Hess's law of constant heat summation

The law states that, "Overall the enthalpy change for a reaction is equal to sum of enthalpy changes of individual steps in the reaction".

The enthalpy change for a chemical reaction is the same regardless of the path by which the reaction occurs. Hess's law is a direct consequence of the fact that enthalpy is a state function: the enthalpy change of a reaction depends only on the initial and final states, and not on the path by which the reaction occurs.

To determine the overall equation of a reaction, the reactants and products in the individual steps are added or subtracted like algebraic entities.

Consider the synthesis of NH$_3$:

Adding (the book sums the two steps above a horizontal rule):

$3\,\mathrm{H_2(g) + N_2(g)} \longrightarrow 2\,\mathrm{NH_3(g)}, \quad \Delta_r H^0 = -92.2\ \mathrm{kJ}$ …