Question

Graphite is used as a lubricant in machines because it has a very high melting point and also it:
A. Is crystalline
B. Has layer structure
C. Is a giant molecule
D. Is a liquid at room temperature

Answer 1

Hint: Graphite is an allotrope of carbon, which is chemically, consists of carbon atoms. It is made of hexagonal planes of \[s{p^2} - s{p^3}\] hybridized carbon bonds and has greasing properties. It is a black, soft, flaky solid and a moderate electrical conductor.

Complete step by step answer:
We have learnt that there are three allotrope of carbon namely diamond, graphite and fullerene. They show different properties due to their different structures. Graphite occurs in nature and can be prepared artificially by heating coke at 3273K-3300K in electric furnaces.
In graphite structure, all the carbon atoms have stable chemical bonds with the other three carbon atoms thereby making sheets, which are held by weak Van der Waals’ forces of attraction. Graphite has a two dimensional sheet consisting of benzene rings fused together
Since, only three electrons of each carbon atom are used in making hexagonal rings in the graphite structure, the fourth electron is free to move and is responsible for making graphite a good conductor of heat and electricity.
The carbon atoms in graphite are $sp^{3}$ hybridized and are directed in the same plane forming hexagonal rings. The two successive layers are held together by weak forces of attraction, one layer can slip over the other making the graphite soft and a good lubricating agent.
So, the correct answer is “Option B”.

Note:
It is an important point to remember that graphite is thermodynamically more stable than that of diamond. Although the conversion of diamond to graphite is thermodynamically favorable but does not occur due to high activation energy requirements. The reverse process (graphite to diamond) is thermodynamically not favorable but can be done under 50,000-60,000 atm pressure and 1873K temperature.