Enthalpy change algebraic determination

In this section, you learned how to calculate the enthalpy change of a chemical reaction using Hess s law of heat summation. Enthalpies of reaction can be calculated by combining chemical equations algebraically or by using enthalpies of formation. Hess s law allows chemists to determine enthalpies of reaction without having to take calorimetric measurements. In the next section, you will see how the use of energy affects your lifestyle and your environment. 

EXAMPLE 2 It is impossible to measure accurately the heat liberated when C burns to CO, because the combustion cannot be stopped exactly at the CO stage. We can, however, measure accurately the heat liberated when C burns to CO2 (393.5 kJ per mole), and also the heat liberated when CO burns to CO2 (283.0 kJ per mole of CO). The enthalpy change for the burning of C to CO is determined by treating algebraically the two experimentally determined thermochemical equations. If two chemical equations are added or subtracted, their corresponding enthalpy changes are, by Hess law, to be added or subtracted. Thus... 

The reason we focus on formation reactions is because it is the changes in enthalpy for formation reactions that are tabulated and used to determine enthalpy changes for chemical processes. This is because any chemical reaction can be written as an algebraic combination of formation reactions. Hess s law therefore dictates how the AfH° values are combined. 

---