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Topic: Using Hess's Law to calculate delta H for a reaction  (Read 6668 times)

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Offline skoozy

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Using Hess's Law to calculate delta H for a reaction
« on: May 28, 2011, 10:51:19 PM »
Use Hess's Law to calculate delta H for the reaction: C6H4(OH)2(aq) + H2O2(aq)  → C6H4O2(aq) + 2H2O(l)

Given the following data:

C6H4(OH)2(aq) → C6H4O2(aq) + H2(g)    deltaH1 = +177.4
H2(g) + O2(g) → H2O2(aq)                      deltaH2 = -191.2
H2(g) + 1/2O2 → H2O(g)                         deltaH3 = -241.8
H2O(g) → H2O(l)                                     deltaH4 = -43.8

I know i need to reverse the 2nd equation, and times the 4th equation by 2.
Since the 3rd equation does not include any of the products or reactants of the target equation, do i not need to include it?

Any help would be appreciated!

Offline Schrödinger

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Re: Using Hess's Law to calculate delta H for a reaction
« Reply #1 on: May 29, 2011, 12:07:42 AM »
The 3rd equation is necessary in order to eliminate the O2 that will result from the 2nd equation.
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Offline Vidya

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Re: Using Hess's Law to calculate delta H for a reaction
« Reply #2 on: May 29, 2011, 10:07:19 AM »
If you don't include the 3rd equation you wont be able to get desired equation.

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