* Exothermic reactions release heat into the surroundings. This means the energy of the products (HCl) is lower than the energy of the reactants (H₂ and Cl₂).

* Bond formation releases energy. The strong bond formed in HCl releases more energy than the energy required to break the bonds in H₂ and Cl₂.

In summary: The formation of HCl releases more energy than it takes to break the starting bonds, making the overall process exothermic.