Which factor primarily affects the frequency of collisions between reacting particles?

The frequency of collisions between reacting particles is primarily affected by the concentration of reactants. When the concentration of reactants increases, there are more particles present in a given volume, leading to a higher likelihood that they will collide with one another. This increased number of collisions can enhance the rate of reaction, as more interactions between particles can lead to more successful reactions.

In contrast, the type of reaction mechanism does not directly influence the frequency of collisions; rather, it pertains to the pathway through which a reaction proceeds. The age of the reactants may affect their reactivity or stability but does not fundamentally change the frequency of collisions in a direct manner. Lastly, although light can affect reactions (particularly those that are photochemical), it does not influence the frequency of collisions unless it directly triggers a specific reaction mechanism. Thus, the concentration of reactants is the most significant factor impacting collision frequency in chemical reactions.