In describing rate of reactions, which phrase is essential?

• A. Collisions between particles
• B. Activation energy
• C. Energy changes during collisions
• D. Temperature increases speed

Answer: (A)

Rate of reaction is governed by how often reactant particles collide. Collisions between particles are the starting event that can lead to bonds breaking and forming new ones, so the frequency of these encounters largely controls how fast the reaction proceeds. If collisions are frequent, more opportunities arise for reactions to occur, increasing the rate. Activation energy matters, but it’s about whether a given collision has enough energy to succeed, not about the basic idea that collisions drive the rate. Similarly, what happens to energy during a collision explains why some collisions fail, but it doesn’t define what determines the rate itself. Temperature can change how fast particles move and how often they collide, as well as the proportion that have enough energy, but the fundamental description of rate focuses on collisions between particles.