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Topic: Heat of combustion Question  (Read 4650 times)

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Offline Wsx594

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Heat of combustion Question
« on: October 05, 2013, 02:29:28 PM »
In a problem lab i'm to determine the heat of combustion for magnesium. The reaction is represented by Mg(s) + 1/2 O2 yields MgO(s). The accepted value is -601.8 Kj/mol. SpH of HCl is 4.184 J/g-k. I tested 2 reactions
1)  1g of MgO + 1M of 103.16 g of HCl which gave me a ΔT of 5.3 degrees C which I found q= -2309.77 J, I then found the molar enthalpy as -93.08 Kj/mol MgO 
2) .5 g Mg(s) + 1M of 102.3 g of HCl which gave me a ΔT of 21.7 degrees C which I used to find q= -9339.86 J, I then found the molar enthalpy as -453.91 Kj/mol Mg
Given these steps what would I do onward to find the heat of combustion?

Offline Borek

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Re: Heat of combustion Question
« Reply #1 on: October 05, 2013, 03:00:28 PM »
Apply Hess law.
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Offline Wsx594

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Re: Heat of combustion Question
« Reply #2 on: October 06, 2013, 11:19:50 AM »
Ok so I applied hess's law and got the reactions:
MgCl2+ H2O(l) yields MgO + 2HCl which produces 93.08 Kj

Mg(s)+ 2HCl yields MgCl2+ H2 which produces -453.91 Kj

H2+1/2O2 yields H2O (l) which produces -142.9 Kj
 
when these are added together you get-557.73 Kj when the value should be -601.8 Kj
Is there any math i'm doing wrong? please help

Offline Borek

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Re: Heat of combustion Question
« Reply #3 on: October 06, 2013, 02:37:22 PM »
At first sight logic looks OK. Not checking the numbers - 7% error is not that bad.

Energy unit is kJ, not Kj.
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