Carbon-graphite offers a unique combination of physical, chemical and mechanical characteristics found in no other material. One particular characteristic is its structure.
Graphite has a layered structure. In each layer, each carbon atom is bound to three others. This results in a two-dimensional network of hexagons. Within each layer, there are strong bonds, but between the different layers, the bonds are fragile. Thus, the layers can easily be shifted against each other and separated. This structure is why graphite is very soft and even used as a lubricant. But graphite has other unique characteristics as well:
Electrical Conductivity
The fact that graphite is electrically conductive results from its atomic structure. Each carbon atom in a graphite crystal has four valence electrons, also called outer electrons, which can form bonds with neighboring atoms. However, only three of the four valence electrons enter into a bond, while the fourth electron remains freely mobile and thus allows electricity to be conducted.
Thermal Conductivity
Graphite has excellent thermal conductivity combined with high-temperature resistance. Graphite does not have a melting point; it changes from the solid state directly into the gaseous state. This process is called sublimation. In an inert gas atmosphere, graphite becomes plastically deformable starting at 2500 °C. At temperatures above 3750 °C, graphite sublimates even without oxygen.
Chemical Resistance
Graphite is one of the most chemically resistant materials. It is resistant to almost all media of organic chemistry. These typically include the intermediate and/or end products in the petrochemicals, coal refining, plastics industry, the production of paints, coatings, refrigerants, and antifreeze, but also in the cosmetics and food industries. In addition, it is resistant to most inorganic media, such as non-oxidizing acids, alkalis, aqueous salt solutions, and most technical gasses.
Important Chemical Reactions
Reaction with Air
Carbon in the form of graphite, burns in the air to form Carbon Monoxide and Carbon Dioxide depending upon the availability of air or oxygen. Diamond is a type of carbon that, when heated to 600°C-800 °C, also burns in the air.
- C(s)+O2→CO2(g)
- When the availability of air or oxygen is limited, incomplete combustion occurs, resulting in carbon monoxide, CO.
- 2C(s)+O2(g)→2CO(g)
- This reaction is important in industrial processes, where air is passed through hot coke. The resulting producer gas is a mixture of carbon monoxide, carbon dioxide, nitrogen, along with small amounts of hydrogen (H2), methane (CH4), and oxygen (O2).
Carbon in the form of graphite doesn’t react with water in normal conditions. Under certain circumstances, the given reaction becomes possible and forms water gas which is a mixture of carbon monoxide and hydrogen gas.
- C+H2O→CO+H2
At elevated temperatures, graphite reacts with fluorine (F2) to form carbon tetrafluoride (CF4), along with smaller amounts of C2F6 and C5F12.
- C (s) + excess F2 (g) → CF4 (g) + C2F6 + C5F12
Other halogens do not seem to react with graphite.
Reaction with Acids
Mellitic acid, C6(CO2H)6, is formed when graphite combines with the oxidizing acid hot concentrated nitric acid.