CHEMISTRY COMMUNITY

How do you know when the units are kj or kj/mol?

You must always use kJ/mole for standard enthalpy of formation because it is the formation of 1 mole of a compound from its elements. But you can use KJ for reaction enthalpy.

Generally, the enthalpy you solve for in a problem will be in kJ. However, it can vary depending on the what the problem asks for. For example, if it asks you to find the standard enthalpy of formation, it will be in kJ/mol since standard enthalpy of formation is in kJ/mol by definition. Pay attention to the wording of the problem and you'll know what to do.

Usually, the chemical reaction is in kj because each reaction has a varying ratio and number of moles. The question can ask, however, what the change per mole is for either the product or reactant. In that case, divide the kj quantity accordingly to find kj/mol.

Most of the time when working with reaction enthalpy problems the units will be in kJ. However, if a question contains wording such as "enthalpy per mole" or something of that sort, the answer will be in kJ/mol. Answers are in kJ/mol when working with standard enthalpies of formation, always.

The units would most likely be in kJ instead of kJ/mole. When multiplying the different enthalpies, the moles unit cancel out.