What is the typical rate pattern at the start of a reaction?

• A. It is fast at the start.
• B. It is slow at the start.
• C. It stays the same.
• D. It increases throughout.

Answer: (A)

At the start, reactant concentrations are highest, so there are many collisions between reacting particles per second. The rate of a reaction depends on how often these collisions occur and have enough energy to proceed, so with plenty of reactants present the rate is fast. As the reaction continues, reactants are consumed and their concentrations drop, leading to fewer collisions and a slower rate. That initial burst of speed followed by a slowdown is why the correct idea is that the rate is fast at the start.