Calculate the change in enthalpy #( DeltaH)# for the chemical reaction modeled by this chemical equation: #H_2 + Cl_2 rarr 2HCl# Is energy entering or leaving the system? How do you know? What happens to the covalent bonds during this reaction?
The enthalpy change is established by MEASUREMENT.......
And I don't know offhand how they measured the enthalpy change for the reaction,
but such a measurement could conceivably be done and no doubt has been done.
Bond enthalpies are, however, extensively tabulated, and these can be used to estimate the enthalpy change of a given reaction with a high degree of precision. See here for an example.