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You'll need to get the overall change in enthalpy for each equation used. If you're adding equations together to calculate the overall reactionary enthalpy, you'll need to scale and/or flip your equations with the calculated enthalpies to find the final answer.
For Hess's Law, you need to get the total sum of the entropies/enthalpies/Gibbs Free Energy (only because they're state functions/extensive properties) of the products and then subtract from it the total sum of the reactants. Don't forget to include the coefficients.
Usually, you'll be given a couple or more equations that are related to the main one given and then you'll want to algebraically solve to get the main equation through the other ones. Whatever changes made on those equations you make onto the delta H's. Then you add them up to get the net change in enthalpy.
Depending on what you are solving for(enthalpy, entropy... etc) find the values for both the products and reactants, making sure to multiply by coefficients if necessary and to find the total, sum the products and then sum the reactants. Subtract the reactants from the products for the final answer.
In order to use Hess's Law, you should be given multiple reactions so that you can add/subtract them as needed. Maybe if you aren't given multiple equations, you could use the reaction enthalpies for the formation of all the products and reactants, and create the equations yourself (ie creating an equation that includes C3H8 could be found by considering the combustion of C3H8 to form CO2 and H20).
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