Units for Change in Enthalpy [ENDORSED]

[Jonny Zhao 1G]

Hello all!

In discussion I think my TA mentioned how the change in enthalpy () associated with a reaction is only measured in units of energy (for example, kJ). I initially took this to mean that the reaction requires/releases amount of energy to proceed in the forward direction one time.

However, a the previous lecture, Lavelle was calculating enthalpy using the bond enthalpies, which are measured in units of energy per moles (i.e., kJ/mol). Does this mean that the value associated with a reaction resembles the energy needed for the reaction going in the forward direction 6.0221*10²³ times? If not, is there any criteria (such as key words/phrases in the prompt) that can denote that a value is supposed to be "per mole of reaction"? Thank you!!!

[Chem_Mod]

Hi Jonny

Awesome question. Enthalpy is measured in kj/mol or J/mol, depending on what units you need. If you see an enthalpy given in just kJ then you can assume it's per mole. Usually when the question doesn't give you kJ/mol and kJ instead, you probably aren't being asked to calculate the total heat change.


To think about why we have per mole for enthalpy, think about a fire. If you have a matchstick (let's say 1 g of wood), then you're going to get a small flame. But if you have a forest burn down (let's say 500000 kg of wood), you're going to have a much bigger fire, and a lot more heat produced. So since enthalpy is used to calculate the change in heat energy, the amount of reactant that you have does matter.

Keep it up. Great job on thinking about the units.
-Vivek