Heat summation, Hess’s law

In situations such as this, you can calculate AH for the formation of sulfur trioxide using Hess s law of heat summation. Hess s law states that if you can add two or more thermochemical equations to produce a final equation for a reaction, then the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction. Hess s law enables you to calculate enthalpy changes for an enormous number of chemical reactions by imagining that each reaction occurs through a series of steps for which the enthalpy changes are known. 

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