[Howard610]

I have a question in the Free Energy section of our textbook that is giving me problems. Here goes "When carbon and oxygen combine to form carbon dioxide, the reaction releases more free energy when diamond is used as a source of carbon than when graphite is used as the carbon source. Based on this information, will diamond spontaneously decompose into graphite? If it does why do we not see our diamond jewelry become pencil points? If it does not, then does this imply that diamonds will last forever?

thanks

[Mitch]

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[Howard610]

Well i know that Diamonds will not decay to graphite. Based on previous chapters in chem class, i know that Diamond's structure differs from Graphite, Diamond has more carbon-carbon bonds thus is the reason for the its high melting temp. It takes a lot of energy to break all the bonds in Diamond. But,as we are just starting the chapter on free energy, i dont see the relationship.

[Yggdrasil]

Here's something to think about.  There is a difference between thermodynamic stabilty and kinetic stability.  As Mitch puts it

2. Don't confuse thermodynamic stability with chemical reactivity.

Let me explain further:

For us to observe a reaction, it must satisfy two criteria:
1)  The reaction must be spontaneous or we must supply enough energy for the reaction to be spontaneous (i.e. we don't see water decomposing into H₂ and O₂ unless we apply a current to the water).
2)  The reaction must proceed at a reasonable rate (i.e. if the compound decomposes one molecule every year, the reaction for all intents and purposes is not happening).

So, given the information you provided and the above explanation is the decomposition of diamond to graphite thermodynamically favorable (i.e. satisfying criterion 1)?  Is the decomposition of diamond kinetically favorable (i.e. satisfying criterion 2)?  Therefore, will we see diamond decompose to graphite?

[mdlhvn]

Here's something to think about.  There is a difference between thermodynamic stabilty and kinetic stability.  As Mitch puts it

    From the thermaldynamic point of view, a reaction is spontaneous if its free energy, or deltaG<0. There are many deltaG<0 reactions which canot actually occur without providing initial energy such as the burning of carbon, hydrogen.., the oxidation of some metals...The reason is the refusal of kinetic.

For us to observe a reaction, it must satisfy two criteria:
1)  The reaction must be spontaneous or we must supply enough energy for the reaction to be spontaneous (i.e. we don't see water decomposing into H₂ and O₂ unless we apply a current to the water).

It is not totally exact because the decomposition of water is not a spontaneous reaction. We have to provide energy continuously to make this occur. If you cut off the current, the reaction stops.

[Mitch]

Applying a current takes the chemical reaction out of thermodynamic equilibrium. Probably best not to use that example for a new student leaning the finer nuances between thermodynamic stability and chemical reactivity.