some thermochemistry help.

[JohnDoeDDS]

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Hello everyone. I have a big mid-term coming up tomorrow (well tuesday), and I have some questions I am trying to figure out. I would appreciate any help you guys can give me. Also I will most likely post some threads tomorrow in the afternoon on things Im not sure about. I hope you guys don't mind. Anyways here is the question that's giving me trouble.

How much energy in joules is released when you burn 454 grams (1 pound) of fat, C63H122O6 in your body? (The heat of combustion of fat is -36700kJ/mole).

Do you just figure out how many moles you have by doing tthe weight/FW and then multiply by the heat of combustion of fat?

If so I got .466 moles of fat multiplied by (-36700kJ) and got -17106.57 kJ.

Is this correct?
--------------------------------------------------------------------------

And the second question:

H2(g) + 1/2O2(g) -->H2O(l) Delta H = -285.85 kJ
C3H4(g) + 4O2(g) -->3CO2(g) + 2H2O(l) Delta H = -1941 kJ
C3H8(g) + 5O2(g) -->3CO2(g) + 4H2O(l) Delta H= -2220 kJ

Calculate Delta H for the reaction:

C3H4(g) + 2H2(g) -->C3H8(g)
--------------------------------------------------------------------------

Thank you everyone so much for your help. Sorry about the long post but it's very important for me to understand this and Im having some trouble cuz my prof. sucks. Thanks again everyone and please explain how you arrived at your answer.

 

[victorria82]

Hello everyone. I have a big mid-term coming up tomorrow (well tuesday), and I have some questions I am trying to figure out. I would appreciate any help you guys can give me. Also I will most likely post some threads tomorrow in the afternoon on things Im not sure about. I hope you guys don't mind. Anyways here is the question that's giving me trouble.

How much energy in joules is released when you burn 454 grams (1 pound) of fat, C63H122O6 in your body? (The heat of combustion of fat is -36700kJ/mole).

Do you just figure out how many moles you have by doing tthe weight/FW and then multiply by the heat of combustion of fat?

If so I got .466 moles of fat multiplied by (-36700kJ) and got -17106.57 kJ.

Is this correct?
--------------------------------------------------------------------------

And the second question:

H2(g) + 1/2O2(g) -->H2O(l) Delta H = -285.85 kJ
C3H4(g) + 4O2(g) -->3CO2(g) + 2H2O(l) Delta H = -1941 kJ
C3H8(g) + 5O2(g) -->3CO2(g) + 4H2O(l) Delta H= -2220 kJ

Calculate Delta H for the reaction:

C3H4(g) + 2H2(g) -->C3H8(g)
--------------------------------------------------------------------------

Thank you everyone so much for your help. Sorry about the long post but it's very important for me to understand this and Im having some trouble cuz my prof. sucks. Thanks again everyone and please explain how you arrived at your answer.

hello there,

let us see if i can help. if someone sees anything dastardly wrong, please help the man or woman 🙂

here we go:

1) remember, u want the answer in Joules. SO do dimensional analyses here. u want to go from grams ==> Joules. SO here is a look at the units that will lead ur to the right answer:


GRams x (1mole/ grams) x (kJ/mole) = Something in kJoules

2) remember enthalphy change of a reaction is delta H of products - delta H of reactants

in the prob u are given the delta Hs. NOW u need to maniuplate the given three chem eqns to form the desired reaction of interest (ie the last chem reaction u wrote). SO

first, arrange the three given reactions in such a way that they look like the desired reaction. meaning u want the first two chem eqns as is while flipping the third chemical reaction. flipping it will reverse the sign of the delta H for the third reaction. furthermore notice how some "intermediate" reagents like H2O cancel when opposite from each other among three reactions. NOW before u can even add the Delta H's of the three reactions to get the desired OVERALL delta H, you must look at how the three given reactions compare stoichiometrically to the desired chemical reaction. u must adjust accordingly by multiplying by the appropriate factors to the delta Hs before adding them to get ur answer.


hope this helps or reminds u what to do in general for these types of probs

i have a physical chem exam on wed and i need alot of prayers for that.

best
victorria

 

[snoosnoo]

Thats G-chem and you should be able to do this if you're planning on going to Dental School 😛

Answer to first is correct, unless your in a P-Chem class and youre leaving out some other info

To answer the second one, use hess's law. Multiple first rxn by 2. Then flip the last one. Add up the values for delta h and got your answer. Make sure you take care of the signs when flipping.

