why can't enthalpy change be calculate sometimes? eg in this question:
The enthalpy change of formation, ΔHf , of glucose, C6H12O6, cannot be determined directly. The equation for this enthalpy change is shown below.
6C(s) + 6H2(g) + 3O2(g) C6H12O6(s)
Suggest why the enthalpy change of formation of C6H12O6 cannot be determined
directly