Heats of Formation

Scribe Post Author: Serene P.

February 19, 2014

Starting off class today, there was no homework to be checked in. Ms. Friedmann reminded us of our homework and informed us that we should do our best to complete our lab calculations independently.  Our homework due tomorrow includes:

Heats of Formation worksheet(with using the two sheets of Thermodynamic Data)

Webassign 8.3-8.4 due by 11.59 tonight

Watch a video Ms. Friedmann posted on moodle. http://vimeo.com/59855650 This will go over the two prelab questions for the lab to be performed tomorrow in class. If you are to be absent, here is the link to the worksheet of the lab with data. http://gbs-moodle.glenbrook225.org/moodle/file.php/12015/1314_Unit_8_--_Thermochemistry/Unit_8_Labs_and_Activities/2.19_Heat_of_Formation_Lab_with_Data.pdf

Heat of combustion lab write-up due tomorrow

During class today, we took notes on Heat Formation. Here are the notes: (Sorry! The diamond shaped figures with question marks in them were supposed to be triangles. I don't know how it occurred to be like this on the blog. Sorry again!)

Heats of Formation

4 NH3(g) + 7 02 → 4 NO2(g) + 6 H2O(l)

🔺H rxn= -1396KJ

4 NH3(g) + 7 O2 ---> 4 NO2(g) + 6 H2O(l)

*🔺H f (heat of formation) = The energy it takes to ‘form’ a COMPOUND from the elements that make it up.

*🔺H f often times, it’s exothermic. Sometimes its positive!

*🔺H f of an element (in its stable state) = 0KJ/moles

Examples: Na, H2, O2, Mg...etc.

*Units are KJ/moles

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🔺 H reaction= sum of 🔺H f products - sum of 🔺 H f reactants

🔺 H reaction= (4 moles x (+33.2 KJ/moles)) + (6 moles x (-285.8 KJ/moles)) - (4 moles x (-46.1 KJ/moles)) + (7 moles x (0 KJ/moles))

=(132.8 KJ - 1714.8 KJ) - (-184.4 KJ)

=-1582KJ + 184.4 KJ

= -1397.6 KJ

Here is a picture of the Thermodynamic Data (Please remember that not all data tables of these values will be identical):

After completing out notes for the day, Ms. Friedmann gave us time to work on our homework. We may look forward to completing another lab tomorrow that will not require a complete write-up but will have post-lab questions to be completed.

Thanks!

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