Final answer:
Using Hess's Law, the reaction ΔH°rxn is calculated by reversing and adjusting the given reactions such that only the desired reactants and products remain, and then summing the corrected ΔH values.
Step-by-step explanation:
To calculate ΔH°rxn for the reaction A + 2B -> 2C, we must use Hess's Law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps into which the reaction can be divided. The reaction we are looking at is the sum of other reactions for which we have been given ΔH values. By combining these reactions in such a way that only the desired reactants and products remain, we can sum their ΔH values to find the ΔHrxn for our target reaction.
First, we write the reactions provided, reversing any as necessary so that the substances appear on the correct side (reactant or product) and ensuring the stoichiometry matches the desired reaction. Then, we adjust the ΔH values for any reactions that have been reversed or multiplied. Finally, we sum these corrected ΔH values to obtain the ΔHrxn for the desired reaction.