Not hard stuff. 😛 You'll see this on the DAT so make sure you know it.

 

[Mr.E]

Hello everyone. I have a big mid-term coming up tomorrow (well tuesday), and I have some questions I am trying to figure out. I would appreciate any help you guys can give me. Also I will most likely post some threads tomorrow in the afternoon on things Im not sure about. I hope you guys don't mind. Anyways here is the question that's giving me trouble.

How much energy in joules is released when you burn 454 grams (1 pound) of fat, C63H122O6 in your body? (The heat of combustion of fat is -36700kJ/mole).

Do you just figure out how many moles you have by doing tthe weight/FW and then multiply by the heat of combustion of fat?

If so I got .466 moles of fat multiplied by (-36700kJ) and got -17106.57 kJ.

Is this correct?


--------------------------------------------------------------------------

And the second question:

H2(g) + 1/2O2(g) -->H2O(l) Delta H = -285.85 kJ
C3H4(g) + 4O2(g) -->3CO2(g) + 2H2O(l) Delta H = -1941 kJ
C3H8(g) + 5O2(g) -->3CO2(g) + 4H2O(l) Delta H= -2220 kJ

Calculate Delta H for the reaction:

C3H4(g) + 2H2(g) -->C3H8(g)
--------------------------------------------------------------------------

Thank you everyone so much for your help. Sorry about the long post but it's very important for me to understand this and Im having some trouble cuz my prof. sucks. Thanks again everyone and please explain how you arrived at your answer.

For the first problem multiply the moles by (-36700kJ) this will give you KJ. If you are looking to get your answer in joules you need to convert KJ to Joules by multiplying by 1000.

 

[yohan520]

Just to add on to the 2nd equation remember that if

--------------------
A --> B = H
B --> A = -(H)
--------------------
A --> B = H
2A -> 2B = 2(H)
--------------------
A --> B = H
A/2 --> B/2 = H/2

 

[smile87]

For the Hess' Law problem - Here's the answer I got. To start, multiply the first equation by 2 - this means you'll have to multiply the deltaH for this first reaction by 2, too - or (-285.85 KJ* 2). Remember - the equation is now 2 H2 + 1 O2 -> 2 H20. The second reaction remains the same - do nothing to it. So, deltaH = -1941 kJ. The third and final reaction has to be switched, which means making the products the reactants and vica versa. The 3rd reaction becomes 3CO2 + 4H20 -> C3H8 + 502 and the deltaH for this reaction instead of being -2220 kJ becomes +2220 kJ. (NB - we switch the sign of the deltaH!) Now, write out all three equations one on top of the other - can you see there are some reactants and products that are the same and can be crossed out? If you have 502's on the left sides of the arrows can you find 502's on the right sides of the arrows? If you're having trouble with this simply combine all three reactions - add up all of the reactants on the left sides of the arrows and products on the right sides of the arrows to make a really long equation- now do you see some similiarities between the two sides? Cross out the 5O2's, 4H20's and 3CO2's and you'll get your target equation of C3H4 + 2H2 -> C3H8. Now to calculate the final deltaH for this reaction add up all of the individual deltaH's of the reactions you just worked with. deltaH total = deltaH1 + deltaH2 + deltaH3. DeltaH total = (-285.85kJ*2) + (-1941kJ) + 2220kJ = -292.7 kJ. These problems are all about finding how to manipulate the reactions given in order to get your target equation. For me, that was always the tricky part. I bet in the back of your text you can find a lot of good practice questions - give them a shot. Also please, by all means, correct me if I am wrong! 🙂 Good luck on your test.

 

[JohnDoeDDS]

Guys thansk so much for your help. I skimmed through the answers as I just begun to study. I am going to try and figure it out myself, perhapos on a fresh day Ill get it. And then only look at the answers that you posted. I am taking Gen Chem I, and I know I need to do this to get into dental school. Its just that my proessor sucks balls and the textbook that we use is just rediulous and only gives one example. I plan to study like crazy for dats so Ill get it but I need to learn how to do it the professors way if you know what I mean...

Yea this Hess' Law crap is giving me a headache. Well back to studying. Thakns everyone who helped out. Ill let you know how it goes.

 

[xxJuiceDxx]

I think I learned more G.Chem concepts as I was studying for the DAT than I did when I actually took the course. 😱 Yes, I am a ******. 😀