Combustion of propane, burning of methane and the decomposition of hydrogen peroxide into water and oxygen are all exothermic reaction because heat is given off from the system to the environment. However, only formation of table salt from Sodium and Chlorine is endothermic reaction, as the reaction is a simple combination reaction.

The formation of sodium chloride (NaCl) is an endothermic reaction because it requires energy in several steps, including:



Sublimation: Sodium metal sublimates, or turns directly from a solid to a gas, without becoming a liquid first. This is an endothermic process.


Ionization: Sodium atoms lose an electron to become Na+ ions, which is also endothermic.


Dissociation: The chlorine molecule splits into two chlorine atoms, which is endothermic.



Conversely, the formation of sodium chloride is not entirely endothermic, however, because the final step of the process is exothermic:


Ionic lattice formation: The Na+ and Cl- ions come together to form the ionic lattice of sodium chloride, releasing a large amount of energy.


There is an explanation video available below.