Problem 11 - Chemistry A Guided Inquiry

Question

Provide two reasons (at least) why all collisions between molecules might not be effective in causing a reaction.

Step-by-Step Solution

Answer

Two reasons why all collisions between molecules may not be effective in causing a reaction are: 1) the colliding molecules may not have enough energy to overcome the activation energy of the reaction (Energy Criterion), and 2) the molecules may not be properly oriented at the time of collision to allow the reaction to occur (Orientation Criterion).

Step 1: Understanding Effective Collisions

To initiate a chemical reaction, it's not only essential for molecules to collide with each other but also, these collisions have to be effective. A collision is considered effective under two conditions: i) Molecules should possess adequate energy that at least matches the activation energy of the reaction - this is also referred to as Energy Criterion and ii) Molecules must have proper orientation at the time of collision - known as the Orientation Criterion. So, all collisions between molecules may not be effective because they may fail to meet one or both of these criteria.

Step 2: Reason One: Energy Criterion

One of the primary reasons why all molecular collisions do not result in a chemical reaction pertains to the Energy Criterion. This means, at the time of collision, the total kinetic energy of the molecules should at minimum meet the reaction’s activation energy. Unless the colliding molecules possess the minimum required energy (activation energy), they can't overcome the energy barrier associated with the reaction - in short, no reaction takes place.

Step 3: Reason Two: Orientation Criterion

The second reason is related to the Orientation Criterion. For a collision to be effective and result in a chemical reaction, molecules should have a proper orientation during collision. Some reactions require a specific alignment of atoms at the time of collision for the reaction to take place. In other words, if the reactive parts of the molecules are not properly aligned at the time of collision, the reaction fails to occur, even if the collision has sufficient energy.