Don’t get confused here. Hess did not DISCOVER that elements in their elemental form have zero enthalpy. He DEFINED it to be that way. Remember, we have no way of measuring the absolute enthalpy of a substance. Thus, there is no way he could have discovered this. Instead, he said that since all we can do is measure the DIFFERENCE in enthalpy for a reaction, let’s say that all elements in their elemental form have zero enthalpy. That way, all enthalpies we measure will be RELATIVE to the potential energy of the elements.

In other words, Hess is not saying that an element has no stored energy. He is saying that we will define zero to be the stored energy of an element in its elemental form. That way, all enthalpies we measure will be relative to the energy stored in the elements.

If you don’t like the idea of “defining” a zero level, think about an analogous situation. Suppose you are in an airplane flying at 10,000 feet and you decide to stand on top of your seat. The seat is 2 feet above the floor of the airplane. How high are you standing? Well, that depends on your definition of zero. If your definition of zero is the ground, then you are standing at a height of 10,002 feet. Instead, if your definition of zero is the floor of the airplane, you are standing at a height of 2 feet. If we have no idea how high the plane is flying, we cannot measure absolute height from the ground. We do, however, know where the floor is. Thus, we can measure height from the floor of the airplane. Anything above the floor has a positive height, anything below the floor has a negative height.

This is what Hess did. He said that because we cannot measure absolute potential energy, we will simply define it to be relative to the elements. Anything with stored energy less than the elements which make it up has a negative enthalpy, while anything that has more stored energy than the elements which make it up will have a positive enthalpy.