1. Reactants collide with sufficient energy:

* Molecules need to have enough kinetic energy to overcome the activation energy barrier, which is the minimum energy required for the reaction to occur.

* This energy can come from heat, light, or other forms of energy.

2. Reactants collide with the correct orientation:

* Molecules need to collide in a way that allows the atoms to interact and rearrange.

* This means the reactive parts of the molecules need to be facing each other.

3. The reaction is thermodynamically favorable:

* The products of the reaction must be more stable than the reactants, meaning the reaction releases energy (exothermic) or requires less energy to maintain (endothermic).

In summary:

A chemical reaction occurs when reactants collide with enough energy and in the correct orientation, and the reaction is thermodynamically favorable. This results in the formation of new chemical bonds and the breaking of old ones, leading to the formation of products with different chemical properties than the reactants.