What causes an increase in effective collisions without increasing average energy?

An increase in the reactant concentration

The correct answer is option 1. An increase in the reactant concentration

Increasing the concentration of reactant molecules leads to an increase in effective collisions without increasing the average energy. Here's how it works:

When the concentration of reactant molecules is increased, there are more molecules per unit volume. This results in a higher frequency of collisions between the reactant molecules. With more collisions occurring, the chances of effective collisions also increase.

Effective collisions are those that have sufficient energy and proper orientation to result in a chemical reaction. Increasing the concentration of reactant molecules enhances the likelihood of reactant molecules colliding in the correct orientation for a reaction to occur. While the average energy of the collisions remains the same, the increased concentration leads to a greater number of successful collisions per unit of time.

On the other hand, the remaining options do not directly lead to an increase in effective collisions without increasing average energy:

2. A decrease in pressure: Lowering the pressure reduces the number of gas molecules in a given volume. This decreases the frequency of collisions and, therefore, the likelihood of effective collisions. It does not increase the number of effective collisions without increasing the average energy.

3. Catalysis: Catalysis does not directly increase the number of effective collisions. Instead, catalysts provide an alternative reaction pathway with lower activation energy. They lower the energy barrier required for a reaction to occur, thereby increasing the rate of successful collisions.

4. An increase in temperature: Increasing the temperature does increase the average energy of collisions. Higher temperatures lead to increased molecular motion and kinetic energy, resulting in more energetic collisions. While an increase in temperature can enhance the rate of effective collisions, it does so by increasing the average energy, which is not what the question is asking for.