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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: voong on June 26, 2013, 06:05:12 PM

Title: Thermal decomposition
Post by: voong 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 ΔH^Θ = +400 kJmol^-1 and ΔS^Θ = +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!

Title: Re: Thermal decomposition
Post by: magician4 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 (http://en.wikipedia.org/wiki/Le_Chatelier%27s_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

Title: Re: Thermal decomposition
Post by: voong on June 27, 2013, 12:25:19 AM

I understand now, thank you!