Which would have the fastest initial rate of reaction?

• A. 20 cm3 of 1.0 moldm-3 HCl
• B. 5 cm3 of 2.0 moldm-3 HCl
• C. 20 cm3 of 0.5 moldm-3 HCl
• D. None

Answer: (B)

The starting speed of a reaction is set by how often reacting particles collide at the surface where the reaction occurs. This collision rate scales with concentration: a higher concentration of the acid means more acid particles are present near the surface, so collisions happen more frequently and the reaction starts faster. Volume changes don’t matter for the initial rate unless they change concentration; it’s the concentration that drives how quick the initial burst of reaction is. Among the options, the mixture with the highest acid concentration will have the fastest initial rate, while the one with the lowest concentration will be slower, with the middle concentration giving an intermediate rate.