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Topic: Thermal decomposition  (Read 1885 times)

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Offline voong

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Thermal decomposition
« on: June 26, 2013, 06:05:12 PM »
Hi,

I have a question that gives me the equation

Cs2CO3 (s)  :rarrow: Cs2O (s) + CO2(g)

and tells me that ΔHΘ = +400 kJmol-1 and ΔSΘ = +161 J mol-1K-1, and therefore decomposition takes place at ≥2483K.

Then in the next section it proceeds to say "However, Cs2CO3 decomposes at 883K, explain the difference in decomposition temperature."

This is all rather confusing, but I'm thinking it's got to do with polarisation of the anion? I don't know, any help is appreciated, thanks!

Offline magician4

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Re: Thermal decomposition
« Reply #1 on: June 26, 2013, 08:13:44 PM »
2483 K denotes the thermodynamic equilibrium where ΔG for this reaction will be exact zero :

Cs2CO3  ::equil:: Cs2O + CO2


now, for a given equilibrium there's more than one way to shift it to one side or another: changing temperature is only one of several options

another option in this case is the permanent removal of carbon dioxide  (just let it escape from the setup) : you  let LeChatelier's principle do the job for you

... and as the above equilibrium is a steady state equilibrium, with the reverse reaction (i.e. holding the counterbalance) not (fully) taking place due to the loss of carbon dioxide, the net shift of the situation (i.e. "net-reaction") will take place at significant lower temperatures

regards

Ingo
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Offline voong

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Re: Thermal decomposition
« Reply #2 on: June 27, 2013, 12:25:19 AM »
I understand now, thank you!

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