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Units of Reaction Enthalpies

Posted: Thu Jan 12, 2017 6:08 pm
by Pratyush Kandimalla 2O
When we calculate reaction enthalpies for reaction that have more than one product or the balanced reaction has multiple moles of a product; what unit should we use?
Should we write kJ/mol or just kJ (since there are multiple moles)?

Re: Units of Reaction Enthalpies

Posted: Thu Jan 12, 2017 6:37 pm
by Nerissa_Low_2F
I think reaction enthalpies are usually written in kJ/mol.

Re: Units of Reaction Enthalpies

Posted: Fri Jan 13, 2017 7:24 am
by Pratyush Kandimalla 2O
But what if the product generated is not one mole in quantity?
Or if there are multiple products? (so multiple moles of each)

Re: Units of Reaction Enthalpies

Posted: Fri Jan 13, 2017 10:00 am
by Chem_Mod
I will answer this in detail in all my classes today, Friday.

Reaction enthalpy is often kJ.
There are times when it is kJ.mol-1.

If someone wants to post a summary here of what I discuss in class that would be great.

Also see examples I already covered in class. If someone wants to quote examples (page numbers) from lecture notes for others to see when I used kJ and kJ.mol-1 that would also be a helpful summary.

Re: Units of Reaction Enthalpies

Posted: Fri Jan 13, 2017 12:24 pm
by Jasleen Kaur 1J
When the question specifies, that it is per mole you would use KJ/mol. Also in addition to that by definition the standard enthalpy of formation is KJ/mol.

Re: Units of Reaction Enthalpies

Posted: Fri Jan 13, 2017 2:22 pm
by Chem_Mod
Jasleen Kaur 1J wrote:When the question specifies, that it is per mole you would use KJ/mol. Also in addition to that by definition the standard enthalpy of formation is KJ/mol.


Yes and yes.

Re: Units of Reaction Enthalpies

Posted: Sun Jan 15, 2017 10:01 pm
by Jata_Kavya_3A
The units used are dependent on the situation. Typically, it is given in kj/mol unless otherwise stated.

Re: Units of Reaction Enthalpies

Posted: Mon Jan 16, 2017 10:51 am
by csebastiani_1B
Only for standard enthalpy of formation is it uniform to use kj/mol but if the also depends on what the question specifies. Its usually kj though

Re: Units of Reaction Enthalpies

Posted: Mon Jan 16, 2017 3:22 pm
by Ryan Petrecca 1L
It can be either kJ/mol or kJ, just keep in mind the coeffecients of the products and what the question wants you to give as a final answer.

Re: Units of Reaction Enthalpies

Posted: Wed Jan 18, 2017 11:50 am
by Janice Kim 3I
To give an example,
we used kj/mol for the standard enthalpy of formation of ethanol for the example (page 17 of the course reader) because the definition of the standard enthalpy of formation has to do with 1 mol of the product that was formed from the reactants. The same goes for the second example on page 18.
On page 16, we used kJ for the total enthalpy change because each reaction has a deltaH. The kJ in this case is not in context of moles, but of the reaction, and we use the kJ for each reaction to find the total enthalpy change in kJ.

Re: Units of Reaction Enthalpies

Posted: Thu Jan 19, 2017 3:38 pm
by Thomas Hui 2J
In last Friday's lecture by Dr. Lavelle, it was stated that the units of reaction enthalpies depends strictly on what is being asked. For example, if the question asks for the enthalpy of a formation reaction for a single molecule, it would be kJ/mol. If it does not happen like this, it will simply be kJ.

Re: Units of Reaction Enthalpies

Posted: Thu Jan 26, 2017 9:44 pm
by Jata_Kavya_3A
So basically to summarize:

q is generally in kJ
Delta H usually in kJ
Standard enthalpy of formation is only kJ/mol