Question

Why do diamonds and graphite, the two allotropic forms of carbon evolve different amounts of heat on combustion?

Answer 1

Combustion reaction:

A type of chemical reaction in which a substance burns or oxidized in the presence of air or oxygen is called a Combustion reaction.

For example:

When methane$\left({\mathrm{CH}}_4\right)$ undergoes a Combustion reaction in excess of Oxygen$\left({\mathrm{O}}_2\right)$ upon heating.

${\mathrm{CH}}_4\left(\mathrm{g}\right)+2{\mathrm{O}}_2\left(\mathrm{g}\right)\stackrel{\mathrm{heat}}{\to }{\mathrm{CO}}_2\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+\mathrm{heat}$

Diamond and Graphite:

1. Diamond and Graphite are allotropes of carbon.
2. They consist of carbon atom atoms but they have different physical properties.
3. They are polymorphs having the same chemistry but the crystalline structures are different.
4. They both have carbon atoms bounded with a strong covalent bond but they are arranged in different arrangements in them.

Since the structure and arrangements are different in graphite and diamond, the attractive forces between the atoms are different in both.

The melting point of the Graphite is more than that of the Diamond and the enthalpy of Carbon in Graphite is negative. This implies that Graphite is thermodynamically more stable.

Combustion reaction of Diamond and Graphite:

$$\begin{gathered} \mathrm{C}\left(\mathrm{diamond}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right)+393.5\mathrm{kJ} \\ \mathrm{C}\left(\mathrm{graphite}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right)+395.4\mathrm{kJ} \end{gathered}$$

Hence the evolution of heat is different in the amount in Graphite and Diamond.