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# how to calculate the entropy

how to calculate the DG for CACO3 given  :
c+ 02  -- c02     DG = -394.4 kj
CaO + CO2 -- CaCO3 DG = -130.4 kj
Ca + 1/2 O2 -- CaO   DG = - -604 Kj

a.  -1128.8 kj
b. -340 kj
c. -130.4 kj
d. 868 kj
e. 1128.8 kj

any ideas ?
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c_hockland
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1 Solution

Commented:
I'd have to break out the text book to be sure I'm correct...

But you need to manipulate the equations and arrive at the etropy through addition / subtraction / multiplication / division of the dG.

Again, until I can get home and pull out text books, I hesitate to go further.

(Its been awhile).
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Author Commented:
i still have 6 hrs before class time...so i can wait.
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Commented:
While you're anxiously waiting for class:

http://en.wikipedia.org/wiki/Hess's_Law

-- Been to long for me, need my old texts...

You know the end result is reactants = CaCO3

Hence:

C  +  O2 --> CO2
CaO + CO2 --> CaCO3
Ca + 1/2 O2 --> CaO

The CaO's cancel out
the CO2 cancel out

Take the link and run with it... You'll use the dG's supplied to calculate the new equation.

--> Sorry, but EE policy does not allow Experts to provide solutions to homework.
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Commented:
EE Engine didn't capture that link correctly... Select HESS'S LAW for the explanation and process.
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Commented:
What does DG stand for?
ie were your starting components CA, C, and O or CaO, CO2, or?
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Commented:
aburr -- DG (Delta G) -- change in Gibbs Free Energy.

They are given the equations provided, and through cancelation (and in some cases) manipulation of the equations and G given for equation, arrive at the needed equation and G for new equation.

C + O2 + Ca + 1/2 O2 --> CaCO3

If memory serves, then use the dG for equation against values found in table (back of Chem text) for an explicit value for CaCO3.
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Author Commented:
that is correct
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Commented:
If you need more help, feel free... but this should give you enough to solve the problem on your own. We are prevented from providing the solution to you.

After all ... you need to learn the material! :-)
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Project Engineer, ElectricalCommented:
c+ 02  --> CO2     DG = -394.4 kj
CaO + CO2 --> CaCO3 DG = -130.4 kj
Ca + 1/2 O2 --> CaO   DG = - -604 Kj

Ca + .5 O2 + C + O2 -> CaCO3
=-604kJ + -394.4kJ - -130.4 kJ     ====> 340kJ

Granted I haven't had chemistry in 4years, but I can make stuff up!
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Commented:
MrBullWinkle...

Actually, I suspect the - -604kj is a typo... likely simply -604 Kj

(Unless the original equation provided prior to posting anything was inversed, and student is displaying the sign change).

Also 340Kj is not an answer provided in the choice set.
(b) is *-* 340
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Project Engineer, ElectricalCommented:
(a) Ya, I didn't even notice the double negation there, so i assumed it to be negative (overlooked the typo).
(b) Ya, forgot to put the negative.
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