Would the change in enthalpy change of chemical reaction in space be considered as change in internel enrgy?
Well, in principle yes, for example if you had a blob of liquid floating in empty space and wanted  to consider the whole blob as a thermodynamic system.

But it may be worth mentioning that wouldn't be a typical use of thermodynamics in astrophysics.  Usually you'd consider your thermodynamic system to be some small part of a larger system and the larger system would be a thermal and volume reservoir, so T and P would both be nonzero.

For example, it's common to consider the thermodynamics of stars, but what you do is consider a small packet of gas to be your system, and the rest of the star to be surroundings that establish a constant T and P for your packet.