The true statement about reaction rates is:
Changing the surface area affects the reaction rate.
Increasing the surface area of a reactant (e.g., by grinding a solid into a powder) typically increases the reaction rate because it allows more particles to collide with each other.
To clarify the other statements:
- A catalyst does not slow down the reaction rate; rather, it speeds up the reaction by lowering the activation energy.
- Changing the amount of reactants can affect the reaction rate; typically, increasing the concentration of reactants increases the reaction rate.
- Changing the temperature can indeed speed up a reaction rate, as higher temperatures generally increase the kinetic energy of the molecules, leading to more frequent and effective collisions.