How does solvent viscosity affect diffusion-controlled reactions?

• A. Higher viscosity lowers diffusion rates, reducing the rate
• B. Higher viscosity increases diffusion rates
• C. Viscosity has no effect
• D. Viscosity only affects equilibrium constants

Answer: (A)

In diffusion-controlled reactions, how fast the reactants meet each other sets the pace of the overall reaction. The move of molecules through the solvent is governed by diffusion, and diffusion slows down when the solvent is more viscous. The diffusion coefficient D is inversely related to viscosity (D is roughly proportional to 1/η). Since the encounter rate of reactants—and thus the reaction rate in the diffusion-limited regime—depends on D, increasing solvent viscosity reduces diffusion, which lowers the overall rate. So, higher viscosity lowers diffusion rates, reducing the rate. Viscosity does not leave the rate untouched, and it doesn’t normally set equilibrium constants in this context, so the other ideas don’t fit as well.