Question

why does changing the concentration of Hydrogen peroxide affect the rate at which it decomposes?

Answer 1

The rate-limiting step of the kinetic mechanism for decomposition probably involves the collision of two H2O2 molecules. (The experimental rate law is bimolecular in H2O2.) When the concentration goes up, collisions between H2O2 molecules are more common, so the decomposition reaction goes faster.

Answer 2

The higher the concentration of peroxide, the more molecules there are to collide, the faster the reaction occurs. The lower the concentration, the fewer molecules will collide in a given period of time, so the reaction moves more slowly.