Help with Collision theory???

Question

Use Collision theory to provide the explanation:

(a) Magnesium metal reacts much more rapidly in concentrated HNO3(aq) than in dilute HNO3(aq).

(b) Food spoils more rapidly on the counter than in a refrigerator

Answer 1

For reactions to occur, the two reactants must collide.  Some fraction of collisions will cause a reaction, and so increasing the rate of collisions between reactants increases the rate of reaction.

a. The magnesium atoms will collide more frequently with H+ ions when there are more of them around.  At a given temperature, the magnesium atoms will be colliding with an approximately constant number of other particles (other magnesium atoms, water, H+, NO3-).  If we increase the concentration of the species that Mg reacts with (H+), we will increase the fraction of collisions that lead to a reaction, and the reaction rate will increase.

b. This is sort of a bad example of the temperature dependence of rate.  With an ordinary chemical reaction, increasing the temperature will increase the average speed of the molecules/atoms, and thus the collision rate.  So if the food is spoiling by reacting with oxygen in the air, or by the reaction of 2 chemicals in the food, this will obviously increase the rate.  However, much food spoilage occurs through the action of bacteria and fungi.  The cold slows the growth of most of these microorganisms - due to the lower rate of chemical reactions at lower temperatures, it is easy to see why the organisms will grow more slowly.  However, there are some such organisms that grow better at lower temperatures, so the relationship is not as basic as it is for a simple chemical system.