Java applet In the activity (upper right) move the sliders to find the amount of energy needed to break the bonds in the hydrogen and chlorine molecules changing them into free gaseous atoms.

In the activity (lower right) move the chlorine atom (green)
towards the hydrogen molecule to find the amount of energy released in the formation of one mole of hydrogen chloride molecules. 

The next step enables you to draw an energy diagram representing the energy content of the species before, during and after the reaction. 

Atoms in a molecule are held together by covalent bonds. Bond energy is needed to break these strong bonds.
This energy source may be heat, electricity or mechanical.