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Author Topic: enthalpy and stability!  (Read 27422 times)

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pink_ska_punk

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enthalpy and stability!
« on: April 11, 2006, 02:39:35 AM »

hey im doing some revision for my chem AS level and im struggling with most of the questions- but the answers arent in the book, or i dont understand them. there probably an obvious anser but i dont understand it so heres the question! :-\
a)How is the enthalpy of formation of a substance connected to the stability of the substance?
b)the enthalpy of combusion of rhombic sulphur is -296.6 kjmol   but monoclinic is -297, they both form suphur dioxide when burnt. why are these two values different and which of te two allotropes is more thermodynamically stable!! ??? ???
be grateful for any information or helpful links:)
thanks!
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fadingrainbows

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Re: enthalpy and stability!
« Reply #1 on: April 11, 2006, 03:39:32 AM »

a) the higher the enthalpy of formation of a substance, the higher the energy level of the substance.
The higher the energy level of the substance, the less stable it becomes, as it can lose more energy by reacting to from lower energy products.
This, however, only refers to thermodynamic stability, not kinetic stability, which is unaffected by enthalpy of formation.
b)I'n not sure on the difference, but the more thermodyamically stable allotrope is the one which is at a higher energy level, and will therefore have a larger enthalpy of combustion.
Hope that helps.
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pink_ska_punk

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Re: enthalpy and stability!
« Reply #2 on: April 11, 2006, 04:33:45 AM »

a) the higher the enthalpy of formation of a substance, the higher the energy level of the substance.
The higher the energy level of the substance, the less stable it becomes, as it can lose more energy by reacting to from lower energy products.
This, however, only refers to thermodynamic stability, not kinetic stability, which is unaffected by enthalpy of formation.
b)I'n not sure on the difference, but the more thermodyamically stable allotrope is the one which is at a higher energy level, and will therefore have a larger enthalpy of combustion.
Hope that helps.

thanks for the info im abit confused by the part b bit though... i see that a higher energy substance is less thermodynamically stable; so in part b do you mean that the more stable allotrope is the one which is at a lower energy level or is it a totally different concept?
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fadingrainbows

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Re: enthalpy and stability!
« Reply #3 on: April 11, 2006, 04:47:53 AM »

Absolutely right, the more stable allotrope is the one at a lower energy level, and therefore has a smaller enthalpy of combustion, exactly oppopsite to what I wrote in part b.  Sorry, that was a silly mistake on my part.
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Mitch

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Re: enthalpy and stability!
« Reply #4 on: April 11, 2006, 04:48:30 AM »

This is a horrible question and you should tell your teacher so. Both allotropes are stable or they would not exist! The enthalpy of combustion has absolutely nothing to do with stability, it only lets you determine how energetically favorable a given combustion reaction is. A better method for determining the overall stability of an allotropic state would be with enthalpy of formation not enthalpy of combustion.
« Last Edit: April 11, 2006, 04:50:50 AM by Mitch »
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pink_ska_punk

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Re: enthalpy and stability!
« Reply #5 on: April 11, 2006, 04:50:30 AM »

haha, the teacher didnt give it to me- its a book that did. but, ill make sure i tell her anyway!!! thanks:)
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