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To know which method to use regarding enthalpy changes involving different reactions you must look at what information you are provided and then decide which method is best for the given information. Usually there is only one method that works with what is given. If you are given the reaction and are given bond enthalpies you need to evaluate which bonds are broken and which are formed and add the given bond enthalpies. Remember if the bond is formed energy is released so enthalpy is negative while it is positive for bonds that break. For the method deltaHf(products)-deltaHf(reactants) you will be given the individual enthalpies of formation for each molecule in the chemical equation. The 3rd method is when you use several reactions and their overall reaction enthalpies to find the reaction enthalpy of another chemical equation. Normally, these problems will be formatted in such a in which you are asked to find the reaction enthalpy for a given equation and are provided with several related equation and there enthalpies. Your goal is to rearrange these equations in such way that when you add them, they result in the chemical equation we need to find the reaction enthalpy for. To do this you may need to reverse certain reactions are change the stoichiometric coefficients(such as changing a equation with coefficients of 2 and 3 to 2/3 and 1). Just make sure when you do this that you multiply the reaction enthalpies by whatever you multiplied the coefficients by. Also, if you reverse the reaction, the reaction enthalpy will have the opposite sign. So basically if you are given specific bond enthalpies, use the bonding method. If given enthalpies of formation for the specific reactants and products use the formula deltaHf(products)-deltaHf(reactants). if you are given a reaction and several other ones and their reaction enthalpies use the rearrangin method.
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