Question

Suppose two molecules that can react collide. Under what circumstances do the colliding molecules not react?

Short answer

Colliding molecules will not react if their physical properties are incompatible, they do not possess enough kinetic energy to meet the activation energy requirement, their orientation upon collision is incorrect, or the temperature is too low for the reaction to proceed.

Step-by-step solution

Understand the properties of the molecules

To determine whether a reaction will occur or not when two molecules collide, we should first consider their physical properties, such as their size, shape, and chemical composition. Molecules with incompatible properties may not react with each other, even upon collision.

Evaluate the activation energy

Activation energy is the minimum energy required for a reaction to occur. If the colliding molecules do not possess enough kinetic energy upon collision to meet or exceed the required activation energy, the reaction will not take place.

Analyze molecular orientation

For a reaction to occur, the molecules must collide with the correct orientation, meaning that their reactive sites must be properly aligned. If the colliding molecules do not have the appropriate orientation upon collision, they will not react.

Consider the effect of temperature

Temperature plays a significant role in the occurrence of chemical reactions. Higher temperatures generally increase the chances of a reaction, as they provide the colliding molecules with more kinetic energy. If the temperature is too low, the molecules may not possess enough energy to overcome the activation energy barrier, and the reaction will not take place.

Conclusion

Colliding molecules will not react under the following circumstances: their physical properties are incompatible, they do not possess enough kinetic energy to meet the activation energy requirement, their orientation upon collision is incorrect, or the temperature is too low for the reaction to proceed.

Key concepts

Understanding Activation Energy

Activation energy is a fundamental concept in collision theory that helps explain whether chemical reactions occur when molecules collide. But what is activation energy? It is the minimum amount of energy required for reactants to form products in a chemical reaction. Think of it as a barrier that molecules need to overcome to transform into a different substance.
Just like an athlete needs a certain amount of energy to jump over a hurdle, molecules need activation energy to react. If colliding molecules do not possess enough kinetic energy to reach or exceed this activation energy threshold, the reaction simply won't take place. Therefore, for a successful reaction:

• Molecules must have sufficient energy when they collide.
• Energy levels must meet or surpass the activation energy barrier.

The greater the activation energy, the slower the reaction tends to be, because fewer molecules have the necessary energy at any given time. Lowering this energy barrier can speed up reactions, which is often achieved by using a catalyst.

The Importance of Molecular Orientation

Molecular orientation refers to the specific arrangement or alignment of molecules when they collide. For a reaction to successfully take place, the reactants must collide in a way that allows their reactive sites—specific areas on the molecules—to come into contact. If the molecules approach each other at the wrong angle or side, the reactive sites might not meet, and the reaction will not occur.
Imagine trying to stack blocks: if they're not aligned correctly, the tower can't be built. Similarly, if molecules are not correctly oriented, even if they have enough energy, they still won't react. For a successful molecular collision:

• Correct alignment of reactive sites is essential.
• Molecules must approach each other in a way that favors interaction.

Chemistry often involves spatial considerations and this geometric arrangement truly matters at a molecular level. Thus, orientation is just as crucial as the energy of collision.

Temperature's Effect on Reaction Rates

Temperature plays a vital role in chemical reactions and how often they occur. When the temperature increases, molecules move faster and thus, have more energy. This energy increase means:

• More collisions between molecules occur.
• The collisions possess higher energy, increasing the chances of surpassing the activation energy barrier.

In simple terms, higher temperatures raise the overall number of effective collisions, leading to a faster reaction rate. Conversely, at lower temperatures, molecules move slower, resulting in fewer collisions and potentially insufficient energy to overcome activation energy, causing the reaction rate to drop.
Let's remember that not all reactions are benefited by increased temperatures, as some might lead to undesirable reactions. However, in most scenarios, heat boosts reaction speeds by energizing the molecules involved.