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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: skoozy on May 28, 2011, 10:51:19 PM

Title: Using Hess's Law to calculate delta H for a reaction
Post by: skoozy 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!

Title: Re: Using Hess's Law to calculate delta H for a reaction
Post by: Schrödinger 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.

Title: Re: Using Hess's Law to calculate delta H for a reaction
Post by: Vidya on May 29, 2011, 10:07:19 AM

If you don't include the 3rd equation you wont be able to get desired equation.