Chemistry 2008 JAMB Past Questions - Question 29: the rate of a reaction usually decreases with a decrease in the concentration of reactants because

Correct Answer

D

Explanation

**1. Collision frequency:**

* Chemical reactions occur when reactant molecules collide with enough energy to overcome a barrier called the activation energy.
* Lowering the concentration of reactants means there are fewer molecules present in a given volume.
* Consequently, there are fewer opportunities for collisions between reactant molecules, leading to a decrease in the rate of successful collisions and, ultimately, the overall reaction rate.

**2. Effective collisions:**

* Not all collisions between reactant molecules result in a reaction. Only collisions with sufficient energy transfer and proper orientation lead to product formation.
* While the individual energy of each molecule might remain unchanged, the decrease in concentration reduces the chances of two colliding molecules having the specific energy and orientation needed for a successful reaction.
* This further contributes to the decline in the overall reaction rate as the concentration of reactants decreases.

Here's an analogy to help visualize this concept:

Imagine throwing darts at a dartboard. Higher dart concentration (more darts) translates to more throws and potentially more hits. Additionally, each throw has a chance of hitting the bullseye (successful reaction). As the number of darts (concentration) decreases, both the number of throws and the hit probability diminish, leading to fewer bullseyes (products) formed.

It's important to note that other factors like temperature, pressure, and the presence of catalysts can also influence the rate of a reaction. However, the decrease in concentration is a fundamental reason why reaction rates typically slow down as reactant molecules become less abundant.