Enthalpy diagram illustrating Hesss law. The net reaction is the same as in Figure 5.21 in the textbook, but here we imagine different reactions in our two-step version. As long as we can write a series of equations that add up to the equation we need, and as long as we know a value for H for all intermediate reactions, we can calculate the overall H.
Suppose the overall reaction were modified to produce 2H2O(g) rather than 2H2O(l). Would any of the values of H in the diagram stay the same?