The Damköhler number is the ratio of the rate of reaction to the rate of mass transfer. Which statement best describes its interpretation?

• A. It indicates whether reaction or transport controls the overall rate
• B. It measures heat transfer coefficient
• C. It is equivalent to the Reynolds number
• D. It only applies to gas-phase reactions

Answer: (A)

The key idea is that the Damköhler number compares how fast the chemical reaction would proceed to how fast species can be transported to the reaction zone, effectively matching the timescales of kinetics and mass transfer. When this ratio is large, transport is the bottleneck—the reaction is fast relative to how quickly reactants arrive, so overall rate is controlled by mass transfer. When the ratio is small, the reaction is slow compared to transport, so the chemical kinetics themselves limit the rate. If the ratio is around one, both processes contribute to the overall rate. This is why the statement describing whether reaction or transport controls the overall rate is the best interpretation. It isn’t a heat transfer coefficient, not equivalent to the Reynolds number, and it applies to both gas and liquid systems, not exclusively to gas-phase reactions.