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Topic: Hess's Law help  (Read 3718 times)

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Offline yoyoils

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Hess's Law help
« on: August 19, 2011, 08:56:29 PM »
Use the thermochemical equations shown below to determine
the enthalpy for the reaction: 3Fe2O3(s)  +  CO(g)=>2Fe3O4(s) +  CO2(g)


Fe2O3(s) + 3CO(g)=>2Fe(s) +  3CO2(g)       H=-5.7KJ

Fe3O4  +  CO(g)=>3FeO(s) +  CO2(g)       H=-4.5KJ

Fe(s)  +  CO2(g)=>FeO(s)  +  CO(g)       H=-0.3KJ

The rules I'm trying to follow are
Reactant must appear on reactant side, and product belongs on product side then multiple or divide following by summing up of the enthalpy but the thing I don't understand is equation #2 how CO & Fe3O4 are both on the same side so I'm not sure how to change it. Thank you in advance for your time! I'm in the middle of working this out as I post this.

Offline opti384

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Re: Hess's Law help
« Reply #1 on: August 19, 2011, 09:31:25 PM »
It's also possible to reverse the equation. In this case you have to change the sign of  :delta:H.

Offline yoyoils

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Re: Hess's Law help
« Reply #2 on: August 19, 2011, 09:43:50 PM »
It's also possible to reverse the equation. In this case you have to change the sign of  :delta:H.

Maybe if you tried to answer the question with the given equations, you'd see my problem and its gay cuz it makes me nervous that i cant get 40% of these basic thermodynamic questions correct

Offline opti384

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Re: Hess's Law help
« Reply #3 on: August 19, 2011, 09:50:41 PM »
Since Fe3O4 is the product in the overall equation, why don't you start with reversing equation 2?

Offline yoyoils

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Re: Hess's Law help
« Reply #4 on: August 19, 2011, 10:59:13 PM »
Oh wow thats a good idea.... lol I don't know why I didn't think of that... I just closed the practice window with it so now I can't even see if that works out! >,< thank you, I'll add that to my set of rules/steps.

I know this is off topic, but I'm running into a problem right now with the Arrhenius Equation, my book says:

k = 1014e -58.4/1.53 = 3.03 x 10-3 s-1

I'm trying to just get the same answer with my calculator except I'm getting -3.81715. I don't know what I'm doing wrong and dont care I just wish I knew how to get  = 3.03 x 10-3 s-1

thank you again

Offline Borek

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Re: Hess's Law help
« Reply #5 on: August 20, 2011, 03:58:07 AM »
Yes it is off topic and yes, you should start another thread. That being said...

k = 1014e -58.4/1.53 = 3.03 x 10-3 s-1

I'm trying to just get the same answer with my calculator except I'm getting -3.81715. I don't know what I'm doing wrong and dont care I just wish I knew how to get  = 3.03 x 10-3 s-1

Please elaborate, your notation is unclear, and the result I get from I think you mean is different from what you and/or your book states.

$$ 10^{14} e^{\frac {-58.4}{1.53}} = 0.00265 /$$
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