units for enthalpy problems

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units for enthalpy problems

Postby 304751905 » Tue Jan 17, 2017 3:44 pm

In homework question 8.59 it asks to calculate the standard rxn enthalpy for a given reaction and the answer is in KJ/mol, and 8.63 asks for the standard rxn of enthalpy for another equation but the answer is just given in KJ. What units should we use?

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Re: units for enthalpy problems

Postby Tara_Hekmati_3B » Tue Jan 17, 2017 4:09 pm

When finding standard rxn enthalpy it doesn't really matter if you use KJ/mol or just KJ (pretty sure my TA confirmed in class that both can be used and in the course reader it uses just KJ sometimes when finding the standard rxn enthalpy), but what you do have to make sure is labeled in KJ/mole is the enthalpy of formation.

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Re: units for enthalpy problems

Postby YuniLee_1K » Wed Jan 18, 2017 1:29 am

For standard reaction enthalpy, it's assumed that the amount of substance in question is 1 mole. So while the unit is typically written as kJ/mol, for standard rxn enthalpy this would be the same as writing kJ/1 mol = kJ, so they would be interchangeable. I think as long as you make sure the problem is referring to standard rxn enthalpy and not just enthalpy, either should be fine. Or just keep the unit consistent with what's given in the question!

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Re: units for enthalpy problems

Postby Kiana_Shibata_2G » Wed Jan 18, 2017 10:03 am

If you follow the units in the math that you are performing you should end up with the correct units, whether that is kJ or kJ/mol. Just make sure that if you are scaling and multiplying by mols you cancel the units and end up with kJ. I am pretty sure most of the time the units are kJ, but if the question specifically asks for kJ/mol then those will be the units.

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Re: units for enthalpy problems

Postby Chem_Mod » Wed Jan 18, 2017 10:42 am

Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

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