Question

can someone explain combination reaction with some examples

Answer 1

Combination reactions are usually exothermic because when the bond forms between the reactants, heat is released.
Example
The reactants, A and B, combine to form a new single product, AB, which is always a compound. An example of a combination reaction is when hydrogen and chlorine react to form hydrochloric acid: The subscripts in the chemical reaction in this image refer to the states of matter.

Answer 2

A combination reaction (also known as a synthesis reaction) is a reaction where two or more elements or compounds (reactants) combine to form a single compound (product). Such reactions may be represented by equations of the following form: X + Y → XY.
For example- 2H+O2---->2H2O