• B. Temperature
• C. Surface Area
• D. Color of reactants

Answer: (D)

In chemical kinetics, the rate of a reaction depends on how often reactant molecules collide and how energetic those collisions are. Higher concentration means more molecules present, so collisions happen more often. Higher temperature gives molecules more kinetic energy, increasing the fraction of collisions that have enough energy to overcome the activation barrier. Greater surface area provides more contact sites for collisions in reactions involving solids. The color of the reactants, however, does not influence how often collisions occur or how energetic they are, so it does not affect the rate under ordinary conditions. (Light-driven or photochemical effects are a special case, but color alone isn’t a general rate factor.)