Both enthalpyA thermodynamic state function, symbol H, that equals internal energy plus pressure x volume; the change in enthalpy corresponds to the energy transferred as a result of a temperature difference (heat transfer) when a reaction occurs at constant pressure. and the internal energyA thermodynamic function corresponding to the energy of a system; represented by the symbol U or E. are often described as state functions. This means that they depend only on the state of the system, i.e., on its pressureForce per unit area; in gases arising from the force exerted by collisions of gas molecules with the wall of the container., temperatureA physical property that indicates whether one object can transfer thermal energy to another object., composition, and amount of substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula., but not on its previous history. Thus any solutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture. of NaCl at 25°C and 1 bar (100 kPa) which contains a mixtureA combination of two or more substances in which the substances retain their chemical identity. of 1 mol NaCland 50 mol H2O has the same internal energy and the same enthalpy as any other solution with the same specifications. It does not matterAnything that occupies space and has mass; contrasted with energy. whether the solution was prepared by simply dissolving NaCl(s) in H2O, by reacting NaOH(aq) with HCl(aq), or by some more exotic method.
The fact that the internal energy and the enthalpy are both state functions has an important corollary. It means that when a system undergoes any change whatever, then the alteration in its enthalpy (or its internal energy) depends only on the initial state of the system and its final state. The initial value of the enthalpy will be H1, and the final value will be H2. No matter what pathway we employ to get to state 2, we will always end up with the value H2 for the enthalpy. The enthalpy change ΔH = H2 – H1 will thus be independent of the path used to travel from state 1 to state 2. This corollary is of course the basis of Hess' law. The change in enthalpy for a given chemical process is the same whether we produce that change in one or in several steps.