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Erin_6227
#1 Posted : Wednesday, May 19, 2021 9:49:20 PM
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Hi,

I was wondering if you could explain when enthalpy has units of kJ/mol and joules (or kJ). I can see in my notes that enthalpy is energy so it is typically in joules; however, it seems that there are instances, such as a change in enthalpy, when the units are kJ/mol.

Here is a list of all the differences I have noticed so far. Are there any more I should know about and can you explain why there is a difference in units?
Enthalpy: J
Standard molar enthalpy of formation: kJ/mol
Standard heat of reaction: kJ
Standard reaction enthalpy: kJ

Thanks,

Erin
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INSTR_Katrina_128
#2 Posted : Friday, May 21, 2021 3:05:32 PM
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Hi Erin.

Great question! We encounter a lot of units through the MCAT, and it can start to get really confusing to sort through all of them.

Enthalpy is measured in units of ENERGY, which are joules (J).

Sometimes, chemists deal with things that have A LOT OF ENERGY, so they'll switch to writing enthaply in kilojoules (kJ).

This is analogous to humans, who measure their masses in kg rather than g. Even though grams are a perfectly good unit of measurement for mass, no one goes around saying "my mass is 60000 g" because that number is annoying to deal with. Instead, we say "my mass is 60 kg".

Same with enthalpy. Chemists will say "the standard heat of reaction is 50 kJ" because "50" is a "nicer number" to deal with instead of "50000 J". That's the only reason why.

Now... why the kJ/mol?

Well, when we're talking about enthalpy of FORMATION, we are interested in figuring out the enthalpy (energy) required to "do" this reaction PER MOLE of a reference species. If the enthalpy of formation is POSITIVE, then we need to add energy to the reaction for it to "go", and if NEGATIVE, the reaction will release energy when the reaction happens.

Consider the following example:

The enthalpy of formation of CO2(g) is -393.5 kJ/mol.

That means that producing 1 mole of gaseous carbon dioxide RELEASES 393.5 kJ of energy.

Chemists like to present this data on a per mole basis. It's like a recipe! If you have 1 mole of CO2, you get 393.5 kJ of energy. Then, if you have 2 moles of CO2, you get 2x393.5 kJ of energy.

It's similar to how we present density. If I asked you to get me 1 cup of water and 2000 L of water, and I asked you "what is the density of this water?", you would say "1000 kg/m³". The density doesn't change. Same thing with the enthalpy of formation.

If I asked you to get me 3 L of CO2 and 4000 m³ of CO2 (that's bad for the environment), and I asked you "what is the enthalpy of formation for each of these amounts of CO2", you can say "-393.5 kJ/mol", and I can't argue with you! That is a correct statement.

Let me know if this helps. 😀
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