# If the standard state symbol means that the substance is pure (and at 1 bar) how is it possible to have a standard REACTION enthalpy?

For the reaction one mole of substance $A$ in equilibrium with one mole of substance $B$ the standard reaction enthalpy is defined: $$\triangle_rH^{\theta}(T)=H_m^{\theta}(B;T)-H_m^{\theta}(A;T)$$ However, I have two issues with this.

Firstly: the standard state of a substance (denoted by the symbol $\theta$) is the pure material at a pressure of 1 bar. How can the reaction enthalpy be standard - the reactant have to be mixed (thus are not pure) to react.

Secondly: what is $H_m^{\theta}$? Surely nothing can have a molar enthalpy because only changes in enthalpy can be measured not absolute values. Am I right in thinking that is is because the enthalpy scale has no zero value?

One last point is that enthalpy is absolutely defined as a thermodynamic quantity .. it is the internal energy plus the product of pressure and volume, $H=U + pV$. The issue is that the internal energy of a substance is hard to define absolutely .. are you going to include the binding energies of the core electrons to their atoms? How about the strong forces holding the nuclei together? Thus for chemical processes it almost always makes sense to deal with just changes in enthalpy, since most of the really low level stuff I mentioned doesn't change between reactants and products. I think that may have been part of what you were getting at with your second question.