A Diamond is Forever...Or Maybe Not

"A diamond is forever" is a phrase synonymous with one of the most successful advertising campaigns in history. This has been the slogan De Beers has used since 1948 to promote the sale of diamonds to couples world wide. It even inspired an adventure of Ian Flemming's 007. De Beers also has told us that diamonds are rare and exceptionally beautiful, neither of which is true. This leads to the question: are diamonds actually forever? Or is this another myth? Are diamonds a stable chemical compound?

Stability: in chemistry stability has a very specific definition. It is determined by the free energy of the product compared with the reactant of a reaction. This is a thermodynamic property. If the free energy of the product is less than the free energy of the reactant, in other words the free energy is a negative value, then the product is considered stable. Now taking a look at two allotopes of carbon (an allotrope is different structural form of the same element): graphite and diamond, we see that by converting diamond into graphite, the free energy is -2.9 KJ/mol. This means that diamonds are higher in energy than graphite, and also means that diamonds are unstable compounds. Hmm starting to look like a diamond is NOT forever.

However, since billions of dollars are tied up in the diamond industry, this must mean that they do not rapidly turn into graphite. This brings us to the concept of lability.

Lability: in chemistry lability describes the speed at which are reaction occurs. This is governed by the activation energy of the reaction. This is a kinetic property. The larger the activation energy, the more energy needed to be put into a reaction to drive it to completion. This is usually accomplished by adding heat to the reaction. In the case of diamonds, there is an extremely large activation energy.  Because of this, diamonds will not be converted into graphite without the addition of an IMMENSE amount of heat. 

So while diamonds are not stable, they are also not labile. This means that De Beers can get away with the slogan "a diamond is forever," even if it is not chemically true because on the time scale of the human lifespan we will never see the conversion to graphite. 

Another example of this that may be important to know during the holiday baking season is table sugar, aka sucrose! Sucrose, by the definitions above, is unstable because upon heating it has free energy of -5650 KJ/mol! But again, sucrose is also not labile and therefore requires the addition of heat. But the activation energy is not nearly as high as it is for diamonds, which is why sugar is so easy to burn: it doesn't require too much heat and causes the release of over 5000 KJ of energy. Keep that in mind before you ever try to microwave icing (I am looking at you Bridget).

References:

Petrucci, R. H.; Harwood, W. S.; Herring, F. G. General Chemistry 8th ed. 2002, Prentice Hall Inc. Upper Saddle River, NJ.

Campbell, G. Blood Diamonds 2004, Basic Books, Cambridge, MA.

Gray, T. The Elements 2009, Black Dog & Leventhal Publishers Inc. New York, NY.