Collision energy requirement.

• A. They must collide with zero energy
• B. They must collide with enough energy to react
• C. They must collide with energy less than activation energy
• D. They must not collide at all

Answer: (B)

The essential idea is that chemical reactions in collisions require enough energy to overcome the activation energy barrier. When two molecules collide, their combined energy in the encounter must be at least as large as that barrier; only then can bonds break and new bonds form to produce products. If the collision energy is less than the activation energy, the molecules simply bounce apart, and no reaction occurs. If they don’t collide at all, there’s obviously no chance for a reaction. Even when energy is sufficient, orientation also matters, so not every high-energy collision leads to products, but having at least the activation-energy energy is the minimum requirement to react. Increasing temperature raises the fraction of collisions with enough energy, which explains why reaction rates typically rise with temperature.