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

A decrease in reactant concentration reduces the frequency of successful collisions.

Explanation:

The rate of any chemical reaction is the change in the concentration of the reactants with time.

Rate = - d[Reactants]/d(time).

Also, the rate of a chemical reaction can be given by:

Rate = k[Reactants]ˣ,

where, k is the rate constant of the reaction.

x is the no. of moles that the rate of the reaction depends on.

So, it is clear that the rate of the reaction is directly proportional to the concentration of the reactants.

The concentration of the reactants is at its maximum value at the time of the start of the reaction "mix up", so, the rate of the reaction is maximum at the start of the reaction.So, the rate of most reactions decrease over time due to there is a decrease in the concentration of the reactants with time and so, the chance for collision of the reactants decrease.So, the right choice is:

A decrease in reactant concentration reduces the frequency of successful collisions.

The correct answer is d

True, the standard unit for volume is liters which can be represented by either a lower case "l" or a capital "l"