When thermochemical data are quoted, often the units are #DeltaH*mol^-1#. In the photosynthesis reaction, why is #DeltaH_"rxn"^@=2800*kJ*mol^-1#?

#6CO_2(g) + 6H_2O(l) rarr C_6H_12O_6(aq) + 6O_2(g)#

#DeltaH=2800*kJ*mol^-1#

You have grasped why this reaction is endothermic. It is a bond breaking reaction, that requires the cleavage of strong #C=O# and #H-O# bonds. Thermodynamic data are always quoted per mole of reaction as written. That is the reaction of 6 mol quantities of carbon dioxide and water gives molar quantities of sugar and 6 mol quantities of dioxygen gas, with the consumption of 2800 kJ of energy.

Note that it is not only carbon dioxide cleavage that requires this energy; the reaction requires the breaking of strong #O-H# bonds, and this adds to the energy budget of the reaction.

When we do the reverse reaction, i.e. the combustion reaction, conservation of energy requires that for,

# C_6H_12O_6(aq) + 6O_2(g)rarr 6CO_2(g) + 6H_2O(l)#

#DeltaH=-2800*kJ*mol^-1#,

i.e. a molar quantity of glucose gives this energy on combustion (by formation of 6 molar quantities of carbon dioxide and water).

Of course, I am just writing this fact down here. But I can assure you that this combustion reaction has been experimentally measured ad infinitum, and #2800*kJ*mol^-1# (of glucose) results from the combustion.