Thermochemical equation adding algebraically

Reality Check Notice that thermochemical equations can be added in exactly the same manner as algebraic equations in this case 1C02 and 02 canceled when the equations were added. 

The overall enthalpy change is the same as that indicated earlier. An useful consequence of Hess s law is that thermochemical equations can be added and subtracted just like algebraic equations. This facilitates the calculation of enthalpy changes for reactions which cannot be studied experimentally. 

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... 

Thermochemical equations may be added and subtracted as ordinary algebraic equations. This generalization is known as HESS LAW. For example, the addition of the two thermochemical equations... 

You can see that any number of chemical equations can be treated by algebraic methods, and the corresponding standard heats of reaction can be added or subtracted in the same fashion as are the equations. By carefully following these rules of procedure, you will avoid most of the common errors in thermochemical calculations. 

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