Which of the following best explains why most chemical reactions proceed more quickly when the concentrations of reactants are increased? (12 points) a. the increased concentration increases the number of collisions between molecules. b. the products of chemical reactions are more stable at higher concentrations. c. at higher concentrations, reactant molecules move more quickly. d. at higher concentrations, product molecules are able to catalyze the reaction.

Among many factors, the concentration of reactant is also responsible for effecting the rate of reaction. And it is found that the rate of reaction is directly proportional to the rate of reaction.

Explanation:
                     Higher concentration means greater number of reactants in the system. Greater the number greater will be the probability of reactants to colloid. And according to collision theory reactants must colloid with optimum kinetic energy and orientation. So, in higher concentrated system the rate of collisions increases with increasing the rate of reaction.

Result:
           Option A is correct answer. (The increased concentration increases the number of collisions between molecules.)