Every chemical reaction has a DH value. What does the symbol "DH" mean?

Every chemical reaction is either endothermic (heat
absorbing) or exothermic (heat producing.)  Capital H in chemistry (not to be confused
with the element Hydrogen!!) will be found with reaction formulas and refers to the Heat
of Formation, or the amount of thermal energy absorbed or released in the chemical
reaction.  The change in the Heat of Formation occurs when the
reaction proceeds; the concept of the thermal change in the reactants is usually
expressed as "Delta" H, or ∆H.