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Topic: Help with Hess's Law problem  (Read 3127 times)

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Offline ding0

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Help with Hess's Law problem
« on: November 22, 2009, 01:48:52 PM »
Fe2O3 (s) + 3CO (g)  :rarrow: 2Fe (s) + 3CO2 (g)  :delta: H = -23KJ

3Fe2O3 (s) + CO (g)  :rarrow: 2Fe3O4 (s) + CO2  :delta: H = -39KJ

Fe3O4 (s) + CO (g)  :rarrow: 3 FeO (s) + CO2 (g)  :delta: H = +18KJ

calculate delta H for the reaction

FeO(g) + CO(g)  :rarrow: Fe(s) + CO2(g)

Offline cliverlong

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Re: Help with Hess's Law problem
« Reply #1 on: November 22, 2009, 02:55:33 PM »
What do you know of Hess' Law? Have you studied it? If not, try section 1.2a here: http://www.docbrown.info/page07/delta1H.htm

What does the concept "alternate path" of a chemical reaction mean to you?

Clive

Offline Porochaz

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Re: Help with Hess's Law problem
« Reply #2 on: November 25, 2009, 08:19:39 AM »
Hi there,

Have you missed out a line of your equation or mistyped something because as far as I can see you can't get it from the information you have provided.

Also in regards to how to work it out. Its a bit like a jigsaw, if you look for your starting piece (or pieces) or reactants in your overall equation, in this case FeO (g) and CO (g) and your ending pieces or your products  - Fe (s) and CO2 and try and fit in the equations you have been given to get from start to finish. To do this you may have to reverse a reaction (for instance 3FeO (s) + CO2  :rarrow: Fe3O4 + CO) However remember to reverse your enthalpy (:delta:H) values as well. So in this example it would be -18kJ. It is also okay to multiply your equations to fit as well providing you change your enthalpy numbers to support this

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