HW Ch.8 #59 and 63

[Michelle Dong 1F]

For #59, the answer writes the standard reaction enthalpy in kJ/mol. However, in #63, the answer key for standard reaction enthalpy is in kJ only. Why are some reaction enthalpies in kJ/mol and some in just kJ?

[Mike Vinci 2B]

Hey Michelle,
With the reaction in general it would be kJ/mol, yet if the question asks to find the enthalpy change associated with only one mole of a certain reactant then you just multiply delta H by moles and get units of only kJ. It is helpful to look at the stoichiometric coefficients to see what you need to divide by.

[Jana Sun 1I]

I'm still a little confused. The answer book gives our answer in kJ without dividing our standard reaction enthalpy by any mol value. For instance, in #63 part a, we're looking at the production of nitric acid. If we wanted our standard reaction enthalpy in kJ, wouldn't we need to divide by 2 (because there are two moles of nitric acid produced) to get ride of the mol^-1?

[Michelle Dong 1F]

I think that if it's asking for the standard reaction enthalpy, it's the enthalpy change for the entire equation so it doesn't matter how many moles of the substance itself because it's kind of like one mole of the entire reaction. I'm assuming this is why it can be in kJ and not kJ/mol. However, if it's asking for the standard enthalpy of formation, it must be forming only one mole of the substance itself.

[Christina Bedrosian 1B]

I am confused about this as well because in both questions 59 and 63 they are asking for the standard reaction enthalpy for the reactions and 59 uses kJ/mol while 63 only uses kJ. The questions are basically identical except for the reactions so how do we know when to use one or the other?

[Wenting Hu 2H]

I'm confused as well. Can someone explain?

[soniatripathy]

kJ/mol and kJ just denote whether the enthalpy given applies to one mole or reactant or multiple. Usually kJ is more all the moles of reactants while kJ/mole specifies for one mole. In those problems I think it is just referring to the same concept.

[Abel Thomas 2C]

I think generally we have to give the answer in kJ. You only have to give the answer in kJ/mol when the question explicitly asks you to.