Question about calculating enthalpy change
The approach to estimating the enthalpy change in a reaction by using bond dissociation energies assumes homolytic cleavage of the bonds: each atom gets one electron from the covalent bond. 

The bond dissociation energy of the $\ce{H-H}$ bond is reported as 436 kJ/mol.  The relevant reaction is:

$$\ce{H2(g) -&gt; 2H.(g)}$$

Note that this reaction is not the same as $\ce{H2 -&gt; 2H+ + 2e-}$. The homolytic cleavage   reaction produces hydrogen atoms, not hydrogen ions. If we look up the standard enthalpy of formation for hydrogen atoms, it is +218 kJ/mol. Since the standard enthalpy of formation for $\ce{H2}$ is defined as zero, using the enthalpies of formation we also get $\Delta H = +436$ kJ/mol for this reaction. 

It is generally hard to use bond dissociation energies for heterolytic reactions (reactions that produce ions from molecules).