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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Controlled Substance on May 12, 2008, 11:00:01 PM

Title: S8 -> 4 S2 (Enthalpy Change and Mechanism)
Post by: Controlled Substance on May 12, 2008, 11:00:01 PM

Quote

"Multi-Concept Problem"

Consider the reaction S₈(g) --> 4 S₂, for which (Delta)H = +99kJ.

(b) The average S-S bond dissociation energy is 225kJ/mol. Using the value of (Delta)H given above, what is the S=S double bond energy in S₂(g)?

It mentions in question (a) that S₈ is cyclic. (cyclooctasulphur I believe)

The solutions manual multiplies the S-S diss. energy by 8 when solving for the S=S bonds according to:
(Delta)H = D(reactant bonds) - D(product bonds)

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Why?

This is what I was going to do at first, but after drawing the structure it appeared to me as incorrect.
It seems to translate as: S₈(g) --> 8 S(g) --> 4 S₂

Why not S₈(g) --> 4 S₂ all in one shot?
Couldn't only 4 S-S bonds be broken, and four new bonds be made from the electron gained by each S atom?

So which mechanism is correct?

All other examples I have seen involve diatomic molecules only... *yawn!*  ;)

EDIT: Ok. I'm tired. I just realized the why the calculation involves all the bonds. Just need insight on mechanism for my curiosity. Thanks.

Title: Re: S8 -> 4 S2 (Enthalpy Change and Mechanism)
Post by: mark reda on January 23, 2013, 11:38:52 AM

I am having the same problem now, but looking at the structure, it should be 8 S-S, its similar to a ring