Explain, in terms of particles, why the rate decreases during the reaction between magnesium and hydrochloric acid.

There are 2 ideas that need to be included here :The concentration of acid decreases over time So collisions are less frequentAs the reaction proceeds more and more molecules would have collided successfully. This means that there are less reactants left in the system, so the successful collisions happen less frequently. Since the rate of the reaction depends on the frequency of collisions, we expect the rate to decrease in time.