ADP
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Enthaphy calculations for oxidation and reduction reactions
Hello I was curious to the method of calculating the kJ of energy produced by the flash reaction (redox):
3KClO4 + 8Al --> 3KCl + 4Al2O3
Do I need to know enthalpy of formation or something? Thanks for the help. Also I didn't think this fit in 'beginnings" or does it?
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vulture
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formation enthalpy of products minus formation enthalpy of reagents.
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ADP
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In the equation:
3KClO4 + 8Al --> 3KCl + 4Al2O3 + energy (heat + light)
- Wouldn't enthapy only give me the heat energy of that reaction? ( Light is given also )
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neutrino
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Enthalpy calculations will tell you the amount of heat released during the reaction. This includes thermal and radiant energy. There is no simple way
of determining the amount of each.
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vulture
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Radiant energy output is directly related to thermal output as it is caused by excited atoms/ions/molecules.
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neutrino
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I'm assuming that he wants to calculate the amounts of radiant vs. thermal energy released in the combustion of a sample of flash powder under a
certain set of conditions. Wouldn’t this depend on more complex factors like geometry, casing, and so forth?
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vulture
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Well, modeling the flame as a sphere and using emission values for Al2O3, Wiens law, etc, will get you a good guess of the wavelengths and the energy
output.
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neutrino
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Fair enough. It really depends on how accurate he wants his results to be.
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ADP
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Well actually my plans are to do a chem class demo with a two gram sample of stoichiometricly correct flash powder. What I want to do is to determine
how much energy is released along with the reaction.
Using enthalpy I would assume that would only calculate total heat energy as opposed to total energy all together (heat + light). Finding only heat
energy released would be fine.
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Nicodem
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Enthalpy is not the "total heat energy" (unless one performs the reaction at zero Kelvins). I think what you are looking for is the Gibbs free energy which gives you the energy that a reaction can release at the given temperature. To calculate it you would need to know the
entropy change of the reaction.
Light emited or radiant heat is the consequence of high temperature reached during the reaction and is thus part of the heat energy released (as
Neutrino and Vulture already explained). It is already included in the Gibbs energy.
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ADP
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Ahh I see so I would use the delta G<sub>f</sub><sup>o</sup> values that are in the CRC handbook along with the Gibb's
equation? I'm guessing this would determine total heat energy of reaction.
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blogfast25
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Quote: Originally posted by ADP  | | Ahh I see so I would use the delta G<sub>f</sub><sup>o</sup> values that are in the CRC handbook along with the Gibb's
equation? I'm guessing this would determine total heat energy of reaction. |
Apart from the CRC handbook, the NIST Web Book lists countless values for Heat of Formation, heat capacity and Entropy at various temperatures, for
large numbers of pure substances:
http://webbook.nist.gov/chemistry/form-ser.html
(input chemical formula to initiate search, then choose 'condensed phase data')
I had a quick peek and KClO4, KCl and Al2O3 are all represented, if you want to calculate the change in Gibbs Free Energy (ΔG) and Reaction
Enthalpy (ΔH) for this particular reaction.
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