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Diamonds are forever, or at least so says Ian Fleming. While they are the hardest material known and seem like they should last for a zillion years, they are actually not the most thermodynamically stable form of carbon. In fact, that one-carat diamond you've been eyeing might possibly turn into graphite. It would be uncool if your gem suddenly became a pencil tip, no?
What exactly is the difference between diamonds and graphite?
First things first: Diamonds are very hard whereas graphite is very soft. Why is that? Well in a diamond there is a three dimensional network of strong covalent bonds. All of these crazy strong covalent bonds make the diamond very hard. So hard diamonds are used in cutting and grinding tools.
Graphite, on the other hand, is made up of flat layers of carbon atoms. These layers are held together by very weak forces called van der Waal's forces. These weak forces allow the layers to slip over one another. Can we guess what a consequence of these weak forces are? You guessed it! Graphite is extremely soft and slippery.
If diamonds are not the most thermodynamically stable form of carbon, then why do they seem so stable? It's not like you see diamonds turn into black graphite over a period of days, weeks, months, or even years. The answer is that diamonds are kinetically stable. Basically, the activation energy for the conversion between diamonds to pencil tips is ginormous. Basically, all of the carbon-carbon bonds in a diamond have to break to form the carbon bonds in graphite.
This can be a constant source of frustration for students. Thermodynamics and equilibrium do not necessarily happen in an eight-hour work day. Equilibrium may not occur except over the life of the universe. Kinetics can help tell us how long we have to wait for a reaction to reach equilibrium. It may be so fast that diffusion limits it from happening faster. It may happen over seconds, hours, or billions of years.
Sometimes what is important is not what atoms are bonded covalently, but how molecules are held together loosely. Intermolecular forces are often unappreciated, however, they are the glue that holds our molecular machinery together. Not to get too biology on us, but proteins, DNA, and cell membranes are all held together by intermolecular forces.
Intermolecular forces work well for biological things. They are not overly strong, so cell processes can yank them apart and put them back together with relative ease. For example, DNA double helices must be pulled apart for cells to reproduce. If the DNA double helix was held together strongly with covalent bonds, then cells would have to work extra hard to reproduce. Instead, DNA is held together with hydrogen bonds, which are much weaker than covalent bonds. This makes it easier to unzip DNA, and then rezip it.