Question

how a chemical reaction occurs??

Answer 1

For a chemical reaction to take place, the reactants must collide. The collision between the molecules in a chemical reaction provides the kinetic energy needed to break the necessary bonds so that new bonds can be formed.
The molecules must also collide in the right orientation, or hit at the right spot, in order for the reaction to occur.
However, many a times even after colliding with proper orientation reaction does not occur, in this type of reaction some activation energy is also provided.