Heat of Formation

On Friday, we first started off with a Heat of Combustion lab. So basically, we added 100 mL of water into the calorimeter which is the can. We also had to mass the candle. We then quickly lit then candle and placed the calorimeter over the fire. Then we waited for the water to reach 35 celsius. Then we blew out the candle and recorded the highest temperature. Then we massed the candle once more to find out how much wax was used in the reaction. The lab is due on Tuesday with the rest of the worksheets and book problems. Ok, this is not cool. It's not letting me upload the pictures, so just look at Flickr to see the pictures of the lab.

http://www.flickr.com/photos/hc1011/5334103360/
http://www.flickr.com/photos/hc1011/5333487119/
http://www.flickr.com/photos/hc1011/5334103324/
http://www.flickr.com/photos/hc1011/5333486983/

After doing the lab, we started learning about Heat of Formation! How exciting. So basically heat of formation is the change in enthalpy that accompanies the formation of one mole of a compound from its elements. Here's a link if you want to learn more about enthalpy! http://en.wikipedia.org/wiki/Enthalpy
By the way, there was no notes sheet about this, so I'll try my best at explaining this. So there's an equation for finding the heat of formation.



So for calculating the Hf (heat of formation), you either have to look it up online, or in the chemistry text book. It's on page 209 or pg 638. Here's a table of what it looks like.
I'm pretty sure Mr. Lieberman said we don't have to memorize it. You only need to use this if it's a compound of two elements. For elements, the heat of formation in its stardard state is 0. Here's an example problem.
Calculate Delta H for reaction Kj/mole. Here's something that you should also consider. The form of the compound (example: gas, liquid, solid) matters.

So first, we find each heat of formation. The heat of formation for NH3 is (-46 kj/mole) You multiply it by 4 because in the equation, there are 4 moles of NH3. Then the heat of formation for O2 is just 0, because oxygen is is in its standard state. Then NO2 is (34 kj/mole) then multiply that by 4. Then finally, H20 is (-286 kj/mole) then multiply that by 6. So then when you multiple it out, the 6 moles of H20=-1716, 4 moles of NO2=136, O2=0, and then 4 moles of NH3=-184. You then take the products-reactants=delta H equation, so (H20+NO2)-(0+ -184)= delta H. Then your answer should equal to -1396. Remember, you always have to do products- reactants, not reactants-products.

After our notes, we had a quick discussion on rare earth metal magnets. To answer Mr. Lieberman's question "do magnets release or absorb energy when pulling them apart, or connecting them together", you have to think about it. Well, you have to use up energy when pulling apart the magnets, and then release energy to combine the magnet. Now think about this. Breaking apart two magnets is like breaking bonds which is an endothermic reaction and combining the magnets is like forming bonds which is an exothermic reaction...
It seems like we'll be learning more about this some other time. Okay, that's it for me. The homework is the pre-lab, webassign, post lab of the heat of combustion, worksheets, AND book problems. Cool. The next scribe is AnnMarie C.