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Enthalpy Equation! Need By Tonight Please!!!

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Rookie
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(University of Nevada - Reno)
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Date Posted: 6/26/2008 11:19:40 PM  Status: Live
Enthalpy Equation! Need By Tonight Please!!!
Course Textbook Chapter Problem
General Chemistry N/A N/A N/A
Question Details:
For the reaction: 4 Fe(s) + 3 O2 (g) --> 2 Fe2O3 (s) the change in enthalpy is 1652 kJ. How many kJ of heat must be absorbed when 10.0 g of Fe and 4.00 g of O2 are reacted?

I have tried to work through it many times and only got an answer of 66.8 kJ. This isn't the right answer so whoever can help me out, I would most greatly appreciate it!

Answers:

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Sage
Karma Points: 3,963
Date Posted: 6/27/2008 1:56:11 AM  Status: Live
Asker's Rating: Lifesaver   
Response:
first of all we have to determine the limiting reactant,
the given reaction is
4Fe(s) + 3O2(g) -----> 2Fe2O3(s)
species          molecular wt.        mass(g)               moles
   Fe                    55.85                 10                    0.179
   O2                      32                     4                     0.125
from the reaction stoichiometry
4moles of Fe requires 3 moles of O2
0.179moles of Fe ---->? 
moles of O2 required = 0.179*3/4 = 0.134moles
but actually present = 0.125moles (less )
hence , O2 is the limiting reactant
so, for 3moles of O2 the change in energy is 1652KJ
for 0.125moles of O2 energy absorbed = (0.125moles O2*1652KJ)/3moles O2
amount of heat absorbed = 68.833KJ
ZMalz07's Comment:
Thank you so much! Very helpful!

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Expert
Karma Points: 971
Date Posted: 6/27/2008 2:02:45 AM  Status: Live
Asker's Rating: Helpful   
Response:
First convert the masses you are given into moles:
(10.0g Fe) / (55.85 g/mol) = 0.179 mol Fe
(4.00g O2) / (32.00 g/mol) = 0.125 mol O2

Then convert the number of moles into equivalents (divide each amount by the stoichiometric coefficient from the balanced equation):
0.179 mol Fe / 4 = 0.0448 eq Fe
0.125 mol O2 / 3 = 0.0417 eq O2

This tells you that the oxygen is the limiting reagent.  Therefore 0.0417 eq of each reagent will react, giving 0.0417 eq of product.

The change in enthalpy given is for exactly one equivalent of all reagents reacting (4 moles Fe and 3 moles O2).  Because only 0.0417 eq reacted, multiply this by 1652 kJ to get the amount of heat absorbed: 68.9 kJ

Depending on where you round, you also might get 68.8 kJ as an answer.
ZMalz07's Comment:
Thank you so much! So helpful!



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