Question

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

Answer 1

This is a good question. Thermodynamic data are quoted per mole of REACTION as WRITTEN.

Explanation:

`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.