Why don't all collisions between reactant molecules lead to product formation?

Not all collisions between reactant molecules lead to product formation because they may lack the necessary energy or correct orientation.

In a chemical reaction, the reactant molecules must collide with each other with sufficient energy and in the correct orientation for a successful reaction to occur. This is known as the collision theory. However, not all collisions result in a reaction, and there are two main reasons for this.

Firstly, the colliding molecules must have enough kinetic energy to overcome the activation energy barrier. The activation energy is the minimum energy required to break the bonds in the reactant molecules so that new bonds can form in the product molecules. If the colliding molecules do not have enough energy to overcome this barrier, the reaction will not occur, even if the molecules collide. The proportion of molecules with sufficient energy to react is determined by the temperature. At higher temperatures, more molecules have the necessary energy, so the reaction rate increases.

Secondly, the molecules must collide in the correct orientation for a successful reaction. This is because only certain parts of the molecules (usually the reactive sites) are involved in the reaction. If these parts do not come into contact during the collision, the reaction will not occur. This is often the case in complex molecules where the reactive site may be shielded by other parts of the molecule.

In addition, even when the molecules collide with sufficient energy and in the correct orientation, there is still a probability factor involved. Not every collision, even under ideal conditions, will result in a reaction. This is due to quantum mechanical effects, which introduce an element of randomness into the process.

In summary, while collisions between reactant molecules are necessary for a chemical reaction to occur, they are not sufficient. The molecules must also have enough energy to overcome the activation energy barrier and must collide in the correct orientation. Even then, not every collision will result in a reaction due to the probabilistic nature of quantum mechanics.