Why does the rate of most reactions decrease over time?
a) a decrease in reactant concentration reduces the frequency of successful collisions.
b) a decrease in temperature reduces the average kinetic energy of the particles.
c) an increase in product concentration interferes with the effectiveness of the catalyst.
d) an increase in pressure of the system reduces the movement and energy of particles.

ExplanaA) A decrease in reactant concentration reduces the frequency of successful collisions.  tion:

As a reactant gets used up, there are fewer molecules present to collide with others. A reaction depends on molecular collisions. When there are fewer collisions, the reaction slows down.

B) is wrong. A reaction is usually carried out at constant temperature,

C) is wrong. Not all reactions need catalysts.

D) is wrong. If the temperature is constant, pressure will have little effect on the movement and energy of molecule.

a compound is a molecule made of atoms from different elements.

a. by the combination of an unlimited number of elements