Different Units for Different Reaction Enthalpy Methods

[SarahOMalley1D]

Hi! I was working on the textbook problems for bond enthalpy, and I noticed that all the answers were listed in the units kJ/mol instead of kJ. Is there a reason for this? Each question had similar instructions: "Use the bond enthalpies in Tables 4E.2 and 4E.3 to estimate the reaction enthalpy for...".

[Bryan Cheng 1H]

Reaction enthalpies are usually given in kJ/mol of reaction, for the reason that this makes them standardized and intensive. To take heat values in kJ to enthalpies in kJ/mol, simply divide by the least amount of moles for a complete reaction (this pdf explains it better than I can: http://dpuadweb.depauw.edu/harvey_web/Chem260/Chem260pdfs/CourseEssays/molRxn.pdf).

[ashna kumar 3k]

It's listed in terms of kj/mol because we use stoichiometric coefficients of a reaction when solving for enthalpy of a reaction. In other words, we compare the molar ratios of the formation of products from the amount of reactants, so when we break different bonds in reactants and products of a reaction, we need to put it over moles as it is the standard unit. Like Bryan said, it makes the unit intensive, meaning enthalpy won't change based on the amount of product (where bonds are breaking) or product (where bonds are forming).

[Daniel Li 3C]

It is listed in kj/mol because that is the unit for comparing the amount of heat per product vs the amount of heat from reactants. So, the kJ is the measurement of heat and it is over moles because we are trying to find the amount of heat is required or released per mole.

[Meghan Terrell 1B]

Hi,

Reaction enthalpies are usually given in kJ/mol so we can use the reaction enthalpies to find the enthalpy for different amounts of moles. For instance, if you are trying to find the enthalpy for a reaction with 5 g of a substance, you convert to moles and then can use the reaction enthalpy in kJ/mol to find the corresponding enthalpy for that specific amount of the substance. Enthalpies given next to reactions are also often in kJ/mol, or assumed to be. That way you can use it to find the enthalpy for various amounts of a substance.