Standard enthalpy of reaction from standard enthalpies of formation

The standard enthalpies of formation of compounds are used to determine the standard enthalpies of reactions. Calculations of $\Delta_r H^0$ from the $\Delta_f H^0$ of compounds are based on the following:

i. Standard enthalpies of formation of an element are zero:

ii. The standard enthalpy of formation of a compound is equal to its standard enthalpy:

Consider the reaction

The standard enthalpy of the reaction is given by

$\Delta_r H^0 = (cH^0_C + dH^0_D) - (aH^0_A + bH^0_B)$ …