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Author Topic: Hess's Law Calculation  (Read 153 times)
hipbear
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« on: February 07, 2010, 11:48:36 PM »

I spent forever trying to figure out the following calculation but not too sure if I am doing things correctly.  Can someone please tell me if I am on the right track.  Thanks!

 Use Hess’s law to calculate ∆H for: C6H6(l) + 15/2O2(g) --> 6CO2(g) +
 3H2O(l) given:

       6C(graphite) + 3H2(g) -->C6H6(l)   delta H = -49.0kJ
       C(graphite) + O2(g) --> CO2(g)   delta H -393.5 kJ
       H2(g) + 1/2O2(g) -->H2O(l)  delta H -285.8kJ

       1.  C6H6(l) --> 6C(graphite) + 3H2(g)
            delta H = 49.0kJ

       2.  6C(graphite) + O2(g) --> 6CO2(g)
             delta H2 = 6(-393.5k/J) = -2361kJ

       3. 3H2(g) + 3O2(g) --> 3H2O(l)
             delta H3 = 3(-285.8kJ) = -857.4kJ

       4.  C6H6(l) + 15/2O2(g) --> 6CO2(g) + 3H2O(l)             
               delta H4 = -3169.4kJ
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kingchemist
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« Reply #1 on: February 08, 2010, 02:41:30 AM »

Use Hess’s law to calculate ∆H for: C6H6(l) + 15/2O2(g) --> 6CO2(g) +
 3H2O(l) given:

       6C(graphite) + 3H2(g) -->C6H6(l)   delta H = -49.0kJ
       C(graphite) + O2(g) --> CO2(g)   delta H -393.5 kJ
       H2(g) + 1/2O2(g) -->H2O(l)  delta H -285.8kJ

       1.  C6H6(l) --> 6C(graphite) + 3H2(g)
            delta H = 49.0kJ

       2.  6C(graphite) + O2(g) --> 6CO2(g)
             delta H2 = 6(-393.5k/J) = -2361kJ

       3. 3H2(g) + 3/2O2(g) --> 3H2O(l)
             delta H3 = 3(-285.8kJ) = -857.4kJ

       4.  C6H6(l) + 15/2O2(g) --> 6CO2(g) + 3H2O(l)             
               delta H4 = -3169.4kJ

APart from the typo in red, I agree with the answer
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