Quick questions on Hess's law and bond energies: TCE Chemistry (Tasmania)

The standard enthalpy of formation $\Delta H_f^\circ$ is the enthalpy change when one mole of a compound forms from its elements in their standard states. Elements in their standard state have $\Delta H_f^\circ = 0$.

A reaction breaks bonds in the reactants (requiring energy) and makes bonds in the products (releasing energy):

When formation data are not directly available, you can construct an energy cycle from any set of reactions that share common intermediates, typically combustion reactions or formation reactions. Write the target reaction, then arrange the known equations (reversing or scaling as needed) so that adding them reproduces the target exactly. The sum of the adjusted enthalpies is the target $\Delta H$. This is the method used in the benzene and propan-1-ol questions above, and it works because enthalpy is a state function: the route does not matter, only the start and end states.